Re: [Xen-devel] [PATCH v2 0/4] xen: enable EPT A/D bit feature

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Jun 26, 2012 at 6:31 AM, Hao, Xudong <xudong.hao@xxxxxxxxx> wrote:
>> -----Original Message-----
>> From: Tim Deegan [mailto:tim@xxxxxxx]
>> Sent: Monday, June 25, 2012 5:02 PM
>> To: Hao, Xudong
>> Cc: xen-devel@xxxxxxxxxxxxx; Shan, Haitao; keir@xxxxxxx; Zhang, Xiantao;
>> JBeulich@xxxxxxxx
>> Subject: Re: [Xen-devel] [PATCH v2 0/4] xen: enable EPT A/D bit feature
>>
>> At 09:57 +0800 on 20 Jun (1340186263), Xudong Hao wrote:
>> > Changes from v1:
>> > - Move hap_has_dirty_bit and hap_has_access_bit definition from patch 3 to
>> patch2.
>> > - define them as bool_t instead of int.
>> >
>> > Extended Page Tables introduce two bit: access bit and dirty bit, A/D bits
>> > enable VMMs to efficiently implement memory management and page
>> classification
>> > algorithms to optimize VM memory operations.
>> >
>> > This series of patches enable EPT dirty bit feature for guest live 
>> > migration.
>>
>> Thanks for this.  I have a few high-level questions:
>>
>> - You're proposing this for after 4.2, right?
>>
>
> Yes, they are not targeted to 4.2.
>
>> - Have you measured what difference this makes?  I take it it improves
>>   performance during live migration but it would be good to know how much
>>   before taking on extra code.
>>
>
> We did live migration performance testing with patches, it's embarrassed to 
> say but the result show there is no performance gain and no performance loss 
> indeed.

What exactly did you measure?  If you measured the speed of the
migration itself on an empty system, it might not be noticeable; but
if, for instance, it increased the performance of a workload running
*inside* the guest during the migration, or if it decreased the cpu
usage of qemu during the migraiton, that might be worth it.

 -George

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