[bitwize]

This is a recurring pattern in Windows development. Unix devs look at the Windows API and go "This syscall takes 11 parameters? GROAN." But the NT kernel is much more sophisticated and powerful than Linux, so its system calls are going to be necessarily more complicated.

[trentnelson]

Curiosity got the better of me recently when I re-read Russinovich's [NT and VMS - The Rest Of The Story](http://windowsitpro.com/windows-client/windows-nt-and-vms-re...), and I bought a copy of [VMS Internals and Data Structures](http://www.amazon.com/VAX-VMS-Internals-Data-Structures/dp/1...).

Side-by-side, comparing VMS to UNIX, and VMS's approach to a few key areas like I/O, ASTs and tiered interrupt levels are simply just more sophisticated. NT inherited all of that. It was fundamentally superior, as a kernel, to UNIX, from day 1.

I haven't met a single person that has understood NT and Linux/UNIX, and still thinks UNIX is superior as far as the kernels go. I have definitely alienated myself the more I've discovered that though, as it's such a wildly unpopular sentiment in open source land.

Cutler got a call from Gates in 89, and from 89-93, NT was built. He was 47 at the time, and was one of the lead developers of VMS, which was a rock-solid operating system.

In 93, Linus was 22, and starting "implementing enough syscalls until bash ran" as a fun project to work on.

Cutler despised the UNIX I/O model. "Getta byte getta byte getta byte byte byte." The I/O request packet approach to I/O (and tiered interrupts) is one of the key reasons behind NT's superiority. And once you've grok'd things like APCs and structured exception handling, signals just seem absolutely ghastly in comparison.

[filereaper]

Since we're going into the history of Windows NT, VMS and Dave Cutler. I'd like to highlight this classic book on the history of all three of the above[1]

It follows the same line of narrative as The Soul of a New Machine

[1] https://www.amazon.com/Showstopper-Breakneck-Windows-Generat...

[trentnelson]

I freaking love Showstopper! Such a great book. It's probably the most information available on David Cutler anywhere.

The author also e-mailed me saying thanks when I tweeted him how much I liked the book, which I thought was super nice.

[jen20]

I've never met a single person who understood what they were talking about and referred to a "UNIX kernels". It may be true that Linux was once less advanced than NT - this is no longer the case, despite egregious design flaws in things like epoll. It has simply never been true (for example) for the Illumos (nee Solaris) kernel.

[ambrop7]

Which design faults do you think epoll specifically has?

I know there are lots of file descriptors not usable with epoll or rather async i/o in general and that sucks (e.g. regular files).

For networking, I find epoll/sockets nicer to work with than Windows' IOCP, because with IOCP you need to keep your buffers around until the kernel deems your operation complete. I think you have 3 options:

1) Design the whole application to manage buffers like IOCP likes (this propagates to client code because now e.g. they need to have their ring buffer reference-counted).

2) You handle it transparently in the socket wrapper code by using an intermediate buffer and expose a simple read()/write() interface which doesn't require the user to keep a buffer around when they don't want the socket anymore.

3) You handle it by synchronously waiting for I/O to be cancelled after using CancelIo. This sounds risky with potential to lock up the application for an unknown amount of time. It's also non-trivial because in that time IOCP will give you completion results for unrelated I/Os which you will need to buffer and process later.

On the other hand, with Linux such issues don't exist by design, because data is only ever copied in read/write calls which return immediately (in non-blocking mode).

[geocar]

The cool thing is that by gifting buffers to the kernel as policy, the gets to make that choice.

In Linux, you don't, and userspace and kernelspace both end up doing unnecessary copying, and it means with shared buffers, something might poll and not-block, but actually block by the time you get around to using the buffer.

This is annoying, and it generally means you need more than two system calls on average for every IO operation in performance servers.

As a rule, you can generally detect "design faults" by the number of competing and overlapping designs (select, poll, epoll, kevent, /dev/poll, aio, sigio, etc, etc, etc). I personally would have preferred a more fleshed out SIGIO model, but we got what we got...

One specific fault of epoll (compared to near-relatives) is that the epoll_data_t is very small. In Kqueue you can store both the file descriptor with activity (ident) as well as a few bytes of user data. As a result, people use a heap pointer which causes an extra stall to memory. Memory is so slow...

[ambrop7]

> something might poll and not-block, but actually block by the time you get around to using the buffer

On Linux if a socket is set to non-blocking it will not block. I don't really understand your point with shared buffers. You wouldn't typically share a TCP socket since that would result in unpredictable splitting/joining of data.

> In Linux, you don't, and userspace and kernelspace both end up doing unnecessary copying

I'm not so sure the Linux design where copies are done in syscalls must be inherently less efficient. I'm pretty sure with either design, you generally need at least one memcpy - for RX, from the in-kernel RX buffers to the user memory, and for TX, from user memory to the in-kernel TX buffers. I think getting rid of either copy is extremely hard and would need extremely smart hardware, especially the RX copy (because the Ethernet hardware would need to analyze the packet and figure out where the final destination of the data is!). Getting rid of TX copy might be easier but still hard because it'd need complex DMA support on the Ethernet card that could access potentially unaligned memory addresses. On the other hand, I also don't think you need more than one copy, if you design the network stack with that in mind.

> you need more than two system calls on average for every IO operation in performance servers.

