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Date:      Tue, 1 Nov 2016 22:49:56 -0700
From:      Kevin Oberman <rkoberman@gmail.com>
To:        Jason Harmening <jason.harmening@gmail.com>
Cc:        FreeBSD-STABLE Mailing List <freebsd-stable@freebsd.org>
Subject:   Re: huge nanosleep variance on 11-stable
Message-ID:  <CAN6yY1v4%2Bf3PCGhcQXVsJYTOpdAN7yK9o-dG99hmX4dkmYBNOw@mail.gmail.com>
In-Reply-To: <a6c4ecc0-c084-8ecd-c9ee-4dfabf5a88e9@gmail.com>
References:  <c88341e2-4c52-ed3c-a469-6446da4415f4@gmail.com> <6167392c-c37a-6e39-aa22-ca45435d6088@gmail.com> <1c3f4599-8aef-471a-3a39-49d913f1a4e5@gmail.com> <CAN6yY1vKr_PAHp3bL-iiHndPxq58kz_qFqmjbEcK1CbmhywVZg@mail.gmail.com> <a6c4ecc0-c084-8ecd-c9ee-4dfabf5a88e9@gmail.com>

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On Tue, Nov 1, 2016 at 10:16 PM, Jason Harmening <jason.harmening@gmail.com>
wrote:

>
>
> On 11/01/16 20:45, Kevin Oberman wrote:
> > On Tue, Nov 1, 2016 at 2:36 PM, Jason Harmening
> > <jason.harmening@gmail.com <mailto:jason.harmening@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.
>
> I figured it out: r282678 (which was never MFCed to 10-stable) added
> support for the MWAIT instruction on the idle path for Intel CPUs that
> claim to support it.
>
> While my CPU (2009-era Xeon 5500) advertises support for it in its
> feature mask and ACPI C-state entries, the cores don't seem to respond
> very quickly to interrupts while idling in MWAIT.  Disabling mwait in
> acpi_cpu.c and falling back to the old "sti; hlt" mechanism for C1
> completely fixes the responsiveness issues.
>
> So if your CPU is of a similar vintage, it may not be ULE's fault.
>
>
You are almost certainly correct. My system is circa 2011; i5-2520M, Sandy
Bridge. While it might have the same issue, I'd be surprised. It's
possible, but probably completely different from what you are seeing.
Reports of the problem I was seeing definitely pre-date 11, but 11 made
things much worse, so it could be a combination of things. And I can't see
how ULE could have anything to do with this issue.

Congratulations on some really good sleuthing to find this.
--
Kevin Oberman, Part time kid herder and retired Network Engineer
E-mail: rkoberman@gmail.com
PGP Fingerprint: D03FB98AFA78E3B78C1694B318AB39EF1B055683



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