Messages in this thread | | | Date | Tue, 10 Dec 2013 22:06:21 -0800 | From | Howard Chu <> | Subject | Re: sched: RT throttling activated, 3.12.3 |
| |
Howard Chu wrote: > Howard Chu wrote: >> Li Zefan wrote: >>> On 2013/12/11 10:59, Howard Chu wrote: >>>> I just upgraded a system from a 3.5 kernel to 3.12.3 and attempted to run some new benchmarks on it. I see my test program ramps up in CPU usage for a few seconds and then it gradually tails off. There's nothing obvious in the user code to trigger this behavior, so I check dmesg, and see this: >>>> >>>> [ 55.037057] JFS: nTxBlock = 8192, nTxLock = 65536 >>>> [163591.807470] perf samples too long (2758 > 2500), lowering kernel.perf_event_max_sample_rate to 50000 >>>> [164061.362762] perf samples too long (5204 > 5000), lowering kernel.perf_event_max_sample_rate to 25000 >>>> [167969.339513] [sched_delayed] sched: RT throttling activated >>>> [182741.484637] perf samples too long (294588 > 10000), lowering kernel.perf_event_max_sample_rate to 12500 >>>> [182741.484726] INFO: NMI handler (perf_event_nmi_handler) took too long to run: 36.665 msecs >>>> [182822.633084] perf samples too long (292359 > 20000), lowering kernel.perf_event_max_sample_rate to 6250 >>>> [182905.606119] perf samples too long (290291 > 40000), lowering kernel.perf_event_max_sample_rate to 3250 >>>> [199384.293514] perf samples too long (288142 > 76923), lowering kernel.perf_event_max_sample_rate to 1750 >>>> [208507.301027] perf samples too long (285964 > 142857), lowering kernel.perf_event_max_sample_rate to 1000 >>>> [208528.976208] perf samples too long (283799 > 250000), lowering kernel.perf_event_max_sample_rate to 500 >>>> >>>> Why is the kernel throttling my server? >>>> >>> >>> Because that is the default setting of the kernel. >> >> Apparently a "new" default that didn't exist in 3.5? The code in question is >> not a realtime process. This behavior also wasn't seen in 3.10 or any older >> kernels. > > I just downgraded to 3.10.23 to doublecheck - everything is running normally > there, although a few percent slower than I expected. (Last time I tried 3.10 > it was 3.10.11.) > For comparison, here's a "normally" behaving benchmark run: http://highlandsun.com/hyc/linux3.10/
The result is a fairly steady 15,000 ops/sec and CPU usage is around 190% (this is a quadcore machine).
On the 3.12.3 kernel: http://highlandsun.com/hyc/linux3.12/
The CPU usage is initially around 180% but quickly plummets to about 7% and stays there. This is a pretty major regression for a "default" kernel setting. And given that the target process isn't running with realtime scheduling priority, this can only be considered a bug. (Btw, setting both sched_rt_period_us and sched_rt_runtime_us to -1 has no effect on this behavior.)
-- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/
| |