> > Which CPU is that?
>
> Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
> ---
> I've access to some VM (Not KVM) with:
> Intel(R) Xeon(R) CPU E5-2420 0 @ 1.90GHz
> PAGE_SIZE: 4096, loop count: 1048576
> jhash2: 0x15433d14 time: 3661 ms, th: 1173.144082 MiB/s
> xxhash32: 0x3df3de36 time: 1163 ms, th: 3691.581922 MiB/s
> xxhash64: 0x5d9e67755d3c9a6a time: 715 ms, th: 6006.628034 MiB/s
>
> As additional info, xxhash work with ~ same as jhash2 speed at 32 byte
> input data.
> For input smaller than 32 byte, jhash2 win, for input bigger, xxhash win.
Please put that information into the changelog when you repost.
>
>
> >> So replace jhash with xxhash,
> >> and use fastest version for current target ARCH.
> >
> > Can you do some macro-benchmarking too? Something that uses
> > KSM and show how the performance changes.
> >
> > You could manually increase the scan rate to make it easier
> > to see.
>
> Try use that patch with my patch to allow process all VMA on system [1].
> I switch sleep_millisecs 20 -> 1
>
> (I use htop to see CPU load of ksmd)
>
> CPU: Intel(R) Xeon(R) CPU E5-2420 0 @ 1.90GHz
> For jhash2: ~18%
> For xxhash64: ~11%
Ok that's a great result. Is a speedup also visible with the default
sleep_millisecs value?
> >> @@ -51,6 +52,12 @@
> >> #define DO_NUMA(x) do { } while (0)
> >> #endif
> >>
> >> +#if BITS_PER_LONG == 64
> >> +typedef u64 xxhash;
> >> +#else
> >> +typedef u32 xxhash;
> >> +#endif
> >
> > This should be in xxhash.h ?
>
> This is a "hack", for compile time chose appropriate hash function.
> xxhash ported from upstream code,
> upstream version don't do that (IMHO), as this useless in most cases.
> That only can be useful for memory only hashes.
> Because for persistent data it's obvious to always use one hash type 32/64.
I don't think it's a hack. It makes sense. Just should be done centrally
in Linux, not in a specific user.
>
> > xxhash_t would seem to be a better name.
> >
> >> - u32 checksum;
> >> + xxhash checksum;
> >> void *addr = kmap_atomic(page);
> >> - checksum = jhash2(addr, PAGE_SIZE / 4, 17);
> >> +#if BITS_PER_LONG == 64
> >> + checksum = xxh64(addr, PAGE_SIZE, 0);
> >> +#else
> >> + checksum = xxh32(addr, PAGE_SIZE, 0);
> >> +#endif
> >
> > This should also be generic in xxhash.h
>
> This *can* be generic in xxhash.h, when that solution will be used
> somewhere in the kernel code, not in the KSM only, not?
Yes.
>
> Because for now i didn't find other places with "big enough" input
> data, to replace jhash2 with xxhash.
Right, but we may get them.
-Andi
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