Re: Performance regression in write() syscall

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <andi@firstfloor.org> wrote:

> On Mon, Feb 23, 2009 at 06:03:04PM -0800, Salman Qazi wrote:

> > -	return __copy_user_nocache(dst, src, size, 1);
> > +	if (likely(size >= PAGE_SIZE))
> > +		return __copy_user_nocache(dst, src, size, 1);
> > +	else
> > +		return __copy_from_user(dst, src, size);
> 
> I think you disabled it completely, the kernel never really 
> does any copies larger than page size because all its internal 
> objects are page sized only.

No, look again, it's not disabled completely - the check now 
basically special-cases 4K writes _only_, and makes them 
non-temporal. That still covers the big/midsize file case.

And that kind of 4K limit makes a lot of sense. A small file 
write will unlikely to have a perfect 4K sized copy. Big file 
writes (and raw/direct IO related copies, etc.) will be chunked 
down to 4K sized units.

	Ingo