Messages in this thread | | | Subject | Re: [patch] voluntary-preempt-2.6.8.1-P0 | From | Lee Revell <> | Date | Sun, 15 Aug 2004 22:08:41 -0400 |
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On Sun, 2004-08-15 at 20:25, Florian Schmidt wrote: > On Sun, 15 Aug 2004 13:56:49 +0200 > Ingo Molnar <mingo@elte.hu> wrote: > > > > > i've uploaded the -P0 patch: > > > > http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8.1-P0 > > I haven't tried this patch yet, but i have a question regarding the > mlockall issue: > > Jackd also uses IPC mechnisms for remote procedure calls [i think, > please correct me] and makes heavy use of shared memory. Might > mlock(all) have influence of this? is jackd maybe producing xruns > because some IPC stuff blocks when mlockall is used?
I reworked the jackd latency histogram to report the time elapsed between entering and exiting the poll() of PCM fd's in alsa-driver.c. This is a different metric than the previous max_usecs histogram, which I believe indirectly measured latency, this one measures it directly.
http://krustophenia.net/testresults.php?dataset=2.6.8.1-P0
The peaks on this graph should correspond directly to the length of the non-preemptible critical section reported by Ingo's latency tracer. I think the large peak around 580-600usecs is caused by the extract_entropy issue (which can be hit by regular processes and ksoftirqd), and the large peak around 80-100 by the XFree86 unmap_vmas issue, as the times match and these are by far the most common reported in latency_trace.
There are a number of samples above 700us. I am working with a period time of 666 usecs, and since there are 2 periods per buffer, we would have to hit two > 666 usec latencies in a row for an xrun - it appears that there are many individual latencies above 666, certainly more than there are xruns. So, maybe the mlockall issue is not a result of triggering a single large latency, but of increasing the frequency of these higher latencies so that we are more likely to hit 2 in a row.
IIRC ksoftirqd will defer more work under load, and ksoftirqd is one of the more common offenders to hit the extract_entropy latency. Maybe mlockall causes more softirqs to be deferred, thus increaing the change that we will have to do more than 666 usecs worth of work on 2 successive wakeups.
Lee
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