True but it's not obvious that this is a performance bottleneck. Consider that a single epoll wait can return many ready sockets. I think theoretically it would hurt latency rather than throughput.

> As a rule, you can generally detect "design faults" by the number of competing and overlapping designs.

On Linux, I think for sockets, there are only: blocking, select, poll, epoll. And the latter three are just different ways to do the same thing. On Windows, it's much more complicated - see this list of different methods to use sockets (my own answer): http://stackoverflow.com/questions/11830839/when-using-iocp-...

> One specific fault of epoll (compared to near-relatives) is that the epoll_data_t is very small.

Pretty much universally when you're dealing with a socket, you have some nontrivial amount of data associated with it that you will need to access when it's ready for I/O, typically a struct which at least holds the fd number. Naturally you put a pointer to such a struct into the epoll_data_t. I don't see how one could do it more efficiently outside of very specialized cases.

[geocar]

> I'm not so sure the Linux design where copies are done in syscalls must be inherently less efficient.

Windows overlapped IO can map the user buffer directly to the network hardware, which means that in some situations there will be zero copies on outbound traffic.

> especially the RX copy I also don't think you need more than one copy, if you design the network stack with that in mind.

When the interrupt occurs, the network driver is notified that the DMA hardware has written bytes into memory. On Windows, it can map those pages directly onto the virtual addresses where the user is expecting it. This is zero copies, and just involves updating the page tables.

This works because on Windows, the user space said when data comes in, fill this buffer, but on Linux the user space is still waiting on epoll/kevent/poll/select() -- it has only told the kernel what files it is interested in activity on, and hasn't yet told the kernel where to deposit the next chunk of data. That means the network driver has to copy that data onto some other place, or the DMA hardware will rewrite it on the next interrupt!

If you want to see what this looks like, I note that FreeBSD went to a lot of trouble to implement this trick using the UNIX file API[0]

> On Linux, I think for sockets, there are only: blocking, select, poll, epoll. And the latter three are just different ways to do the same thing.

Linux also supports SIGIO[1], and there are a number of aio[2] implementations for Linux.

epoll is not the same as poll: Copying data in and out of the kernel costs a lot, as can be seen by any comparison of the two, e.g. [3]

Also worth noting: Felix observes[4] SIGIO is as fast as epoll.

> I don't see how one could do it more efficiently

Dereferencing the pointer causes the CPU to stall right after the kernel has transferred control back into user space, while the memory hardware fetches the data at the pointer. This is a silly waste of time and of precious resources, considering the process is going to need the file descriptor and it's user data in order to schedule the IO operation on the file descriptor.

In fact, on Linux I get more than a full percent improvement out of putting the file descriptor there, instead of the pointer, and using a static array of objects aligned for cache sharing.

For more on this subject, you should see "what every programmer should know about memory"[4].

[0]: http://people.freebsd.org/~ken/zero_copy/

[1]: http://davmac.org/davpage/linux/async-io.html#sigio

[2]: http://lse.sourceforge.net/io/aio.html

[3]: http://lse.sourceforge.net/epoll/dph-smp.png

[4]: http://bulk.fefe.de/scalability/

[5]: https://www.akkadia.org/drepper/cpumemory.pdf

[trentnelson]

I qualified it as "Linux/UNIX kernel" because I wanted to emphasize the kernel and not userspace.

Solaris event ports are good, but they're still ultimately backed by a readiness-oriented I/O model, and can't be used for asynchronous file I/O.

[binarycrusader]

Solaris event ports most certainly can be and are used for async I/O. I'm not sure how you can claim otherwise:

https://blogs.oracle.com/dap/entry/libevent_and_solaris_even...

https://blogs.oracle.com/praks/entry/file_events_notificatio...

And Solaris, (unlike Linux historically at least), supports async I/O on both files and sockets. Linux (historically) only supported it for sockets. I have no idea if Linux generally supports async I/O for files at this point.

[trentnelson]

Let me rephrase it: there is nothing on any version of UNIX that supports an asynchronous file I/O API that integrates cleanly with the file system cache -- you can do signal based asynchronous I/O, but that isn't anywhere near as elegant as having a single system call that will return immediately if the data is available, and if not, sets up an overlapped operation and still returns immediately to the caller.

This isn't a terrible recap of async file I/O issues on contemporary operating systems: http://blog.libtorrent.org/2012/10/asynchronous-disk-io/

[wahern]

They're called threads. Overlapped I/O on Windows is based on a thread pool, which is why you can't cancel and destroy your handle whenever you want--a thread from the pool might be performing the _blocking_ I/O operation, and in that critical section there's no way to wake it up to tell it to cancel. Just... like... on... Unix.

The difference between Windows Overlapped I/O and POSIX AIO is that on Windows it's a black box, so people can pretend it's magical. Whereas on Linux there hasn't been interest (AFAIK) to merge patches that provide a kernel-side pool of threads for doing I/O, and the decades-long debates have spilled out onto the streets. If you view userspace code as somehow inelegant or fundamentally slow, then of course all the blackbox Windows API and kernel components look appealing. What has held Linux back regarding AIO is that Linux (and Unix people in general) have historically preferred to keep as much in userspace as possible.

