One thing to keep in mind when looking at GNU programs is that they're often intentionally written in an odd style to remove all questions of Unix copyright infringement at the time that they were written.
The long-standing advice when writing GNU utilities used to be that if the program you were replacing was optimized for minimizing CPU use, write yours to minimize memory use, or vice-versa. Or in this case, if the program was optimized for simplicity, optimize for throughput.
It would have been very easy for the nascent GNU project to unintentionally produce a line-by-line equivalent of BSD yes.c, which would have potentially landed them in the 80/90s equivalent of the Google v.s. Oracle case.
Remember that "yes" is the bottom end of complexity here, not the top. As the utility grows larger that stops looking like "asking for trouble" and starts looking like "often the only sensible solution". Expecting people to extend programs in C is often "asking for trouble", after all!
One point in favor of this simple version is that it's immediately obvious that it doesn't do the same thing as the OpenBSD version. In OpenBSD `yes a b c` will only print "a" while in GNU it prints "a b c". I did not catch that when I was reading the more complicated modern version.
At the risk of sounding like a copyright newbie, shouldn't that be covered by just doing a 'clean room' implementation? As long as you can verifiably prove that you didn't copy the source, it should fall under general use (as there's really only one way of doing such a thing), right? Much like Apple can't patent rectangles, although they tried.
Normally I think readability is more important than speed. But in this particular case, I think GNU is doing the right thing optimizing the code to the limit.
This is the beautiful part of Unix: small tools that do only one thing well. Programs following this philosophy are very good abstractions. They do one very well defined thing so you can use them without having to understand how they work. I have used Unix for years and I've never felt the need to read the source code for `yes`. And because they do a very small thing, even if you need to read them, the overhead of optimization is not that much, for example, the optimized GNU yes is just under 100 LOC if you remove comments and help boilerplate. Yes, it's longer than the BSD version, but it's just a matter of minutes to understands what it does.
I totally disagree. Nobody will ever want to use `yes` at 10 GB/s. They will want it to be reliable, and this sort of over-optimisation increases the risk of bugs.
I've used 'yes' many times to generate huge amounts of data quickly. Back then, it never had the small string optimisation, but you could always run 'yes InsertReallyLongStringHere' to spew out data much faster than /dev/urandom or even /dev/zero
I'm glad it runs fast, and I hope that all OS utilities are optimised (and tested, of course!) instead of making their source code pretty. The fact is, most people want to use programs, not read them.
I agree, all I remember is that when I tried it, /dev/zero sometimes sucked performance-wise. I can't recall the exact circumstances as it was some time ago, and could have been on any of Linux/FreeBSD/SunOS/HP-UX/IRIX - perhaps it was the fastest common way at the time?
On a recent x64 Linux, /dev/zero seems plenty fast enough now:
$ dd bs=8k count=819200 if=/dev/zero of=/dev/null
819200+0 records in
819200+0 records out
6710886400 bytes (6.7 GB, 6.2 GiB) copied, 0.331137 s, 20.3 GB/s
$ yes | dd bs=8k count=819200 of=/dev/null
819200+0 records in
819200+0 records out
6710886400 bytes (6.7 GB, 6.2 GiB) copied, 0.959551 s, 7.0 GB/s
While 'strace yes > test2' is just a constant stream of write() calls.
The difference matters if you're benchmarking e.g. some new SSD compared to a tmpfs on a machine with 100+ GB of RAM. It's always better if the tools have less overhead, because the comparison is more meaningful.
Also consider that it can be faster to write to a local network than to disk. I've never done it, but I imagine that the kernel's not going to want to deal with your /dev/zero calls if it's spending all of its time writing to a 10GB switch. I can imagine some very specialized storage servers that could spend most of their time writing from memory buffers to a network switch, or if you're troubleshooting a slowdown in the networking itself.
