Ketil Malde wrote:
I've recently implemented some benchmarks for my library, and while I expected a slowdown for 64-bit code, I'm a bit bit surprised by the results. In summary:
with 64 bit ghc 6.6.1, my benchmark runs in ~160 seconds with 32 bit ghc 6.6, it runs in ~ 95 seconds
Most of the time is traversing a list of elements, doing a few numerical calculations. Presumably this is due to increased code size due to 8-byte pointers?
Not so much code size, but data size (heap size, to be more precise). The amount of data shuffled around at runtime is doubled when running a 64-bit version of GHC - the GC has to do twice as much work. The cache hit rate drops, for a given cache size. It would be interesting to know how much time is spent in the GC - run the program with +RTS -sstderr.
I'll add some more benchmarks, but just wondered whether this was to be expeced, and, if so, whether I perhaps should be running a 32 bit version of GHC?
I guess it's moderately surprising, I don't expect to see that much difference usually. But I suppose if the memory demands of your program are high, then it could be reasonable. There are benefits to running on 64 bits: more registers in particular, but this doesn't outweight the extra memory overhead for us usually. Cheers, Simon