This is why NT has syscalls taking 11 parameters. In the Unix world you don't design _kernel_ APIs that way, or any APIs, generally. In the Unix world you prefer simple APIs that compose well. read/write/poll compose much better than overlapped I/O, though which model is most elegant and useful in practice is highly context-dependent. As an example, just think how you'd abstract overlapped I/O in your favorite programming language. C# exposes overlapped I/O directly in the language, but doing so required committing to very specific constructs in the language.

As for performance, both Linux and FreeBSD support zero-copy into and out over userspace buffers. The missing piece is special kernel scheduling hints (e.g. like Apple's Grand Central Dispatch) to optimize the number of threads dedicated per-process and globally to I/O thread pools. But at least not too long ago the Linux kernel was much more efficient at handling thousands of threads than Windows, so it wasn't really an issue. That's another thing Linux prefers--optimizing the heck out of simpler interfaces (e.g. fork), rather than creating 11-argument kernel syscalls. IOW words, make the operation fast in all or most cases so you don't need more complex interfaces.

[binarycrusader]

Your rephrasing still doesn't matter, Solaris event ports are not signal-based.

My understanding is that Solaris event ports were intended to offer equivalent functionality to Windows' I/O completion ports, so this should not be surprising.

Solaris also has its own native async I/O API in addition to supporting POSIX async.

[niels_olson]

Here's a nice graphical comparison of syscalls between Linux and Windows

http://www.visualcomplexity.com/vc/project.cfm?id=392

Are you saying the Windows flow looks like spaghetti only because the software tested software (Apache) wasn't designed for Windows?

[trentnelson]

Heh, 10 years old, original link doesn't work, image is tiny. And it sounds like they were comparing Linux and Apache to IIS and Windows.

It's hard to evaluate this in any way more than "yeah that's a cute spaghetti diagram". If I wanted to drag Linux through the mud visually I'd depict how much time every socket I/O op spends in vfs/fsync stuff. (i.e. you can depict anything to make your point)

[niels_olson]

That might be a cool diagram to show! I honestly know nothing about kernel programming, just happened to come across that a long time ago and bookmarked it.

[jsmeaton]

The second image is of IIS running on Windows, not Apache. Different software on different OSes. Regardless, it doesn't seem like parent is making the argument that the NT kernel isn't complicated - just that it is superior.

[bitwize]

It is more complicated because -- surprise -- I/O usage patterns do not fit in one neat little box, and you have to provide special handling of special cases that do occur frequently in the real world.

[1024core]

The joke used to be: VMS++ --> WNT

[CamperBob2]

Yep, that'd be the spiritual predecessor to Bing Is Not Google.

[rmu09]

First there was HAL, superior to IBM.

[adamnemecek]

Can I ask what else is on your reading list? I ended up buying the VMS Internals book.

Also do you have an opinion on BeOS?

[trentnelson]

I was fascinated by BeOS in the late 90s when I had a lot of enthusiasm (and little clue). All their threading claims just sounded so cool. I was also really into FreeBSD from around 2.2.5 so I got to see how all the SMPng stuff (and kqueue!) evolved, as well as all the different threading models people in UNIX land were trying (1:1, 1:m, m:n).

NT solves it properly. Efficient multithreading support and I/O (especially asynchronous I/O) are just so intrinsically related. Trying to bend UNIX processes and IPC and signals and synchronous I/O into an efficient threading implementation is just trying to fit a square peg in a round hole in my opinion.

As for reading list... I've bought so many old books lately. Here's my "makes the short list" bookshelf: http://imgur.com/DfTUVQx

And the more ridiculous one that I use as a cover page on my resume: http://imgur.com/0u9OZcN

What things in particular are you interested in?

[adamnemecek]

Lol, I've been saying something like this for some time.

Thanks for the answer. I guess what I'm interested is somewhat obscure/historical operating systems and also HW that are in some way superior to currently popular solutions. The more comparative the better.

Also your reading list has quite a few Oracle SQL entries so I'm guessing it's your preferred DB of choice. What features are you using that aren't available in MySQL or Postgres?

[trentnelson]

As far as commercial vendors go I really preferred SQL Server from about version 2000 onwards. I got into Oracle again recently for a consulting project with a finance client (who basically have unlimited Oracle licenses) and really quite enjoyed it since the Oracle 7/8 days.

You can do some phenomenally sophisticated things... I extensively leveraged things like partitioning, parallel execution (dbms_parallel_execute!), lots of PL/SQL using the pipelined table cursor stuff, data mining stuff (dbms_frequent_itemset!), index-organized tables, and my god, bitmap indexes were a godsend, direct insert tricks for bulk data loading, external tables were fantastic (you can wrap a .csv in an external table and interact with it in parallel just like any other table -- great for ingesting large amounts of janky .csv data from other parts of the business).

The parallel execution and robust partitioning options were probably the most critical pieces that have no particularly good counterpart in open source land.

[Joeri]

They also have a text mining engine which lets you dump almost any filetype in the database and do sophisticated full text queries, including generating html snippets with highlighted matches. And they have a native column type for storing geometry, so you can do geometric queries and aggregates, for which they then also have a web viewer engine with tiled maps support, so you basically have the core of a GIS engine. There's a ton of cool stuff in there, I haven't even looked at half of it.