When I started this comment, I didn't think you were measuring what you thought you were measuring with those straces. 'strace yes > test2' only watches 'yes', not '> test2' (which is handled by the shell). Here's what your command outputs:
How does this possibly work? File descriptor 1 is supposed to be the terminal, not a file! Of course, the magic of file redirection:
open("/tmp/test2", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3 # Open the file, get FD 3
fcntl(1, F_DUPFD, 10) = 10 # Copy FD 1 (the terminal, STDOUT) to FD 10 temporarily
close(1) = 0 # Close original FD 1
fcntl(10, F_SETFD, FD_CLOEXEC) = 0 # Close FD 10 (the terminal, copy of STDOUT) when exec() is called
dup2(3, 1) = 1 # Copy FD 3 (the file) to FD 1 (STDOUT)
close(3) = 0 # Close FD 3 (the file's original descrptor)
I had the impression that you thought "yes" pipes the output to shell, and then the shell writes to disk. That's incorrect. " > file" means redirecting to a file, and therefore all write system calls actually write to disk.
That's exactly what I thought. Now, had you asked me how redirection worked I would have said "the redirection operators cause the shell to attach a file to the descriptor," but I'd never actually thought through the implications of that in terms of what syscalls get made, and the output of strace presented a rather visceral demonstration of the implications of this clever bit of design.
The ability to get 10 GB/s dummy date into a pipe from the command line could come in handy at some point, for stress testing or something. I am not sure if it is over-optimized. (And even with the optimizations the risk of bugs should be very small.)
yes will give repeated data though, not just zeros, seems like it's more useful here - as others have pointed out, also uses less syscalls than /dev/zero.
If you want to maximize readability and simplicity then writing `yes` in C is a bad choice. It is much easier, cleaner and shorter to write it in python or just use the shell which would normally be using the `yes` in the first place. Since `yes` is used in shells and builtins can be considered very reliable, here is a implementation as a shell function:
yes(){ while :; do echo "${1:-y}"; done; }
Python:
import sys
while True:
if len(sys.argv) > 1:
print(sys.argv[1])
else:
print("y")
And if you don't need the features of `yes` and only need a loop that prints 'y', then there really is hard to beat the simplicity of:
The C example has three includes, two conditionals, two loops, and one function definition. The python example has a single include, conditional, and loop.
For readability purpose it is easier to go through each lines of the python program than the OpenBSD C code. Its not massively different, but its distinguishable enough that I would choose the python version if I wanted to maximize readability, minimize syntax requirement and did not want to use shell script.
The Shell function is in my view the superior choice if the audience is a programmer than know the shell script syntax. It is just a single loop and is written in the environment that the program is intended to be used in. The only drawback there is the speed.
Most of what makes the C program bigger comes from the fact that the C program does more. Your python example doesn't call pledge(). Remove that from the C program and it drops to one include, one conditional, and two loops. Further, counting the two loops against C doesn't make any sense: it's entirely up to the programmer whether to have a conditional containing two loops, or a loop containing a conditional. Both languages could naturally do it either way.
If the speed of yes is bounded by memory speed, doing anything useful will almost certainly consume that data at a far lower rate. Putting it on a disk, pushing it over a network, etc. will almost always be slower than yes is able to generate data.
The typical use case is piping it into another running program. Maybe someone wants to do that really quickly rather than putting it on disk or pushing it over a network.
It's not about running 'yes' at 10GB/s, it's about less overhead to do a simple job. If this version of yes is 100x faster, that implies it using 1% of a cpu to do the same work that would otherwise occupy 100% of a cpu. This leaves more of the machine to do what is likely to be the intended task.
Unix userland tools have evolved over decades to be as efficient as possible because they have historically underpinned everything the operating system does. The faster each tool works, the faster the other tools that depend on them work. If increased efficiency results in a bug, that bug can then be fixed, making it a net gain for system stability.
Easy! Write a program that does a brute force check of Goldbach's conjecture on all integers. For each integer that passes the check, print a line. If you can prove this correct (or incorrect) you'll probably win a Fields medal.
I agree with the general point that you could prove such a simple program correct relatively easily but that does still have a cost, which is always a concern in an open-source project. You still need someone to step up and do that work and continue to verify it periodically in the future – if that code is doing complicated things with buffering, that opens up possible odd bugs due to stdlib, gcc, maybe even kernel behaviour changes which might not affect simpler programs.
Not a huge bit of work to be sure but for a non-commercial project you might have trouble finding a volunteer who cares about that tedium.
Absolutely, I once trivially proved a 1000,000 line implementation of return 0 to be correct. I don't know why all comments are bothered by how much overkill this yes implementation is. Maybe they don't hold 100,000 PhDs like we do, am I right?
It's not really optimized to the limit – or perhaps it is, but then the limit is fairly easy to reach.
