Re: puzzled re hyperthreaded performance

[tmoran@acm.org]

Further data:

>     Cache thrashing certainly comes to mind as a possible problem with
> multi-threading.  But the program at hand is small.  I cut the problem
> size so there are at most two simultaneous hash tables of Length 4091 for
> 6 character Strings, which, assuming 20 bytes/entry would be 2*4000*20 or
> 160K, plus 50K input String = 210K which I would think would fit well
> inside the cache without thrashing.  But it's still slower (about 10%) to
> run multithreaded than sequential.
   Since this used exclusively 6 character Strings, I changed from
Ada.Containers.Indefinite_Hashed_Maps to
Ada.Containers.Hashed_Maps
Now the parallel version runs 30% faster than the sequential one.
Increasing the string (and thus hash table) size to more probably
thrash the cache brings it back to parallel being slower than sequential.
So part of the problem is cache thrashing, but there's still some
remaining problem when using Indefinite_Hashed_Maps.