[jamespo]

I always thought the external table functionality was a terrible idea, but if the use case is just for import that's more reasonable. Don't you like SQL*Loader or something ;)

[akavel]

Curious if you've ventured into (good/modern? maybe QNX?) microkernels at some point and have some thoughts on them by chance?

[trentnelson]

I have not I'm afraid. Hard enough keeping how all the parts of NT work in my head at the same time ;-)

[tremon]

But the NT kernel is much more sophisticated and powerful than Linux

That does not follow from the example. All it shows is that Microsoft prefers to put a lot of functionality in one interface, while Linux probably prefers low-level functions to be as small as possible, and probably offers things like filtering on a higher level (in glibc, for example).

Neither explanation has anything to do with sophistication. I personally believe that small interfaces are a better design.

[bitwize]

Actually it does, as it mentioned that the extra parameters are for things like async callbacks and partial results.

The I/O model that Windows supports is a strict superset of the Unix I/O model. Windows supports true async I/O, allowing process to start I/O operations and wait on an object like an I/O completion port for them to complete. Multiple threads can share a completion port, allowing for useful allocation of thread pools instead of thread-per-request.

In Unix all I/O is synchronous; asynchronicity must be faked by setting O_NONBLOCK and buzzing in a select loop, interleaving bits of I/O with other processing. It adds complexity to code to simulate what Windows gives you for real, for free. And sometimes it breaks down; if I/O is hung on a device the kernel considers "fast" like a disk, that process is hosed until the operation completes or errors out.

[piscisaureus]

I wrote the windows bits for libuv (node.js' async i/o library), so I have extensive experience with asynchronous I/O on Windows, and my experience doesn't back up parent's statement.

Yes, it's true that many APIs would theoretically allow kernel-level asynchronous I/O, but in practice the story is not so rosy.

* Asynchronous disk I/O is in practice often not actually asynchronous. Some of these cases are documented (https://support.microsoft.com/en-us/kb/156932), but asychronous I/O also actually blocks in cases that are not listed in that article (unless the disk cache is disabled). This is the reason that node.js always uses threads for file i/o.

* For sockets, the downside of the 'completion' model that windows is that the user must pre-allocate a buffer for every socket that it wants to receive data on. Open 10k sockets and allocate a 64k receive buffer for all of them - that adds up quickly. The unix epoll/kqueue/select model is much more memory-efficient.

* Many APIs may support asynchronous operation, but there are blatant omissions too. Try opening a file without blocking, or reading keyboard input.

* Windows has many different notification mechanisms, but none of them are both scalable and work for all types of events. You can use completion ports for files and sockets (the only scalable mechanism), but you need to use events for other stuff (waiting for a process to exit), and a completely different API to retrieve GUI events. That said, unix uses signals in some cases which are also near impossible to get right.

* Windows is overly modal. You can't use asynchronous operations on files that are open in synchronous mode or vice versa. That mode is fixed when the file/pipe/socket is created and can't be changed after the fact. So good luck if a parent process passes you a synchronous pipe for stdout - you must special case for all possible combinations.

* Not to mention that there aren't simple 'read' and 'write' operations that work on different types of I/O streams. Be ready to ReadFileEx(), Recv(), ReadConsoleInput() and whatnot.

IMO the Windows designers got the general idea to support asynchronous I/O right, but they completely messed up all the details.

[trentnelson]

You're completely missing how the NT I/O subsystem works, and how to use it optimally.

> * Asynchronous disk I/O is in practice often not actually asynchronous. Some of these cases are documented (https://support.microsoft.com/en-us/kb/156932), but asychronous I/O also actually blocks in cases that are not listed in that article (unless the disk cache is disabled). This is the reason that node.js always uses threads for file i/o.

The key to NT asynchronous I/O is understanding that the cache manager, memory manager and file system drivers all work in harmony to allow a ReadFile() request to either immediately return the data if it is available in the cache, and if not, indicate to the caller that an overlapped operation has been started.

Things like extending a file, opening a file, that's not typically hot-path stuff. If you're doing a network oriented socket server, you would submit such a blocking operation to a separate thread pool (I set up separate thread pools for wait events, separate to the normal I/O completion thread pools), and then that I/O thread moves on to the next completion packet in its queue.

> * For sockets, the downside of the 'completion' model that windows is that the user must pre-allocate a buffer for every socket that it wants to receive data on. Open 10k sockets and allocate a 64k receive buffer for all of them - that adds up quickly. The unix epoll/kqueue/select model is much more memory-efficient.

Well that's just flat out wrong. You can set your socket buffer size as large or as small as you want. For PyParallel I don't even use an outgoing send buffer.

Also, the new registered I/O model in 8+ is a much better way to handle socket buffers without the constant memcpy'ing between kernel and user space.

> IMO the Windows designers got the general idea to support asynchronous I/O right, but they completely messed up all the details.

I disagree. Write a kernel driver on Linux and NT and you'll see how much more superior the NT I/O subsystem is.

[haberman]

> The key to NT asynchronous I/O is understanding that the cache manager, memory manager and file system drivers all work in harmony to allow a ReadFile() request to either immediately return the data if it is available in the cache, and if not, indicate to the caller that an overlapped operation has been started.

The Microsoft article cited above (https://support.microsoft.com/en-us/kb/156932) directly contradicts you:

> Be careful when coding for asynchronous I/O because the system reserves the right to make an operation synchronous if it needs to. Therefore, it is best if you write the program to correctly handle an I/O operation that may be completed either synchronously or asynchronously.

