Re: [patch] voluntary-preempt-2.6.8.1-P0

Ingo Molnar
2004-08-16T02:35:33Z

* Lee Revell <rlrevell@joe-job.com> wrote:

> 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.

hm, it seems the mlockall() issue is too deterministic for it to be a
statistical-only phenomenon. Also, isnt that xrun on the order of 15
msecs? That's way too big too.

> 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.

there should be no relation between softirqs and mlockall().

this is truly a mind-boggling latency. mlockall() is fully preemptible.
All it does is to pre-fault the whole range of pages that a process has.

could you try another thing: modify mlockall-test.cc to use mlock() on
the freshly allocated anonymous pages? Does this produce the same
latencies? mlockall() prefaults _all_ pages the process currently has.
Maybe mlockall() touches some page that is mapped both by jackd and
mlockall-test and thus somehow interacts with jackd's scheduling.

the anonymous pages themselves can have no IPC-alike connection to any
page jackd owns. It is unlikely for that to be any connection between
jackd and mlockall-test - other than both map glibc. To further exclude
any possibility of resource sharing between jackd and mlockall-test,
could you compile the later with -static?

Ingo