linux-kernel - Re: High wake up latencies with FAIR_USER

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Date:	Mon, 28 Jan 2008 21:13:53 +0100
From:	"Guillaume Chazarain" <guichaz@...oo.fr>
To:	vatsa@...ux.vnet.ibm.com
Cc:	"Ingo Molnar" <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, dhaval@...ux.vnet.ibm.com
Subject: Re: High wake up latencies with FAIR_USER_SCHED

Unfortunately it seems to not be completely fixed, with this script:

#!/usr/bin/python

import os
import time

SLEEP_TIME = 0.1
SAMPLES = 5
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:
    if os.fork() == 0:
        os.setuid(1)
        while True:
            pass
    else:
        os.setuid(2)
        while True:
            pass
else:
    os.setuid(1)
    print_wakeup_latency()

I get seemingly unpredictable latencies (with or without the patch applied):

# ./sched.py
14.810944 ms
19.829893 ms
1.968050 ms
8.021021 ms
-0.017977 ms
4.926109 ms
11.958027 ms
5.995893 ms
1.992130 ms
0.007057 ms
0.217819 ms
-0.004864 ms
5.907202 ms
6.547832 ms
-0.012970 ms
0.209951 ms
-0.002003 ms
4.989052 ms

Without FAIR_USER_SCHED, latencies are consistently in the noise.
Also, I forgot to mention that I'm on a single CPU.

Thanks for the help.

-- 
Guillaume
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