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MFC r266208: Speed up pmcstat by improving string hash

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In one case generating callgraph output from a 24MB system-wide sampling
  data file took 17.4 seconds on average.  Profiling showed pmcstat
  spending a lot of time in strcmp, due to hash collisions.

  Replacing the XOR-only hash with FNV-1a reduces the run time for my
  test by 40%.
This commit is contained in:
emaste 2014-05-23 17:46:00 +00:00
parent 80044281eb
commit e761352067
1 changed files with 3 additions and 3 deletions
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6
usr.sbin/pmcstat/pmcstat_log.c
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@ -307,10 +307,10 @@ pmcstat_stats_reset(int reset_global)
static int static int
pmcstat_string_compute_hash(const char *s) pmcstat_string_compute_hash(const char *s)
{ {
int hash; unsigned hash;
for (hash = 0; *s; s++) for (hash = 2166136261; *s; s++)
hash ^= *s; hash = (hash ^ *s) * 16777619;
return (hash & PMCSTAT_HASH_MASK); return (hash & PMCSTAT_HASH_MASK);
} }