Re: ULE and current.
From: Andy Farkas (andyf_at_speednet.com.au)
Date: 12/11/03
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Date: Fri, 12 Dec 2003 00:03:06 +1000 (EST) To: Bruce Evans <bde@zeta.org.au>
Bruce Evans wrote:
> On Thu, 11 Dec 2003, Jeff Roberson wrote:
> > On Thu, 11 Dec 2003, Andy Farkas wrote:
> > > Jeff Roberson wrote:
> > ...
> > > And at this point I would expect something like:
> > >
> > > sh #0 using 66.3%,
> > > sh #1 using 66.3%,
> > > sh #2 using 66.3%,
> > > idle: cpu0 to be 0%,
> > > idle: cpu1 to be 0%.
> >
> > This is actually very difficult to get exactly right. Since all processes
> > want to run all the time, you have to force alternating pairs to share the
> > second cpu. Otherwise they wont run for an even amount of time.
>
> Perhaps add some randomness. Busting the caches every second or so
> shouldn't make much difference. It happens anyway if there are more
> processes.
Ah, a fundamental misconceptualisation of how SMP works on my part. I'll
leave it up to you guys to figure out how best to do it.
I also have a quad cpu box to test with, so beware :)
...
> > The vm has an idle thread that zeros pages. This is the third thread.
> >
> > > /0 /10 /20 /30 /40 /50 /60 /70 /80 /90 /100
> > > root idle: cpu0 XXXXXXXXXXXXXXXX
> > > root idle: cpu1 XXXXXXXXXXXXXXXX
> > > <idle> XXXXXXXXXXXXXXXX
>
> No, <idle> is just cp_time[CP_IDLE] scaled incorrectly. It is bogus now that
> we have actual idle processes. The scaling for the idle processes seems to
> be almost correct (it is apparently scaled by the number of CPUs), but the
> scaling or the value for <idle> is apparently off by a factor of the number
> of CPUs.
...
> > > So, where *I* get confused is that top(1) thinks that the system can be up
> > > to 200% idle, whereas systat(1) thinks there are 3 threads each consuming
> > > a third of 100% idleness... who is right?
> >
> > Both, they just display different statistics. ;-)
>
> Neither; they have different bugs :-). top actually seems to be
> bug-free here, except it intentionally displays percentages that add
> up to a multiple of 100%. This seems to be best. You just have to
> get used to the percentages in the CPU stat line being scaled and the
> others not being scaled.
So the almost-bug in top(1) is that some CPU percentages are scaled and
some are not scaled?
> I now understand the case of an idle system:
>
> /0 /10 /20 /30 /40 /50 /60 /70 /80 /90 /100
> root idle: cpu0 XXXXXXXXXXXXXXXX
> root idle: cpu1 XXXXXXXXXXXXXXXX
> <idle> XXXXXXXXXXXXXXXX
>
> This should show 50% for each "idle: cpuN" process.
Yes.
> Instead, it tries
> to show 33.3% for each idle process including the pseudo one, but has
> some rounding errors that make it display 30%. The factor of 3 to get
> 33.3% instead of 2 to get 50% for the real idle processes is from
> bogusly counting the pseudo-idle process. The factor of 3 to get 33.3%
> instead of 1 to get 100% for the pseudo-idle process is from bogusly
> counting the real idle processes.
>
> None of these bugs except the percentages being slightly too high are
> scheduler-dependent.
ps. You mentioned "jitter". Thats why I 'sleep 120' in the above tests.
It tends to take about that long for top(1) to settle down. Why is that
so?
>
> Bruce
--
:{ andyf@speednet.com.au
Andy Farkas
System Administrator
Speednet Communications
http://www.speednet.com.au/
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- Previous message: Dag-Erling Smørgrav: "Re: buildworld doesn't like -O2"
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