RE: [Xen-devel] [PATCH] Fix performance issue brought by TSC-sync logic

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Keir Fraser
>Sent: Monday, February 23, 2009 8:52 PM
>On 23/02/2009 00:21, "Yang, Xiaowei" <xiaowei.yang@xxxxxxxxx> wrote:
>
>> Recently we found one performance bug when doing network 
>test with VTd
>> assigned devices - in some extreme case, the network 
>performance in HVM
>> using new Linux kernel could be 1/20 of native. Root cause 
>is one of our
>> sync-tsc-under-deep-C-state patches brings extra kilo-TSC 
>drift between
>> pCPUs and let check-tsc-sync logic in HVM failed. The result is the
>> kernel fails to use platform timer (HPET, PMtimer) for gettimeofday
>> instead of TSC and brings very frequent costly IOport access VMExit -
>> triple per one call.
>> 
>> We provides below 2 patches to address the issue:
>
>Patch 1 looks reasonable. Patch number 2 I'm less keen on, 
>since patch 1
>should suffice? Also I think regular re-sync across CPUs is a good idea
>anyway. And that also reminds me -- isn't the CONSTANT_TSC 
>logic in time.c
>broken by host S3, and also by CPU hotplug? There's nothing to 
>force sync of
>AP TSC to BP TSC when an AP comes online after boot. Doesn't
>init_percpu_time() need to handle that?
>

Ah, yes, it's broken regarding to S3. We'll work out a patch to handle it.

Thanks,
Kevin
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