linux-kernel - Re: High priority tasks break SMP balancer?

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Message-ID: <473DEBDD.2010706@davidnewall.com>
Date:	Sat, 17 Nov 2007 05:43:33 +1030
From:	David Newall <david@...idnewall.com>
To:	Micah Dowty <micah@...are.com>
CC:	Kyle Moffett <mrmacman_g4@....com>,
	Cyrus Massoumi <cyrusm@....net>,
	LKML Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...l.org>,
	Mike Galbraith <efault@....de>,
	Paul Menage <menage@...gle.com>,
	Christoph Lameter <clameter@....com>
Subject: Re: High priority tasks break SMP balancer?

There are a couple of points I would make about your python test 
harness.  Your program compares real+system jiffies for both cpus; an 
ideal result would be 1.00.  The measurement is taken over a relatively 
short period of approximately a half-second, and you kill the CPU hogs 
before taking final measurements, even wait for them to die first.  You 
repeat this measurement, starting and killing CPU hogs each time.  Why 
do you do that?

What happens if you start the hogs and take the baseline outside of the 
loop?

    from __future__ import division
    import sys, os, time

    def getCpuTimes():
        cpu0 = 0
        cpu1 = 1
        for line in open("/proc/stat"):
            tokens = line.split()
            if tokens[0] == "cpu0":
                cpu0 = int(tokens[1]) + int(tokens[3])
            elif tokens[0] == "cpu1":
                cpu1 = int(tokens[1]) + int(tokens[3])
        return cpu0, cpu1

    pid = os.spawnl(os.P_NOWAIT, "./priosched")
    baseline = getCpuTimes()
    while True:
        time.sleep(0.5)
        current = getCpuTimes()
        print "%.04f" % (current[0] - baseline[0]) / (current[1] -
    baseline[1])

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