When I saw this item here I reached for the terminal and wrote a simple (and non-compliant) alternative that simply initialises 8150 bytes of "y\n" and then loops a write forever. I understand that it is not a fully standard-compliant yes, and that maybe GNU yes is indeed fast, but that awfully simple program that takes all of 10 lines (everything included) and took me all of a minute to write performs just as well as far as pv is concerned.
(I eventually completed a feature complete yes but I still think that simply not using `puts` is hardly optimising to the limit.)
A nitpick, but I notice that the BSD implementation do not catch errors when printing. In theory it could get EINTR and only write partial amounts of argv[1], especially if the argument string is really long and the program runs for a extended amount of time. The GNU version do catch EINTR.
Naturally it could be that the C function puts used by most people in OpenBSD is implemented with built in EINTR catching loop, or that OpenBSD do not interrupts writes.
You'd have to check the various POSIX standards to be sure - and have to then verify that the OS/libc actually follows them, but you can pretty much rely on every libc's I/O wrapper functions to handle interrupted system calls or incomplete writes. I've never seen any code check the return value of a printf() to verify that all the characters were printed.
As you say, the GNU versions definitely handle EINTR - the linux man page for puts() just says it returns a non-negative value on success, it's not even specified whether it returns the number of bytes written or not.
`puts` is an "stdio" function, not a system call. It won't EINTR, it correctly resumes if the underlying write() EINTRs. If it does get an error, it will return EOF, then you'll have to call `ferror()` to find out which error.
But the point stands; there's no error handling in the OpenBSD version. But that could be considered a design decision; the OpenBSD version never gives up on writing data until you kill it; the GNU version bails at the first error.
I think the GNU version is surprisingly readable for what it does. I have seen code that is a lot less readable than this and doesn't have the performance benefits described in the article.
I generally look at some BSD sources when I want to know how some unix tools are working, they are always much much more readable than the GNU equivalent.
It's to make sure the buffer is flushed. Streams are closed, but not explicitly flushed at exit. Stdout only because it's the only filehandle with a buffer in yes.
In normal use, GNU `yes` does unbuffered IO on stdout. However, it does use buffered IO for --help and --version messages; it sets atexit(close_stdout) to cover both of those cases at once, rather than handling them both separately.
According to the article/experiment you don't have to bork up the code much, just copy your stuff into a big buffer before you start printing it (and print using write(2) instead of puts)
This works easily for the default case, which prints "y\n" (two bytes), which is likely to divide BUFSIZ. To handle the general case with the same efficiency, you have to have a buffer size that is a multiple of both BUFSIZ and the length of the string to be printed. It appears that GNU yes will not do that and simply does unaligned writes in this case, which is likely to be considerably slower (possibly slower than write would have been).
> It appears that GNU yes will not do that and simply does unaligned writes in this case, which is likely to be considerably slower (possibly slower than write would have been).
Why would it be slower to do a single, say, 8190 bytes write instead of 2730 3-byte writes?
And it's also a source of many bugs (in the past and likely in the future as well). In most use cases today I'd rather have a slightly slower userland which is easily read (and audited) then one which compromises quality for speed in edge cases.
This work certainly has value but it is frustrating to keep up with everything being changed. And if changes affect me do the fashions and attitudes of those making the changes have synchronicity with the way that I use computers?
In the old days I think we would have left yes in c because the compiler will build it on any platform.
My first experience with Linux is porting land.c to one of the commercial Unix. Now I first met TCP/IP on a 3B2 and later met systems sold by SMI and DEC and SCO and we all mostly constructed packets the same way and a guy called Stevens had written some nice books about this that everyone had. I think I recall the commercial and free BSD also did things the usual way. But whoever figured out this interesting phenomenon land.c demonstrated happened to be a Linux user and this platform had some different ideas about it. I saw rewriting the relevant parts as an annoying, menial task.
The long-standing advice when writing GNU utilities used to be that if the program you were replacing was optimized for minimizing CPU use, write yours to minimize memory use, or vice-versa. Or in this case, if the program was optimized for simplicity, optimize for throughput.
It would have been very easy for the nascent GNU project to unintentionally produce a line-by-line equivalent of BSD yes.c, which would have potentially landed them in the 80/90s equivalent of the Google v.s. Oracle case.