Re: [PATCH 2/3] x86/flush_tlb: try flush_tlb_single one by one in flush_tlb_range

[Rik van Riel <riel@redhat.com>]

On 04/28/2012 04:50 AM, Alex Shi wrote:
> x86 has no flush_tlb_range support in instruction level. Currently the
> flush_tlb_range just implemented by flushing all page table. That is not
> the best solution for all scenarios. In fact, if we just use 'invlpg' to
> flush few lines from TLB, we can get the performance gain from later
> remain TLB lines accessing.
>
> But the 'invlpg' instruction costs much of time. Its execution time can
> compete with cr3 rewriting, and even a bit more on SNB CPU.
>
> So, on a 512 4KB TLB entries CPU, the balance points is at:
> 512 * 100ns(assumed TLB refill cost) =
> x(TLB flush entries) * 140ns(assumed invlpg cost)
>
> Here, x is about 360, that is about 5/8 of 512 entries.
>
> But with the mysterious CPU pre-fetcher and page miss handler Unit, the
> assumed TLB refill cost is far lower then 100ns in sequential access. And
> 2 HT siblings in one core makes the memory access more faster if they are
> accessing the same memory. So, in the patch, I just do the change when
> the target entries is less than 1/16 of whole active tlb entries.
> Actually, I have no data support for the percentage '1/16', so any
> suggestions are welcomed.

The numbers speak for themselves, 1/16th seems to work
fine on current generation CPUs.

> +
> +#define FLUSHALL_BAR	16

However, since this is a somewhat arbitrary number, it
would be good to accompany this #define with a multi-line
comment explaining your reasoning for choosing this number.

That will make it easy to re-evaluate in the future, if neeeded.