Author: emaste
Date: Fri May 16 03:05:53 2014
New Revision: 266208
URL: http://svnweb.freebsd.org/changeset/base/266208
Log:
Speed up pmcstat by improving string hash
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%.
Modified:
head/usr.sbin/pmcstat/pmcstat_log.c
Modified: head/usr.sbin/pmcstat/pmcstat_log.c
==============================================================================
--- head/usr.sbin/pmcstat/pmcstat_log.c Fri May 16 02:21:51 2014
(r266207)
+++ head/usr.sbin/pmcstat/pmcstat_log.c Fri May 16 03:05:53 2014
(r266208)
@@ -307,10 +307,10 @@ pmcstat_stats_reset(int reset_global)
static int
pmcstat_string_compute_hash(const char *s)
{
- int hash;
+ unsigned hash;
- for (hash = 0; *s; s++)
- hash ^= *s;
+ for (hash = 2166136261; *s; s++)
+ hash = (hash ^ *s) * 16777619;
return (hash & PMCSTAT_HASH_MASK);
}
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