Re: Another Performance Regression in write() syscall

Linus Torvalds
2009-02-24T17:31:46Z

On Tue, 24 Feb 2009, Dave Hansen wrote:
>
> Yeah, that's a good point. Are we sure that's what is happening here,
> though? That's one thing a profile would hopefully help with.

One thing to note is that _if_ it's purely an issue of nontemporal stores
vs normal stores, then profiling is very likely going to be almost
entirely useless. You'll get "results", but the results have nothing
what-so-ever to do with reality or anything interesting.

The nontemporal stores may stand out in the profiles, but the actual
performance impact will be all about whether totally unrelated code got
cache misses or not. Quite often those cache misses will also be in user
mode, and very possibly in other processes.

So profiles can certainly be interesting, but if Salman says that his
patch makes a difference for his benchmark, then profiling is almost
certainly not interesting FOR THAT PATCH. It's interesting mainly as a way
to look at whether there are then also _other_ issues that are worth
addressing (ie the whole atime thing is in a whole different dimension
and an independent issue).

Linus