linux-kernel - Re: [PATCH 3/4 changelog-v2] KVM: Switch to srcu-less get_dirty

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Message-Id: <20120307002317.d8682de9c6ccd6af8ccac467@gmail.com>
Date:	Wed, 7 Mar 2012 00:23:17 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>, avi@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4 changelog-v2] KVM: Switch to srcu-less
 get_dirty_log()

Marcelo Tosatti <mtosatti@...hat.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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