Re: [PATCH v2 0/4] Support xxhash64 checksums

[Johannes Thumshirn <jthumshirn@suse.de>]

On Thu, Aug 22, 2019 at 02:28:53PM +0200, Holger Hoffstätte wrote:
> On 8/22/19 1:40 PM, Johannes Thumshirn wrote:
> > Now that Nikolay's XXHASH64 support for the Crypto API has landed and BTRFS is
> > prepared for an easy addition of new checksums, this patchset implements
> > XXHASH64 as a second, fast but not cryptographically secure checksum hash.
> 
> Question from the cheap seats.. :)
> 
> I know that crc32c-intel uses native SSE 4.2 instructions, but so far I have
> been unable to find benchmarks or explanations why adding xxhash64 benefits
> btrfs. All benchmarks seem to be against crc32c in *software*, not the
> SSE4.2-enabled version (or I can't read). I mean, it's great that xxhash64 is
> really fast for a software implementation, but how does btrfs benefit from this
> compared to using crc32-intel?
> 
> Verifying that plugging in other hash impls works (e.g. as preparation for
> stronger impls) has value, but it's probably not something most
> users care about.
> 
> Maybe there are obscure downsides to crc32c-intel like instruction latency
> (def. a problem for AVX512), cache pollution..?
> 
> Just curious.

It's not so much about the performance aspect of xxhash64 vs crc32c. xxhash64
has a lower collission proability compared to crc32c, which for instance makes
it a good candidate to use for de-duplication.

HTH,
	Johannes
-- 
Johannes Thumshirn                            SUSE Labs Filesystems
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