Re: tweaking MemSet() performance - 7.4.5

On Sat, 2004-09-25 at 23:23 +0200, Manfred Spraul wrote:
> 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 ;-)

I think it cannot be problem if you write it to some .h file (in port
directory?) as macro with "#ifdef GCC". The other thing is real
advantage of hacks like this in practical PG usage :-)

Karel

--
Karel Zak
http://home.zf.jcu.cz/~zakkr