Re: tweaking MemSet() performance - 7.4.5

Marc Colosimo wrote:

> Oops, I used the same setting as in the old hacking message (-O2, gcc
> 3.3). If I understand what you are saying, then it turns out yes, PG's
> MemSet is faster for smaller blocksizes (see below, between 32 and
> 64). I just replaced the whole MemSet with memset and it is not very
> low when I profile.

Could you check what the OS-X memset function does internally?
One trick to speed up memset it to bypass the cache and bulk-write
directly from write buffers to main memory. i386 cpus support that and
in microbenchmarks it's 3 times faster (or something like that).
Unfortunately it's a loss in real-world tests: Typically a structure is
initialized with memset and then immediately accessed. 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.

> I could squeeze more out of it if I spent more time trying to
> understand it (change MEMSET_LOOP_LIMIT to 32 and then add memset
> after that?). I'm now working one understanding Spin Locks and
> friends. Putting in a sync call (in s_lock.h) is really a time killer
> and bad for performance (it takes up 35 cycles).
>
That's the price you pay for weakly ordered memory access.
Linux on ppc uses eieio, on ppc64 lwsync is used. Could you check if
they are faster?

--
Manfred