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

["H. Peter Anvin" <hpa@zytor.com>]

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