svn commit: r266208 - head/usr.sbin/pmcstat

[Ed Maste]

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);
 }