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xen-devel
Re: [Xen-devel] [PATCH 1/2] cpu steal time accounting
On 22 Feb 2006, at 14:27, Rik van Riel wrote:
Is accounting user/system time an unnecessary extra? I guess we
already
do it by sampling at tick granularity anyway?
Should 'steal time' include blocked time when the guest had no work to
execute?
No, this is idle time. If the guest had no work to do,
it wasn't suffering from contention of the CPU.
But the 'vcpu_time' you read out of Xen excludes time spent
blocked/unrunnable. Won't you end up accounting that as it it were
involuntary preemption? Also:
1. What if a guest gets preempted for lots of short time periods (less
than a jiffy). Then some arbitrary time in the future is preempted for
long enough to activate you stolen-time logic. Won't you end up
incorrectly accounting the accumulated short time periods?
2. Is the Xen provided 'vcpu_time', divided down into jiffies, even
comparable with the kstats that you sum? What about accumulated
rounding errors in 'vcpu_time' and the kstats causing relative drift
between them over time?
-- Keir
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