Hi Timofey,
>
> Perf numbers:
> Intel(R) Xeon(R) CPU E5-2420 v2 @ 2.20GHz
> ksm: crc32c hash() 12081 MB/s
> ksm: xxh64 hash() 8770 MB/s
> ksm: xxh32 hash() 4529 MB/s
> ksm: jhash2 hash() 1569 MB/s
That is a very nice improvement over jhash2!
> Add function to autoselect hash algo on boot,
> based on hashing speed, like raid6 code does.
Are you aware of hardware where crc32c is slower compared to xxhash?
Perhaps always use crc32c when available?
> +
> +static u32 fasthash(const void *input, size_t length)
> +{
> +again:
> + switch (fastest_hash) {
> + case HASH_CRC32C:
> + return crc32c(0, input, length);
> + case HASH_XXHASH:
> + return xxhash(input, length, 0);
You are loosing half of 64-bit word in xxh64 case? Is this acceptable? May
be do one more xor: in 64-bit case in xxhash() do: (v >> 32) | (u32)v ?
> + default:
> + choice_fastest_hash();
> + /* The correct value depends on page size and endianness */
> + zero_checksum = fasthash(ZERO_PAGE(0), PAGE_SIZE);
> + goto again;
> + }
> +}
choice_fastest_hash() does not belong to fasthash(). We are loosing leaf
function optimizations if you keep it in this hot-path. Also, fastest_hash
should really be a static branch in order to avoid extra load and conditional
branch.
I think, crc32c should simply be used when it is available, and use xxhash
otherwise, the decision should be made in ksm_init()
Thank you,
Pavel
