From: Daniel Axtens
> Sent: 15 March 2017 22:30
> Hi David,
> > While not part of this change, the unrolled loops look as though
> > they just destroy the cpu cache.
> > I'd like be convinced that anything does CRC over long enough buffers
> > to make it a gain at all.
> >
> > With modern (not that modern now) superscalar cpus you can often
> > get the loop instructions 'for free'.
> > Sometimes pipelining the loop is needed to get full throughput.
> > Unlike the IP checksum, you don't even have to 'loop carry' the
> > cpu carry flag.
> Internal testing on a NVMe device with T10DIF enabled on 4k blocks
> shows a 20x - 30x improvement. Without these patches, crc_t10dif_generic
> uses over 60% of CPU time - with these patches CRC drops to single
> digits.
> I should probably have lead with that, sorry.

I'm not doubting that using the cpu instruction for crcs gives a
massive performance boost.

Just that the heavily unrolled loop is unlikely to help overall.
Some 'cold cache' tests on shorter buffers might be illuminating.
> FWIW, the original patch showed a 3.7x gain on btrfs as well -
> 6dd7a82cc54e ("crypto: powerpc - Add POWER8 optimised crc32c")
> When Anton wrote the original code he had access to IBM's internal
> tooling for looking at how instructions flow through the various stages
> of the CPU, so I trust it's pretty much optimal from that point of view.

Doesn't mean he used it :-)


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