Messages in this thread | | | Date | Fri, 16 Nov 2007 17:03:52 -0800 | From | Micah Dowty <> | Subject | Re: High priority tasks break SMP balancer? |
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On Sat, Nov 17, 2007 at 12:26:41AM +0100, Dmitry Adamushko wrote: > Let's say we change a pattern for the niced task: e.g. run for 100 ms. > and then sleep for 300 ms. (that's ~25% of cpu load) in the loop. Any > behavioral changes?
For consistency, I tested this using /dev/rtc. I set the rtc frequency to 16 Hz, and replaced the main loop of my high (-19) priority thread with:
while (1) { unsigned long data;
for (i = 0; i < 3; i++) { if (read(rtc, &data, sizeof data) != sizeof data) { perror("read"); return 1; } }
fcntl(rtc, F_SETFL, O_NONBLOCK); while (read(rtc, &data, sizeof data) < 0); fcntl(rtc, F_SETFL, 0); }
Now it's busy-looping for 62ms, and sleeping for three consecutive 62.5ms chunks totalling 187.5ms.
The results aren't quite what I was expecting. I have only observed this so far in test cases where I have a very high wakeup frequency, so I wasn't expecting this to work. I did, however, still observe the problem where occasionally I get into a state where one CPU is mostly idle.
Qualitatively, this feels a bit different. With the higher clock frequency it seemed like the CPU would easily get "stuck" in this state where it's mostly idle, and it would stay there for a long time. With the low wakeup frequency, I'm seeing it toggle between the busy and mostly-idle states more quickly.
I tried a similar test using usleep() and gettimeofday() rather than /dev/rtc:
while (1) { usleep(300000);
gettimeofday(&t1, NULL); do { gettimeofday(&t2, NULL); } while (t2.tv_usec - t1.tv_usec + (t2.tv_sec - t1.tv_sec) * 1000000 < 100000); }
With this test program, I haven't yet seen a CPU imbalance that lasts longer than a fraction of a second.
--Micah
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