Microsoft is directly saying that it reserves the right to violate the guarantee you are counting on at any time, and it documents several known cases of this. You can try to guess when this will happen and put those I/O operations on a different thread pool, but you're just playing whack-a-mole. And you're violating Microsoft's own recommendations.

[trentnelson]

That's not a particularly good article with regards to high performance techniques.

You wouldn't be using compression or encryption for a file that you wanted to be able to submit asynchronous file I/O writes to in a highly concurrent network server. Those have to be synchronous operations. You'd do everything you can to use TransmitFile() on the hot path.

If you need to sequentially write data, wanted to employ encryption or compression, and reduce the likelihood of your hot-path code blocking, you'd memory map file-sector-aligned chunks at a time, typically in a windowed fashion, such that when you consume the next one you submit threadpool work to prepare the one after that (which would extend the file if necessary, create the file mapping, map it as a view, and then do an interlocked push to the lookaside list that the hot-path thread will use).

I use that technique, and also submit prefaults in a separate threadpool for the page ahead of the next page as I consume records I'm writing to. Before you can write to a page, it needs to be faulted in, and that's a synchronous operation, so you'd architect it to happen ahead of time, before you need it, such that your hot-path code doesn't get blocked when it writes to said page.

That works incredibly well, especially when you combine it with transparent NTFS compression, because the file system driver and the memory manager are just so well integrated.

If you wanted to do scatter/gather random I/O asynchronously, you'd pre-size the file ahead of time, then simply dispatch asynchronous writes for everything, possibly leveraging SetFileIoOverlappedRange such that the kernel locks all the necessary sections into memory ahead of time.

And finally, what's great about I/O completion ports in general is they are self-aware of their concurrency. The rule is always "never block". But sometimes, blocking is inevitable. Windows can detect when a thread that was servicing an I/O completion port has blocked and will automatically mark another thread as runnable so the overall concurrency of the server isn't impacted (or rather, other network clients aren't impacted by a thread's temporary blocking). The only service that's affected is to the client that triggered whatever blocking I/O call there was -- it would be indistinguishable (from a latency perspective) to other clients, because they're happily being picked up by the remaining threads in the thread pool.

I describe that in detail here: https://speakerdeck.com/trent/pyparallel-how-we-removed-the-...

> > Be careful when coding for asynchronous I/O because the system reserves the right to make an operation synchronous if it needs to. Therefore, it is best if you write the program to correctly handle an I/O operation that may be completed either synchronously or asynchronously.

That's not the best wording they've used given the article is also talking about blocking. If you've followed my guidelines above, a synchronous return is actually advantageous for file I/O because it means your request was served directly from the cache, and no overlapped I/O operation had to be posted.

And you know all of the operations that will block (and they all make sense when you understand what the kernel is doing behind the scenes), so you just don't do them on the hot path. It's pretty straight forward.

[4ad]

> Write a kernel driver on Linux and NT and you'll see how much more superior the NT I/O subsystem is.

I wrote Windows drivers and file systems for about 10 years, and Unix drivers and file systems also for about 10 years.

I'd rather practice substance agriculture for the rest of my life than deal with Windows drivers again.

[trentnelson]

Yeah it's not a simple affair at all. It's a lot easier these days though, and the static verifier stuff is very good.

[thwarted]

I disagree. Write a kernel driver on Linux and NT and you'll see how much more superior the NT I/O subsystem is.

Can programming against the userspace interface the I/O subsystem really be compared to programming against the kernel driver interface to I/O subsystem? In Linux, kernel drivers have access to structures, services, and layers that userspace doesn't. And can these be compared between a monolithic and a micro-kernel approach, other than what has been debated ad nauseam for micro/monolithic kernels in general (not just used for I/O)?

[trentnelson]

I didn't make my point particularly well there to be honest. Writing an NT driver is incredibly more complicated than an equivalent Linux one, because your device needs to be able to handle different types of memory buffers, support all the Irp layering quirks, etc.

I just meant that writing an NT kernel driver will really give you an appreciation of what's going on behind the scenes in order to facilitate awesome userspace things like overlapped I/O, threadpool completion routines, etc.

[SwellJoe]

I have never worked with systems-level Windows programming, so I don't know the answer to this...but, how is what you're describing better than epoll or aio in Linux or kqueues on the BSDs?

I'm guessing you're coming from the opposite position of ignorance I am (i.e. you've worked on Windows, but not Linux or other modern UNIX), though, since "setting O_NONBLOCK and buzzing in a select loop, interleaving bits of I/O with other processing" doesn't describe anything developed in many, many years. 15 years ago select was already considered ancient.

[MarkSweep]

I think IO Completion Ports [1] in Windows are pretty similar to kqueue [2] in FreeBSD and Event Ports [3] in Illumos & Solaris. All of them are unified methods methods for getting change notifications on IO events and file system changes. Event Ports and kqueue also handle unix signals and timers.

Windows will also take care of managing a thread pool to handle the event completion callbacks by means of BindIoCompletionCallback [4]. I don't think kqueue or Event Ports has a similar facility.

[1]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa3... [2]: https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2 [3]: https://illumos.org/man/3C/port_create [4]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa3...

