Re: tweaking MemSet() performance - 7.4.5

mcolosimo(at)mitre(dot)org wrote:

>>If the memset
>>bypasses the cache then the following access will cause a cache line
>>miss, which can be so slow that using the faster memset can result in a
>>net performance loss.
>>
>>
>>
>
>Could you suggest some structs to test? If I get your meaning, I would make a loop that sets then reads from the structure.
>
>
>
Read the sources and the cpu specs. Benchmarking such problems is
virtually impossible.
I don't have OS-X, thus I checked the Linux-kernel sources: It seems
that the power architecture doesn't have the same problem as x86.
There is a special clear cacheline instruction for large memsets and the
rest is done through carefully optimized store byte/halfword/word/double
word sequences.

Thus I'd check what happens if you memset not perfectly aligned buffers.
That's another point where over-optimized functions sometimes break
down. If there is no slowdown, then I'd replace the postgres function
with the OS provided function.

I'd add some __builtin_constant_p() optimizations, but I guess Tom won't
like gcc hacks ;-)
--
Manfred