ajax at redhat.com
Thu Jan 26 11:08:35 PST 2012
On Thu, 2012-01-26 at 11:58 -0500, Adam Jackson wrote:
> I think a more complete solution would involve changing the main loop's
> logic slightly: if we get to the end of requests for a single client
> within a single timeslice, start pulling requests from any other clients
> that have pending requests (in priority order) until the timeslice
> expires (at which point we'd loop back around to select and start
> again). This would punish absolute throughput for any one client when
> contended, but I'm willing to take that if it means we stop dropping
Just a quick followup since it was an easy experiment to run. Test
model name : Intel(R) Core(TM)2 Duo CPU P8600 @ 2.40GHz
The reproducer scenario is two apps, one that's throughput-bound and one
that's latency-bound. Easiest way to get this is one x11perf -noop and
one x11perf -pointer.
With the stock tuning on an idle server, -noop performance is about
21.6e6 per second, and -pointer is about 28.7e3 per second. With both
running at once, -noop performance is essentially unchanged but -pointer
drops to a mere 1.02e3/s.
If I change the default timeslice to 2ms (and the max to 16ms, not that
this case would hit it), baseline performance is unchanged. Running
both -noop and -pointer at once, -noop drops to about 20.6e6/s but
-pointer climbs to 3.8e3/s.
So, there's that. You lose about 5% in the pathologically
throughput-bound case, but your peak latency goes way down. Sounds like
a no-brainer to me, honestly. And I'm pretty sure if you wanted that 5%
back that the other code change I proposed would recover it.
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