lkml.org 
[lkml]   [2006]   [Jul]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch 1/2] sLeAZY FPU feature - x86_64 support
From
Date

>
> However I'm not sure 256 is a good number. It seems a bit too high.

it's 256 context switches... if you care about context switch cycles
you'll do many, and 256 isn't a lot ;)

(remember that this is after 5 *consecutive* fpu uses, not just 5 uses
total, to you're really a heavy fpu user if you hit that)

>
> > Index: linux-2.6.17-sleazyfpu/arch/x86_64/kernel/process.c
> > ===================================================================
> > --- linux-2.6.17-sleazyfpu.orig/arch/x86_64/kernel/process.c
> > +++ linux-2.6.17-sleazyfpu/arch/x86_64/kernel/process.c
> > @@ -515,6 +515,10 @@ __switch_to(struct task_struct *prev_p,
> > int cpu = smp_processor_id();
> > struct tss_struct *tss = &per_cpu(init_tss, cpu);
> >
> > + /* we're going to use this soon, after a few expensive things */
> > + if (next_p->fpu_counter>5)
> > + prefetch(&next->i387.fxsave);
>
> Did you measure this prefetch makes a difference? I would expect it to
> be too soon to be really worth while (normally you need hundreds of
> instructions for them to make sense and that's probably not the case here)

s/instructions/cycles/

well there are 4 segment loads, a few msr accesses, a few PDA writes and
optionally even the fxsave of the old task inbetween the prefetch and
the use of the memory; those do add up *bigtime*...



-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-07-01 23:58    [W:0.240 / U:0.072 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site