Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [patch 1/2] sLeAZY FPU feature - x86_64 support | Date | Sat, 1 Jul 2006 23:49:36 +0200 |
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> After 256 switches, this is reset and lazy behavior is returned (until > there are 5 consecutive ones again). The reason for this is to give apps > that do longer bursts of FPU use still the lazy behavior back after some > time.
Cool. This has been on my todo list forever.
However I'm not sure 256 is a good number. It seems a bit too high.
> 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)
> #endif > + /* > + * fpu_counter contains the number of consecutive context switches > + * that the FPU is used. If this is over a threshold, the lazy fpu > + * saving becomes unlazy to save the trap. This is an unsigned char > + * so that after 256 times the counter wraps and the behavior turns > + * lazy again; this to deal with bursty apps that only use FPU for > + * a short time > + */ > + unsigned char fpu_counter;
Putting it at the end is also not good because there are the rarely used cachelines. Probably better in the thread structure -Andi - 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/
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