Re: tweaking MemSet() performance - 7.4.5

Karel Zak wrote:
> 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 :-)

The reason MemSet is a win is not that the C code is great but because
it eliminates a function call.

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