Re: puzzled re hyperthreaded performance

[tmoran@acm.org]

> Now coming back to the threading and speed improvements above, in my
> view it is a matter of which trick to use for getting the bus to work
> in a boosted mode. We obtained it with simple loop unrolling, you
> report threading. Are there other experiences out there?
    Loop unrolling sounds like the machine was able to use multiple
arithmetic units to parallelize work at the sub-instruction level.  If
that happens then perhaps there are no extra resources for the second
virtual CPU, so there would be no performance speedup from
multi-threading, but I'm certainly not expert in the internals of Intel's
hyperthreading etc.  I wouldn't expect an significant slowdown however.
    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.