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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