Messages in this thread | | | Date | Sun, 27 Jan 2008 21:01:15 +0100 | From | "Guillaume Chazarain" <> | Subject | High wake up latencies with FAIR_USER_SCHED |
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Hi,
I noticed some strangely high wake up latencies with FAIR_USER_SCHED using this script:
#!/usr/bin/python
import os import time
SLEEP_TIME = 0.1 SAMPLES = 100 PRINT_DELAY = 0.5
def print_wakeup_latency(): times = [] last_print = 0 while True: start = time.time() time.sleep(SLEEP_TIME) end = time.time() times.insert(0, end - start - SLEEP_TIME) del times[SAMPLES:] if end > last_print + PRINT_DELAY: copy = times[:] copy.sort() print '%f ms' % (copy[len(copy)/2] * 1000) last_print = end
if os.fork() == 0: os.setuid(1) for i in xrange(2): if os.fork() == 0: while True: pass else: os.setuid(2) # <-- here print_wakeup_latency()
We have two busy loops with UID=1. And UID=2 maintains the running median of its wake up latency. I get these latencies:
# ./sched.py 4.300022 ms 4.801178 ms 4.604006 ms 4.606867 ms 4.604006 ms 4.606867 ms 4.604006 ms 4.606867 ms 4.606867 ms 4.676008 ms 4.604006 ms 4.604006 ms 4.606867 ms
Disabling FAIR_USER_SCHED restores wake up latencies in the noise:
# ./sched.py -0.156975 ms -0.067091 ms -0.022984 ms -0.022984 ms -0.022030 ms -0.022030 ms -0.022030 ms -0.021076 ms -0.015831 ms -0.015831 ms -0.016069 ms -0.015831 ms
Strangely enough, another way to restore normal latencies is to change setuid(2) to setuid(1), that is, putting the latency measurement in the same group as the two busy loops.
Thanks in advance for any help.
-- Guillaume
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