[trentnelson]

BindIoCompletionCallback is very old, the new threadpool APIs should be used, e.g.: https://github.com/pyparallel/pyparallel/blob/branches/3.3-p...

Regarding the differences between IOCP and epoll/kqueue, it all comes down to completion-oriented versus readiness-oriented.

https://speakerdeck.com/trent/pyparallel-how-we-removed-the-...

[MarkSweep]

The documentation for the new thread pool APIs in Windows say that it is more performant than the old ones. Do you have any experience transitioning from the old thread pool to the new one, and if so did you notice performance differences?

[trentnelson]

To quote myself:

> The “Why Windows?” (or “Why not Linux?”) question is one I get asked the most, but it’s also the one I find hardest to answer succinctly without eventually delving into really low-level kernel implementation details. >

> You could port PyParallel to Linux or OS X -- there are two parts to the work I’ve done: a) the changes to the CPython interpreter to facilitate simultaneous multithreading (platform agnostic), and b) the pairing of those changes with Windows kernel primitives that provide completion-oriented thread-agnostic high performance I/O. That part is obviously very tied to Windows currently. >

> So if you were to port it to POSIX, you’d need to implement all the scaffolding Windows gives you at the kernel level in user space. (OS X Grand Central Dispatch was definitely a step in the right direction.) So you’d have to manage your threadpools yourself, and each thread would have to have its own epoll/kqueue event loop. The problem with adding a file descriptor to a per-thread event loop’s epoll/kqueue set is that it’s just not optimal if you want to continually ensure you’re saturating your hardware (either CPU cores or I/O). You need to be able to disassociate the work from the worker. The work is the invocation of the data_received() callback, the worker is whatever thread is available at the time the data is received. As soon as you’ve bound a file descriptor to a per-thread set, you prevent thread migration >

> Then there’s the whole blocking file I/O issue on UNIX. As soon as you issue a blocking file I/O call on one of those threads, you have one thread less doing useful work, which means you’re increasing the time before any other file descriptors associated with that thread’s multiplex set can be served, which adversely affects latency. And if you’re using the threads == ncpu pattern, you’re going to have idle CPU cycles because, say, only 6 out of your 8 threads are in a runnable state. So, what’s the answer? Create 16 threads? 32? The problem with that is you’re going to end up over-scheduling threads to available cores, which results in context switching, which is less optimal than having one (and only one) runnable thread per core. I spend some time discussing that in detail here: https://speakerdeck.com/trent/parallelism-and-concurrency-wi.... (The best example of how that manifests as an issue in real life is `make –jN world` -- where N is some magic number derived from experimentation, usually around ncpu X 2. Too low, you’ll have idle CPUs at some point, too high and the CPU is spending time doing work that isn’t directly useful. There’s no way to say `make –j[just-do-whatever-you-need-to-do-to-either-saturate-my-I/O-channels-or-CPU-cores-or-both]`.) >

> Alternatively, you’d have to rely on AIO on POSIX for all of your file I/O. I mean, that’s basically how Oracle does it on UNIX – shared memory, lots of forked processes, and “AIO” direct-write threads (bypassing the filesystem cache – the complexities of which have thwarted previous attempts on Linux to implement non-blocking file I/O). But we’re talking about a highly concurrent network server here… so you’d have to implement userspace glue to synchronize the dispatching of asynchronous file I/O and the per-thread non-blocking socket epoll/kqueue event loops… just… ugh. Sure, it’s all possible, but imagine the complexity and portability issues, and how much testing infrastructure you’d need to have. It makes sense for Oracle, but it’s not feasible for a single open source project. The biggest issue in my mind is that the whole thing just feels like forcing a square peg through a round hole… the UNIX readiness file descriptor I/O model just isn’t well suited to this sort of problem if you want to optimally exploit your underlying hardware. >

> Now, with Windows, it’s a completely different situation. The whole kernel is architected around the notion of I/O completion and waitable events, not “file descriptor readiness”. This seems subtle but it pervades every single aspect of the system. The cache manager is tightly linked to the memory management and I/O manager – once you factor in asynchronous I/O this becomes incredibly important because of the way you need to handle memory locking for the duration of the I/O request and the conditions for synchronously serving data from the cache manager versus reading it from disk. The waitable events aspect is important too – there’s not really an analog on UNIX. Then there’s the notion of APCs instead of signals which again, are fundamentally different paradigms. The digger you deep the more you appreciate the complexity of what Windows is doing under the hood. >

> What was fantastic about Vista+ is that they tied all of these excellent primitives together via the new threadpool APIs, such that you don’t need to worry about creating your own threads at any point. You just submit things to the threadpool – waitable events, I/O or timers – and provide a C callback that you want to be called when the thing has completed, and Windows takes care of everything else. I don’t need to continually check epoll/kqueue sets for file descriptor readiness, I don’t need to have signal handlers to intercept AIO or timers, I don’t need to offload I/O to specific I/O threads… it’s all taken care of, and done in such a way that will efficiently use your underlying hardware (cores and I/O bandwidth), thanks to the thread-agnosticism of Windows I/O model (which separates the work from the worker). >

> Is there something simple that could be added to Linux to get a quick win? Or would it require architecting the entire kernel? Is there an element of convergent evolution, where the right solution to this problem is the NT/VMS architecture, or is there some other way of solving it? I’m too far down the Windows path now to answer that without bias. The next 10 years are going to be interesting, though.

https://groups.google.com/forum/#!topic/framework-benchmarks...

