Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()

[Ingo Molnar]

* Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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)

	Ingo
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