[MPlayer-dev-eng] SFENCE really needed?

[Michael Niedermayer]

Hi

On Tuesday 21 January 2003 17:10, D Richard Felker III wrote:
> On Tue, Jan 21, 2003 at 10:21:23AM +0100, Michael Niedermayer wrote:
> > Hi
> >
> > On Tuesday 21 January 2003 06:22, D Richard Felker III wrote:
> > > Hmm, I was reading some of the fast/agp memcpy code and other related
> > > stuff, and I'm wondering...what's the performance penalty for using
> > > the SFENCE instruction, and is it really necessary after writes to
> > > video memory? I could understand it being useful when writing to
> > > system memory, when another procedure might need to read the data
> > > after you write it. But from what I understand, it's nonsense when
> > > writing to video mem unless you plan on reading from video mem (slow
> > > and pointless!). Any ideas?
> >
> > its needed, btw if u really dont read the data from there then why do u
> > write it in the video mem? (hint the grafix card reads it & displays it
> > on screen, so the last few pixels could be missing ..., i dunno if that
> > would happen in practice, but it could IMHO according to the manuals)
>
> Hmm. I was under the impression that the special 'non-temporal' writes
> or whatever caused the data to be written directly through the cache,
> with only a minimal amount being cached for write combining, and that
> sfence was for making sure the cpu wouldn't read stale values for
> those addresses from the cache. However, you probably know a good deal
> more about it than I do. It's been a long while since I coded much
> asm, and back in the day it was all different -- cpu wasn't 10x as
> fast as the memory so number of cycles actually mattered. :)
well, i only know whats written in the docs from intel & amd and these are not 
completely clear to me, so IMHO its saver to add the sfence stuff ...

[...]

Michael