[wahern]

"The problem with adding a file descriptor to a per-thread event loop’s epoll/kqueue set is that it’s just not optimal if you want to continually ensure you’re saturating your hardware (either CPU cores or I/O)"

Both epoll and kqueue permit multiple threads to poll the same event set. Normally you do this in tandem with edge-triggered readiness (EPOLLET on Linux, EV_CLEAR on BSD) so that only one thread will dequeue an event.

How do think IOCP is implemented in Windows? There's a thread pool in the kernel which _literally_ polls a shared event queue. It's just hidden so you can pretend it's magical. But conceptually it works almost identically to how you would do it in Unix.

The benefit of IOCP is that it's a native API. It's warts and shortcomings notwithstanding, developers never even need to think about how it's actually implemented. Whereas with epoll and kqueue you either have to roll your own framework, or select from various third-party options. Seeing how the sausage is made can turn some people off. But just because you don't see the gory details doesn't mean it's implemented using magical fairy dust.

There's much to recommend Windows, and many things the NT kernel conceptually gets right. But IOCP vs polling? The only real difference architecturally is how much of the stack sits in user-space vs kernel-space, and how much of the stack is delivered by Microsoft (all of if in the case of IOCP) vs other sources (in Linux, glibc does AIO, while all the event loop and callback code is provided by various libraries or written yourself).

Putting more of the stack in kernel-space doesn't magically make it easier to perform optimizations. That's marketing speak and kernel fetishism. You have to first show why those optimizations can't be achieved elsewhere, like in the I/O or process scheduler. Various Linux components traditionally are more performant (e.g. process scheduling) than in Windows, so many of the optimizations wrt IOCP is arguably clawing back performance lost elsewhere in the system.

[zxcvcxz]

http://pyparallel.org/wrk-rps-comparison2.svg

According to your website pretty much every other technology runs better on Linux than it does on Windows, and of course pyparallel runs better than everything you tested.

How can I run these tests my self? I specifically want to test it against golang.

[trentnelson]

See here: https://www.reddit.com/r/programming/comments/3jhv80/pyparal...

Source: https://github.com/tpn/pyparallel-tefb

Go-specific comment thread: https://www.reddit.com/r/programming/comments/3jhv80/pyparal...

[abaines]

I believe Linux since 2.5 has 'proper' asynchronous system calls, io_getevents(2) and co.

Further information: https://www.fsl.cs.sunysb.edu/~vass/linux-aio.txt

[inopinatus]

Based on what you describe here, I'd say your comparative understanding is about two decades out of date. Async I/O has been a capability in various Unix and Unix-alike kernels for that long.

[trentnelson]

As in signal based AIO? Have you ever tried to use it in a high performance network server, where you want to have reads also satisfied from the cache if possible?

Because that is like pulling teeth on UNIX. See: https://groups.google.com/forum/#!topic/framework-benchmarks...

[ckaygusu]

I think the problem here is not a syscall taking 11 parameters, it's a syscall that merely lists what is inside a directory taking 11 parameters. ataylor_284 explained the reasons (how convincingly, I'd argue) but on the first sight that surely smells bloat.

I'd also object NT kernel being more "powerful". Sure unixy kernels and NT has their differences but I don't think either one is superior.

[bigger_cheese]

11 parameters may seem bloated but in some cases Unix syscalls weren't designed with enough parameters whcih caused a bunch of pain necessitating things like

dup->dup2->dup3 pipe->piep2 rename->renameat->renameat2

Best practice nowadays in linux is to allow overloading syscalls via a flags parameter.

see https://lwn.net/Articles/585415/

So modern linux syscalls may be bloated too.

[rbanffy]

I remember the struct I had to populate to start a new process in 1997 or 1998...

[darkengine]

It may be more "sophisticated" (sounds like a more positive synonym of "complex" to me), but I certainly don't think it's more powerful.

[deprave]

Since when is kernel complexity a measure of quality...? :)

[jasonm23]

hmmm...

Usage of the adjective "sophisticated" always precedes an outpouring either ignorance or straight bs.

[pjmlp]

Also UNIX devs seem to forget how cumbersome the X11, Xlib and Motif APIs are.

[pbarnes_1]

This was maybe the case at Linux 2.0, but is not the case now.

Also, Windows development is infinitely more painful than Unix/Linux.

[uudecode]

"... so its system calls are going to be necessarily more complicated."

Are you implying that an increase in "power" can never be achieved through increasing simplicity?

[bitwize]

That's the thing. Just by glancing at the API docs, Windows looks more complicated but where the rubber meets the road in terms of real high-performance application development, Windows is way simpler. In Windows you can do in one syscall what would take several in Linux. You can schedule I/O calls across multiple threads in a completely thread-safe manner without having to manage the synchronization yourself -- and since threads go to sleep entirely while waiting for I/O operations to complete, there is no chewing up CPU cycles in a select/epoll loop. So yes, writing "hello world" or simple filters is simpler in Unix -- but writing multithreaded server applications that maximize throughput is simpler in Windows.

