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SubjectRe: [PATCH 3/4 changelog-v2] KVM: Switch to srcu-less get_dirty_log()
On Wed, Mar 07, 2012 at 12:23:17AM +0900, Takuya Yoshikawa wrote:
> 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?

Yes, only if there is contention (see cond_resched_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.

It was not necessary to read the bitmap under mmu_lock previously.

> 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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