[mrlongroots]

Yes, it doesn't take a benchmark to find out that storage can not be faster than memory.

Even if you had a million SSDs and somehow were able to connect them to a single machine somehow, you would not outperform memory, because the data needs to be read into memory first, and can only then be processed by the CPU.

Basic `perf stat` and minor/major faults should be a first-line diagnostic.

[johncolanduoni]

This was a comparison of two methods of moving data from the VFS to application memory. Depending on cache status this would run the whole gambit of mapping existing memory pages, kernel to userspace memory copies, and actual disk access.

Also, while we’re being annoyingly technical, a lot of server CPUs can DMA straight to the L3 cache so your proof of impossibility is not correct.

[mrlongroots]

> This was a comparison of two methods of moving data from the VFS to application memory. Depending on cache status this would run the whole gambit of mapping existing memory pages, kernel to userspace memory copies, and actual disk access.

Yes, I think maybe a reasonable statement is that a benchmark is supposed to isolate a meaningful effect. This benchmark was not set up correctly to isolate a meaningful effect IMO.

> Also, while we’re being annoyingly technical, a lot of server CPUs can DMA straight to the L3 cache so your proof of impossibility is not correct.

Interesting, didn't know that, thanks!

I think this does not invalidate the point though. You can temporarily stream directly to the L3 cache with DDIO, but as it fills out the cache will get flushed back to the main memory anyway and you will ultimately be memory-bound. I don't think there is some way to do some non-temporal magic here that circumvents main memory entirely.

[johncolanduoni]

I agree, not a well designed benchmark. The mmap side did not use any of the mechanisms for paging in data before faulting each page, which are table stakes for this usecase.

The point of DDIO is that the data is frequently processed fast enough that it can get pulled from L3 to L1 and be finished with before it needs to be flushed to main memory. For a system with a coherent L3 or appropriate NUMA this isn’t really non-temporal, it’s more like the L3 cache is shared with the device.

[alphazard]

> storage can not be faster than memory

This is an oversimplification. It depends what you mean by memory. It may be true when using NVMe on modern architectures in a consumer use case, but it's not true about computer architecture in general.

External devices can have their memory mapped to virtual memory addresses. There are some network cards that do this for example. The CPU can load from these virtual addresses directly into registers, without needing to make a copy to the general purpose fast-but-volatile memory. In theory a storage device could also be implemented in this way.

[mrlongroots]

> The CPU can load from these virtual addresses directly into registers

This requires that device to bring meaningful amounts of its own memory. GPUs do that with VRAM. A storage device does not come with its own RAM, but interesting point!

[johncolanduoni]

On a modern desktop/server CPU, the RAM memory and PCIe device mapped memory do not share a bus. The equivalence is a fiction maintained by the MMU. Some chips (e.g. Apple Silicon) have unified memory such that RAM is accessible from the CPU and devices (GPU) on a shared bus, but this is a little different.

Also, direct access of device memory is quite slow. High throughput usecases like storage or network have relied entirely on DMA to system RAM from the device for decades.

[Thaxll]

I mean even with their memory mapped, the physical access to the memory ( NVME ) will always be slower than the physical access to the RAM, right?

Which external device has memory access as fast or faster than generic RAM?

[usefulcat]

I have heard that some Intel NICs can put received data directly into L3 cache. That would definitely make it faster to access than if it were in main RAM.

If a NIC can do that over PCI, probably other PCI devices could do the same, at least in theory.

[wahern]

For the curious,

> When a 100G NIC is fully utilized with 64B packets and 20B Ethernet overhead, a new packet arrives every 6.72 nanoseconds on average. If any component on the packet path takes longer than this time to process the individual packet, a packet loss occurs. For a core running at 3GHz, 6.72 nanoseconds only accounts for 20 clock cycles, while the DRAM latency is 5-10 times higher, on average. This is the main bottleneck of the traditional DMA approach.

> The Intel® DDIO technology in Intel® Xeon® processors eliminates this bottleneck. Intel® DDIO technology allows PCIe devices to perform read and write operations directly to and from the L3 cache, or the last level cache (LLC).

https://www.intel.com/content/www/us/en/docs/vtune-profiler/...

[mrlongroots]

From a response elsewhere in this thread (my current understanding, could be wrong):

> You can temporarily stream directly to the L3 cache with DDIO, but as it fills out the cache will get flushed back to the main memory anyway and you will ultimately be memory-bound. I don't think there is some way to do some non-temporal magic here that circumvents main memory entirely.

[wahern]

Ah. I guess it's only useful for an optimized router/firewall or network storage appliance, perhaps with a bespoke stack carefully tuned to quickly process and then hand the data back to the controller (via DDIO) before it flushes.

EDIT: Here's an interesting writeup about trying to make use of it with FreeBSD+netmap+ipfw: https://adrianchadd.blogspot.com/2015/04/intel-ddio-llc-cach... So it can work as advertised, it's just very constraining, requiring careful tuning if not outright rearchitecting your processing pipeline with the constraints in mind.

[hinkley]

I’m pretty sure that as of PCI-E 2 this is not true.

It’s only true if you need to process the data before passing it on. You can do direct DMA transfers between devices.

In which case one needs to remember that memory isn’t on the CPU. It has to beg for data just about as much as any peripheral. It uses registers and L1, which are behind two other layers of cache and an MMU.