linux-kernel - Re: [PATCH 00/13] KVM: MMU: fast page fault

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Message-Id: <20120411231441.9d0984672dd252b806f99128@gmail.com>
Date:	Wed, 11 Apr 2012 23:14:41 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 00/13] KVM: MMU: fast page fault

On Wed, 11 Apr 2012 20:38:57 +0800
Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com> wrote:

> Well, my point is that live migration is so very useful that it is worth
> to be improved, the description of your also proves this point.
> 
> What is your really want to say but i missed?

How to improve and what we should pay for that.

Note that I am not objecting to O(1) itself.

Do you remember that when we discussed O(1) issue last year, with Avi,
the agreement was that we should take more time and look carefully
with more measurements to confirm if it's really worthwhile.

The point is whether we should do O(1) now, including near future.

My opinion is that we should do what we can do now and wait for feedback
from real users.

Before making the current code stable, I do not want to see it replaced
so dramatically.  Otherwise when can we use live migration with enough
confidence?  There may be another subtle bugs we should fix now.

In addition, XBRZLE and post-copy is now being developed in QEMU.

What do you think about this Avi, Marcelo?

I am testing the current live migration to see when and for what it can
be used.  I really want to see it become stable and usable for real
services.

> Okay, let us to compare the performance number after O(1) implemented.

>From my experience, I want to say that live migration is very difficult
to say about performance.  That is the problem I am now struggling with.

I developed dirty-log-perf unit-test for that but that was not enough.

Needless to say, checking the correctness is harder.

So I really do not want to see drastic change now without any real need
or feedback from real users -- this is my point.

Thanks,
	Takuya
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