Re: High priority tasks break SMP balancer?

[Micah Dowty <micah@vmware.com>]

On Sat, Nov 17, 2007 at 05:43:33AM +1030, David Newall wrote:
> 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?

The Python test harness is fairly artificial, but this is just the
best way I found to reliably reproduce the problem in a short amount
of time. It was just for convenience while running git-bisect. When
running the C program directly, there seems to be a somewhat random
chance that it will start up in the "bad" state. Once the single CPU
is stuck in this mostly-idle mode, it seems to stay that way for a
while.

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

The problem still occurs then, but killing/restarting the test app
seems to trigger the problem more reliably. As I said in the original
email about this, left to its own devices this problem will occur
seemingly-randomly. In the original VMware code I observed this
problem in, the same process would flip between the "good" and "bad"
states seemingly randomly, every few seconds.

Thanks,
--Micah