Re: [EXTERNAL] Re: [PATCH v2 0/6] Add support for Texas Instruments MCRC64 engine

Eric Biggers
2023-08-22T05:17:17Z

On Fri, Aug 18, 2023 at 02:36:34PM +0530, Kamlesh Gurudasani wrote:
> Hi Eric,
>
> We are more interested in offload than performance, with splice system
> call and DMA mode in driver(will be implemented after this series gets
> merged), good amount of cpu cycles will be saved.

So it's for power usage, then? Or freeing up CPU for other tasks?

> There is one more mode(auto mode) in mcrc64 which helps to verify crc64
> values against pre calculated crc64, saving the efforts of comparing in
> userspace.

Is there any path forward to actually support this?

>
> Current generic implementation of crc64-iso(part of this series)
> gives 173 Mb/s of speed as opposed to mcrc64 which gives speed of 812
> Mb/s when tested with tcrypt.

This doesn't answer my question, which to reiterate was:

How does performance compare to a properly optimized software CRC
implementation on your platform, i.e. an implementation using carryless
multiplication instructions (e.g. ARMv8 CE) if available on your platform,
otherwise an implementation using the slice-by-8 or slice-by-16 method?

The implementation you tested was slice-by-1. Compared to that, it's common for
slice-by-8 to speed up CRCs by about 4 times and for folding with carryless
multiplication to speed up CRCs by 10-30 times, sometimes limited only by memory
bandwidth. I don't know what specific results you would get on your specific
CPU and for this specific CRC, and you could certainly see something different
if you e.g. have some low-end embedded CPU. But those are the typical results
I've seen for other CRCs on different CPUs. So, a software implementation may
be more attractive than you realize. It could very well be the case that a
PMULL based CRC implementation actually ends up with less CPU load than your
"hardware offload", when taking into syscall, algif_hash, and driver overhead...

- Eric