On 11/01/16 20:45, Kevin Oberman wrote: > On Tue, Nov 1, 2016 at 2:36 PM, Jason Harmening > <jason.harmen...@gmail.com <mailto:jason.harmen...@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.
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