On Tue, Nov 1, 2016 at 2:36 PM, Jason Harmening <jason.harmening at
gmail.com>
wrote:
> Sorry, that should be ~*30ms* to get 30fps, though the variance is still
> up to 500ms for me either way.
>
> On 11/01/16 14:29, Jason Harmening wrote:
> > repro code is at http://pastebin.com/B68N4AFY if anyone's
interested.
> >
> > On 11/01/16 13:58, Jason Harmening wrote:
> >> Hi everyone,
> >>
> >> I recently upgraded my main amd64 server from 10.3-stable
(r302011) to
> >> 11.0-stable (r308099). It went smoothly except for one big issue:
> >> certain applications (but not the system as a whole) respond very
> >> sluggishly, and video playback of any kind is extremely choppy.
> >>
> >> The system is under very light load, and I see no evidence of
abnormal
> >> interrupt latency or interrupt load. More interestingly, if I
place the
> >> system under full load (~0.0% idle) the problem *disappears* and
> >> playback/responsiveness are smooth and quick.
> >>
> >> Running ktrace on some of the affected apps points me at the
problem:
> >> huge variance in the amount of time spent in the nanosleep system
call.
> >> A sleep of, say, 5ms might take anywhere from 5ms to ~500ms from
entry
> >> to return of the syscall. OTOH, anything CPU-bound or that waits
on
> >> condvars or I/O interrupts seems to work fine, so this doesn't
seem to
> >> be an issue with overall system latency.
> >>
> >> I can repro this with a simple program that just does a 3ms usleep
in a
> >> tight loop (i.e. roughly the amount of time a video player would
sleep
> >> between frames @ 30fps). At light load ktrace will show the huge
> >> nanosleep variance; under heavy load every nanosleep will complete
in
> >> almost exactly 3ms.
> >>
> >> FWIW, I don't see this on -current, although right now all my
-current
> >> images are VMs on different HW so that might not mean anything.
I'm not
> >> aware of any recent timer- or scheduler- specific changes, so
I'm
> >> wondering if perhaps the recent IPI or taskqueue changes might be
> >> somehow to blame.
> >>
> >> I'm not especially familiar w/ the relevant parts of the
kernel, so any
> >> guidance on where I should focus my debugging efforts would be
much
> >> appreciated.
> >>
> >> Thanks,
> >> Jason
>
This is likely off track, but this is a behavior I have noticed since
moving to 11, though it might have started in 10.3-STABLE before moving to
head before 11 went to beta. I can't explain any way nanosleep could be
involved, but I saw annoying lock-ups similar to yours. I also no longer
see them.
I eliminated the annoyance by change scheduler from ULE to 4BSD. That was
it, but I have not seen the issue since. I'd be very interested in whether
the scheduler is somehow impacting timing functions or it's s different
issue. I've felt that there was something off in ULE for some time, but it
was not until these annoying hiccups convinced me to try going back to
4BSD.
Tip o' the hat to Doug B. for his suggestions that ULE may have issues that
impacted interactivity.