Messages in this thread | | | Date | Thu, 09 Sep 2004 21:31:48 -0500 | From | "K.R. Foley" <> | Subject | Re: [patch] voluntary-preempt-2.6.9-rc1-bk4-R1 |
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Ingo Molnar wrote: > * Mark_H_Johnson@raytheon.com <Mark_H_Johnson@raytheon.com> wrote: > > >>If you look at the date / time of the traces, you will notice that >>most occur in the latter part of the test. This is during the "disk >>copy" and "disk read" parts of the testing. [...] > > > would it be possible to test with DMA disabled? (hdparm -d0 /dev/hda) It > might take some extra work to shun the extra latency reports from the > PIO IDE path (which is quite slow) but once that is done you should be > able to see whether these long 0.5 msec delays remain even if all (most) > DMA activity has been eliminated. > > >>preemption latency trace v1.0.5 on 2.6.9-rc1-VP-R1 >>-------------------------------------------------- >> latency: 550 us, entries: 6 (6) >> ----------------- >> | task: cat/6771, uid:0 nice:0 policy:0 rt_prio:0 >> ----------------- >> => started at: kmap_atomic+0x23/0xe0 >> => ended at: kunmap_atomic+0x7b/0xa0 >>=======> >>00000001 0.000ms (+0.000ms): kmap_atomic (file_read_actor) >>00000001 0.000ms (+0.000ms): page_address (file_read_actor) >>00000001 0.000ms (+0.549ms): __copy_to_user_ll (file_read_actor) >>00000001 0.550ms (+0.000ms): kunmap_atomic (file_read_actor) >>00000001 0.550ms (+0.000ms): sub_preempt_count (kunmap_atomic) >>00000001 0.550ms (+0.000ms): update_max_trace (check_preempt_timing) > > > this is a full page copy, from userspace into a kernelspace pagecache > page. This shouldnt take 500 usecs on any hardware. Since this is a > single instruction (memcpy's rep; movsl instruction) there's nothing > that Linux can do to avoid (or even to cause) such a situation.
I saw this one (or one very similar) on a system that I just started testing on some today. Not quite as high (~219 usec if I remember correctly). I don't have access to the system from here. I will forward the trace tomorrow when I'm there tomorrow. However, I haven't seen this on my slower system running the same stress tests. There are several possible points of interest:
System I saw this on: P4 2.4GHz or 3.0GHz 2GB memory 2.6.9-rc1-bk12-S0 built for SMP (even though hyperthreading is off currently)
System I haven't seen this on: PII 450 256MB memory 2.6.9-rc1-bk12-R6 built for UP
Sorry I don't have more complete data in front of me. I will send the concrete info tomorrow with the trace.
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