Re: [PATCH 11 of 11] x86: defer cr3 reload when doing pud_clear()
From: H. Peter Anvin
Date: Fri Jan 25 2008 - 19:24:55 EST
Ingo Molnar wrote:
* 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)
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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