Messages in this thread |  | | | Date | Wed, 7 Mar 2012 00:23:17 +0900 | | From | Takuya Yoshikawa <> | | Subject | Re: [PATCH 3/4 changelog-v2] KVM: Switch to srcu-less get_dirty_log() |
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Marcelo Tosatti <mtosatti@redhat.com> wrote:
> > If we do not mind scanning the bitmap twice, we can decouple the > > xchg loop and write protection, but it will be a bit slower, and in > > any case we need to hold mmu_lock until TLB is flushed. > > Why is it necessary to scan twice? Simply continuing to the next set > of pages, after dropping the lock, should be enough.
We cannot drop the lock. Do you mean doing TLB flush each time before dropping the lock?
> The potential problem i am referring to is: > > - kvm.git next + srcu-less series > average(ns) stdev ns/page pages improvement(%) > > 8497356.4 16441.0 32.4 256K -29 > > So 8ms for 1GB. Assuming it increases linearly, it would take > 400ms for get_dirty on a 50GB slot (most of that time spent > with mmu_lock held). Is this correct?
Partly yes: my method mainly depends on the number of dirty pages, not slot size.
But it is not a new problem: traversing all shadow pages for that also takes linearly increasing time.
If that 1GB dirty memory is in a 50GB slot, my method will alleviate the latency really a lot compared to the current way. I do not want to imagine checking every shadow page in such a huge slot.
Checking pages found in the dirty bitmap only should be better.
Takuya
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