[PATCH] bdiff: replace hash algorithm

[Gregory Szorc]

> On Nov 6, 2016, at 19:06, Gregory Szorc <gregory.szorc at gmail.com> wrote:
> 
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1478487117 28800
> #      Sun Nov 06 18:51:57 2016 -0800
> # Node ID bb7c6d6f4a10e80ff4bdf88919692f08497d2d66
> # Parent  1c7269484883804b6f960e87309169ef4ae85043
> bdiff: replace hash algorithm
> 
> This patch replaces lyhash with the hash algorithm used by diffutils.
> The algorithm has its origins in Git commit 2e9d1410, which is all the
> way back from 1992. The license header in the code at that revision
> in GPL v2.
> 
> I have not performed an extensive analysis of the distribution
> (and therefore buckets) of hash output. However, `hg perfbdiff`
> gives some clear wins. I'd like to think that if it is good enough
> for diffutils it is good enough for us?

Searching the Internets seems to reveal that xxHash is the state of the art for fast string hashing with great distribution. We'll have a copy of xxHash vendored as part of zstd and it should be relatively easy to plug in then.

Honestly, I'm not sure if we should take the quick win or hold out for xxHash in a few weeks (assuming my compression engine series and zstd vendoring moves forward...).

> 
> From the mozilla-unified repository:
> 
> $ perfbdiff -m 3041e4d59df2
> ! wall 0.053271 comb 0.060000 user 0.060000 sys 0.000000 (best of 100)
> ! wall 0.035827 comb 0.040000 user 0.040000 sys 0.000000 (best of 100)
> 
> $ perfbdiff 0e9928989e9c --alldata --count 100
> ! wall 6.204277 comb 6.200000 user 6.200000 sys 0.000000 (best of 3)
> ! wall 4.309710 comb 4.300000 user 4.300000 sys 0.000000 (best of 3)
> 
> From the hg repo:
> 
> $ perfbdiff 35000 --alldata --count 1000
> ! wall 0.660358 comb 0.660000 user 0.660000 sys 0.000000 (best of 15)
> ! wall 0.534092 comb 0.530000 user 0.530000 sys 0.000000 (best of 19)
> 
> Looking at the generated assembly and statistical profiler output
> from the kernel level, I believe there is room to make this function
> even faster. Namely, we're still consuming data character by character
> instead of at the word level. This translates to more loop iterations
> and more instructions.
> 
> At this juncture though, the real performance killer is that we're
> hashing every line. We should get a significant speedup if we change
> the algorithm to find the longest prefix, longest suffix, treat those
> as single "lines" and then only do the line splitting and hashing on
> the parts that are different. That will require a lot of C code,
> however. I'm optimistic this approach could result in a ~2x speedup.
> 
> diff --git a/mercurial/bdiff.c b/mercurial/bdiff.c
> --- a/mercurial/bdiff.c
> +++ b/mercurial/bdiff.c
> @@ -17,6 +17,10 @@
> #include "bitmanipulation.h"
> #include "bdiff.h"
> 
> +/* Hash implementation from diffutils */
> +#define ROL(v, n) ((v) << (n) | (v) >> (sizeof(v) * CHAR_BIT - (n)))
> +#define HASH(h, c) ((c) + ROL(h ,7))
> +
> struct pos {
>    int pos, len;
> };
> @@ -44,8 +48,7 @@ int bdiff_splitlines(const char *a, ssiz
>    /* build the line array and calculate hashes */
>    hash = 0;
>    for (p = a; p < a + len; p++) {
> -        /* Leonid Yuriev's hash */
> -        hash = (hash * 1664525) + (unsigned char)*p + 1013904223;
> +        hash = HASH(hash, *p);
> 
>        if (*p == '\n' || p == plast) {
>            l->hash = hash;