linux-kernel - Re: [PATCH v2 00/16] KVM: MMU: fast page fault

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Message-Id: <20120414123722.a94af3adb4ca80a6a5f6b477@gmail.com>
Date:	Sat, 14 Apr 2012 12:37:22 +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>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 00/16] KVM: MMU: fast page fault

On Fri, 13 Apr 2012 18:05:29 +0800
Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com> wrote:

> Thanks for Avi and Marcelo's review, i have simplified the whole things
> in this version:
> - it only fix the page fault with PFEC.P = 1 && PFEC.W = 0 that means
>   unlock set_spte path can be dropped.
> 
> - it only fixes the page fault caused by dirty-log
> 
> In this version, all the information we need is from spte, the
> SPTE_ALLOW_WRITE bit and SPTE_WRITE_PROTECT bit:
>    - SPTE_ALLOW_WRITE is set if the gpte is writable and the pfn pointed
>      by the spte is writable on host.
>    - SPTE_WRITE_PROTECT is set if the spte is write-protected by shadow
>      page table protection.
> 
> All these bits can be protected by cmpxchg, now, all the things is fairly
> simple than before. :)

Well, could you remove cleanup patches not needed for "lock-less" from
this patch series?

I want to see them separately.

Or everything was needed for "lock-less" ?

> Performance test:
> 
> autotest migration:
> (Host: Intel(R) Xeon(R) CPU           X5690  @ 3.47GHz * 12 + 32G)

Please explain what this test result means, not just numbers.

There are many aspects:
	- how fast migration can converge/complete
	- how fast programs inside the guest can run during migration:
	  -- throughput
	  -- latency
	- ...

I think lock-less will reduce latency a lot, but not sure about convergence:
why it became fast?

> - For ept:
> 
> Before:
>                     smp2.Fedora.16.64.migrate
> Times   .unix      .with_autotest.dbench.unix     total
>  1       104           214                         323
>  2       68            238                         310
>  3       68            242                         314
> 
> After:
>                     smp2.Fedora.16.64.migrate
> Times   .unix      .with_autotest.dbench.unix     total
>  1       101           190                         295
>  2       67            188                         259
>  3       66            217                         289
> 

As discussed on v1-threads, the main goal of this "lock-less" should be
the elimination of mmu_lock contentions.

So what we should measure is latency.

Thanks,
	Takuya
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