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

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Message-Id: <20120308103545.24b81093.yoshikawa.takuya@oss.ntt.co.jp>
Date:	Thu, 8 Mar 2012 10:35:45 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>, 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:

> What is worrying are large memory cases: think of the 50GB slot case.
> 100ms hold time is pretty bad (and reacquiring the lock is relatively
> simple).
> 

OK, I agree basically.

But let me explain one thing before deciding what I should do next.

With my method, even when we use a 50GB slot, the hold time will be under
10ms -- not 100ms -- if the memory actually updated from the last time is
1GB (256K dirty pages).

> >   8747274.0   10973.3       33.3      -31%    -3%    256K
  Note that this unit-test was done on my tiny core-i3 32-bit host.
  On servers which can install more than 50GB memory, this will become
  much faster: actually my live migration tests done on Xeon saw much
  better numbers.  Unit-test has been tuned for the worst case.

I admit that if the dirty memory size is more than 10GB, we may see over
100ms hold time you are worrying about.

  For that I was proposing introducing a new GET_DIRTY_LOG API which can
  restrict the number of dirty pages we get the log - but this is a long
  term goal.

So, I am OK to try to introduce cond_resched_lock_cb() as you suggested.
But, as I explained above, my current implementation does not introduce
any real regression concerning to mmu_lock hold time:

  Before we could see 10ms hold time in real workloads:
  > funcgraph_entry:      ! 9783.060 us |  kvm_mmu_slot_remove_write_access();

  I have never seen ms hold time with my method in the same workloads.

So, it would be helpful if you can apply the patch series and I can work
on top of that: although I cannot use servers with 100GB memory now,
migrating a guest with 16GB memory or so may be possible later: I need
to reserve servers for that.

I also want to know "mmu_lock -- TLB flush"-decoupling plan.  We will not
need to introduce cond_resched_lock_cb() in sched.h if that is possible.

Thanks,
	Takuya
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