Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons

From: Ingo Molnar <[email-masked]>
Date: Thu, 28 Apr 2011 17:14:09 +0200

* Ingo Molnar <[email-masked]> wrote:

> * Jonathan Nieder <[email-masked]> wrote:
> 
> > Hi,
> > 
> > A side note for amusement.
> > 
> > Erik Faye-Lund wrote:
> > 
> > > --- a/cache.h
> > > +++ b/cache.h
> > > @@ -681,13 +681,17 @@ extern char *sha1_pack_name(const unsigned char *sha1);
> > >  extern char *sha1_pack_index_name(const unsigned char *sha1);
> > >  extern const char *find_unique_abbrev(const unsigned char *sha1, int);
> > >  extern const unsigned char null_sha1[20];
> > > -static inline int is_null_sha1(const unsigned char *sha1)
> > > +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> > >  {
> > > -	return !memcmp(sha1, null_sha1, 20);
> > > +	/* early out for fast mis-match */
> > > +	if (*sha1 != *sha2)
> > > +		return *sha1 - *sha2;
> > > +
> > > +	return memcmp(sha1 + 1, sha2 + 1, 19);
> > >  }
> > 
> > On the off-chance that sha1 and sha2 are nicely aligned, a more
> > redundant
> > 
> > 	if (*sha1 != *sha2)
> > 		return *sha1 - *sha2;
> > 
> > 	return memcmp(sha1, sha2, 20);
> > 
> > would take advantage of that (yes, this is just superstition, but it
> > somehow seems comforting anyway).
> 
> Your variant also makes the code slightly more compact as the sha1+1 and sha2+1 
> addresses do not have to be computed. I'll re-test and resend this variant.

Seems to perform measurably worse:

 #
 # Open-coded loop:
 #
 Performance counter stats for './git gc' (10 runs):

       2358.560100 task-clock               #    0.763 CPUs utilized            ( +-  0.06% )
             1,870 context-switches         #    0.001 M/sec                    ( +-  3.09% )
               170 CPU-migrations           #    0.000 M/sec                    ( +-  3.54% )
            38,230 page-faults              #    0.016 M/sec                    ( +-  0.03% )
     7,513,529,543 cycles                   #    3.186 GHz                      ( +-  0.06% )
     1,634,103,128 stalled-cycles           #   21.75% of all cycles are idle   ( +-  0.28% )
    11,068,971,207 instructions             #    1.47  insns per cycle        
                                            #    0.15  stalled cycles per insn  ( +-  0.04% )
     2,487,656,519 branches                 # 1054.735 M/sec                    ( +-  0.03% )
        59,233,604 branch-misses            #    2.38% of all branches          ( +-  0.09% )

        3.092183093  seconds time elapsed  ( +-  3.49% )

 #
 # Front test + memcmp:
 #
 Performance counter stats for './git gc' (10 runs):

       2723.468639 task-clock               #    0.833 CPUs utilized            ( +-  0.22% )
             1,751 context-switches         #    0.001 M/sec                    ( +-  2.02% )
               167 CPU-migrations           #    0.000 M/sec                    ( +-  1.23% )
            38,230 page-faults              #    0.014 M/sec                    ( +-  0.03% )
     8,684,682,538 cycles                   #    3.189 GHz                      ( +-  0.21% )
     2,062,906,208 stalled-cycles           #   23.75% of all cycles are idle   ( +-  0.60% )
     9,019,624,641 instructions             #    1.04  insns per cycle        
                                            #    0.23  stalled cycles per insn  ( +-  0.04% )
     1,771,179,402 branches                 #  650.340 M/sec                    ( +-  0.04% )
        75,026,810 branch-misses            #    4.24% of all branches          ( +-  0.04% )

        3.271415104  seconds time elapsed  ( +-  1.97% )

So i think the open-coded loop variant i posted is faster.

The key observation is that there's two cases that matter to performance:

 - the hashes are different: in this case the front test catches 99% of the cases
 - the hashes are *equal*: in this case the open-coded loop performs better than the memcmp

My patch addresses both cases.

Thanks,

	Ingo