Re: [patch 1/2] sLeAZY FPU feature - x86_64 support

[Andi Kleen <ak@suse.de>]

> 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
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