Unix is bristling with features designed to "allow you to save me some time". It was designed to make it easy to write quick, "one-off" programs in C. VMS -- the predecessor to Windows NT -- was designed to run long-lasting, high-performance, high-reliability business applications for real users with money on the line (i.e., not just hackers) and Windows NT inherits this legacy.

[uudecode]

"It was designed to make it easy to write quick, "one-off" programs in C."

OK, so it just so happens this is what I love to do. I like writing small programs and continually trying to improve them.

So I guess I should be a UNIX user?

Is NT not good for this too?

BTW, I do like VMS. But despite the NT kernel, using NT feels nothing like using VMS.

[trentnelson]

I love writing NT-style C-level (no CRT, just pure C and whatever the CNF/Cutler Normal Form style is).

[zxcvcxz]

>the NT kernel is much more sophisticated and powerful than Linux

Source?

It's not sophisticated enough or powerful enough to be the most used kernel on super computers (and in the world). Windows pretty much only dominates the desktop market. Servers, super computers, mainframes, etc, mostly use Linux.

A few years ago there was even a bug in Windows that caused degradation in network performance during multimedia playback that was directly connected with mechanisms employed by the Multimedia Class Scheduler Service (MMCSS), this is used on a lot of audio setups. If they can't even get audio setups right how can people consider anything Windows releases "sophisticated"?

It's made to do anything you throw at it I guess, it's definitely complicated, but powerful and sophisticated aren't words I would use to describe NT.

[recursive]

If you're arguing in favor of linux, you probably shouldn't use any arguments that deal with getting audio setups right.

[bitwize]

Indeed.

I would go so far as to say that a large part of why audio is such a CF under Linux is -- wait for it -- lack of real asynchronous I/O.

Audio is asynchronous by nature, and to do that right under Linux you need a "sound server" with all the additional overhead, jank, and shuffling of data between kernel and at least two different user spaces that implies. Audio under Linux was best with OSS, which was synchronous in nature and not suitable for professional applications. JACK mitigated that somewhat, but for an OS to do audio right you need a kernel-level async audio API like Core Audio or whatever Windows is doing these days.

[makomk]

Windows has a sound server too, you know. I believe Core Audio on Mac does too. A large part of why audio is such a CF under Linux is that PulseAudio is incredibly badly written and poorly maintained. My favourite was the half-finished micro-optimization that broke some of the resamplers because the author got bored and never modified them to handle the change, which somehow made it all the way into a release. I dread to think what they'd do with powerful tools like asynchronous I/O.

[cbd1984]

Audio on Linux works fine in my experience.

[tacos]

Hey everyone, we found him!

[madez]

I don't want to take sides in this discussion but share an anecdote. Hey, maybe even someone knows a solution for this.

I have a PC connected via on-board HDMI to a Denon AVR solely for the purpose of getting the audio to the amplifier. Windows doesn't let me use that audio interface without extending or mirroring my desktop to that HDMI port. Since there is no display connected to the AVR I don't want to extend the desktop, and mirroring heavily decreases performance of the system.

On Debian Sid the computer by default allows me to use the HDMI audio without doing anything to my desktop display. It seems the system realizes that there is no display connected to the AVR but it's still a valid sink for audio.

[j_s]

If I remember correctly, NVIDIA did a decent job with their on-board HDMI driver audio-wise; what brand is yours? I'm 100% the functionality is dependent on the driver rather than the OS.

[tacos]

Can you use optical instead of HDMI?

[orbifold]

Well for various definitions of fine I guess.

[cbd1984]

It works fine as in "I can listen to audio on my laptop from multiple programs at once, with a centralized way to control audio volume on a per-application or per-sound-device basis." I literally cannot imagine any audio system doing better than that given the hardware I have to work with.

[tacos]

https://en.wikipedia.org/wiki/BeOS

[zxcvcxz]

Works very well for me too. Don't know why you got downvoted. It's like it's 1994 in here...

[xodjoshd]

But this is exactly what made me switch, windows was preventing me from accessing my sound card directly in order to record a remote interview.

I use Linux regularly to record and edit audio, it's free , it works, and I dont have to worry my OS is active reducing the functionality of my equipment.

[Sanddancer]

They got audio setups right. The reason the network degradation happened is that video and audio playback were given realtime priority so background processes couldn't cause pops, stutters, etc. At the time Vista was released, most home users didn't have a gigabit network, so the performance degradation would only happen on a small number of users, and most would rather prefer good audio and video performance to a slowdown in network performance in a small percentage of users. With today's massively multicore systems, it's even less of an issue, while linux still has a problem with latency on applications like pro audio.

[makomk]

The reason the network degradation happened is that Microsoft couldn't figure out how to stop heavy network activity causing audio glitches on some systems even after giving audio realtime priority, so they hacked around it by adding a fixed 10,000 packets-per-second cap on network activity regardless of system speed or CPU usage (less if you had multiple network adapters). See https://blogs.technet.microsoft.com/markrussinovich/2007/08/... This was just as much of an issue on multicore systems because the cap was unaffected by the system speed and chosen based on a slow single-core machine.

[pbarnes_1]

I don't know why you're getting downvoted since the parent is basically stating random opinions about "power" and "sophistication" without anything to actually back it up.

11 param functions don't say "power" to me. They say "poorly thought out API design". Much can be said for most Windows APIs in general.