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SubjectRe: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()
Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>> Is there any guide about the tradeoff of when to use invlpg vs
>> flushing the whole tlb? 1 page? 10? 90% of the tlb?
>
> i made measurements some time ago and INVLPG was quite uniformly slow on
> all important CPU types - on the order of 100+ cycles. It's probably
> microcode. With a cr3 flush being on the order of 200-300 cycles (plus
> any add-on TLB miss costs - but those are amortized quite well as long
> as the pagetables are well cached - which they usually are on today's
> 2MB-ish L2 caches), the high cost of INVLPG rarely makes it worthwile
> for anything more than a few pages.
>
> so INVLPG makes sense for pagetable fault realated single-address
> flushes, but they rarely make sense for range flushes. (and that's how
> Linux uses it)
>

Incidentally, as far as I can tell, the main INVLPG is so slow is
because of its painful behaviour with regards to large pages which may
have been split by hardware.

-hpa


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