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xen-devel
[Xen-devel] RE: [PATCH] Align periodic vpts
>From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
>Sent: Wednesday, February 11, 2009 9:06 PM
>
>On 11/02/2009 12:00, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
>
>> Think it more, I think that Jimmy's patch is simpler and more
>> accurate for the purpose. It's just a one-time adjustment for
>> periodical timer, and no harm to single-shot timer. It can be
>> enabled by default, while per-domain range has side-effect
>> unless adding more code to differentiate timers which is not
>> worthy.
>>
>> Of course per-domain switch is still required to disable it as
>> your previous comment, for old guest.
>
>I'd actually be interested in knowing how just bumping Xen
>cmdline option
>timer_slop= would influence power usage and guest timers. No new code
>needed, a nice sliding dial (per host) for power usage versus timer
>accuracy.
>
We'll present some in-depth data in near summit. Basically for
a single 2-vcpu HVM RHEL5u1 on a two core mobile platform,
1ms slop, compared to default 50us, could bring 7.5% more power
saving by reducing timer interrupt by a factor of 3 (RHEL5u1 is
by default 1000HZ meaning 3000 virtual interrupts for 1 vPIT and 2
vAPIC, and then 1ms slop roughly drops interrupt to ~1000). By
running SPECpower, power efficiency score is also slightly improved.
However when we run iperf to check latency, the data became
unstable.
So range timer does affects latency, but in general is a power
efficient feature to fit requirement where power matters more. It's
especially useful at cpu over-commitment where more chances
to align timers and reduce interrupts by a higher factor.
While range timer impacts all timers nondistinctively (xen timer
itself is in essential one shot), Jimmy's patch tends to reach
similar effect for periodical timer (since once align at 1st shot,
so does latter), while leaving single shot timer as it is w/o touching
global slop.
To me above two are not identical which reduces power in different
level.:-)
Thanks,
Kevin
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