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On 13/12/2012 08:21, Richter, Jörg wrote:
Strings are stored in a hash table but the string hash value is reduced modulo the size of the hash table and so it helps to compare string hash values for colliding strings before resorting to memcmp. See http://www.lua.org/source/5.2/lstring.c.html#luaS_newlstrAfter reading this I applied the same optimization to my hash table implementation. Are you sure this is worthwhile? My unrepresentative performance tests are somewhat slower (2-3%) than without this extra compare. But I have no performance tests for Lua to double check. Perhaps anyone can run their tests with "h == ts->tsv.hash &&" commented out. Jörg
I'll do some perf tests probably later today. It's for a plain C string pool (don't know if it looks like Lua's, haven't checked). As with lua strings, since they know their sizes and thus I can use memcmp instead of strcmp, size equality must be checked before, so there is a second filter there anyway (after the one due to hash & modulo). This may explain why yet another filter on hash alone possibly does not help much (except maybe if strings commonly have same size).
DenisPS: Do you know of some scheme (or reflexion on the topic) to define a proportion of big strings to hash?