Re: SMP Scheduling

Peter Waltenberg (peterw@dascom.com)
Mon, 09 Aug 1999 13:24:17 +1000 (EST)

On 09-Aug-99 Horst von Brand wrote:
> Peter Waltenberg <peterw@dascom.com> said:
> [...]
>
>> Under 2.0 if you ran a CPU hog it'd pretty well stick to one CPU.
>
> How is that an improvement?
>
>> I.e. if you had xosview running you'd see one CPU at 100%, the other mostly
>> idle. If there was a load burst, it might move to the other CPU, but that
>> was
>> pretty unusual.
>
>> Under 2.2 you see that one CPU hog hopping CPU's and at regular intervals.
>> Using xosview to track load what you see is a picket fence effect.
>> And there are more than "3 processes" running, more like 80 on my machine,
>> so running xosview alone shouldn't be enough to force this to happen and if
>> it were, the other processes should be introducing enough noise to make the
>> CPU swapping more erratic.
>
> If you have that many processes running, your hog will have its state at
> the CPU flushed anyway, so the CPU selected is irrelevant.
>
>> This does seem to be "wrong", not so much that the process is changing
>> CPU's,
>> thats reasonable, but the fact that it's doing it with such regularity
>> now.
>
> File it under "random trivia" then ;-)

I'd expect the process to be flushed, however in that case I'd expect it to be
re-run on some random CPU. However that doesn't seem to happen, the process
swaps CPU's at REGULAR intervals. The scheduler is supposedly designed so that a
process will have a tendency to run on the same CPU.

It's not that the process changes CPU's, it's that it's doing it at
regular intervals that I find worrying.
Yes, that could just be coincidence, or it could be a real problem.

I would file it under random trivia, but the costs of moving a process from
CPU to CPU are (relatively) quite high compared with the other kernel
overheads and it SEEMS to be happening when it's not necessary. It's not
just me seeing this, I think we have 3 or 4 separate reports now.

Anyone want to produce figures for how large a % of our timeslice a cache
refill takes ?. I get a very small number at 100HZ, however if we increase
the scheduling rate that obviously gets worse fairly quickly.

And, is there anyone out there with a box with > 2 CPU's ?, if there's no
scheduler problem then with one CPU hog running I'd expect it to jump back and
forward between two CPU's at most, if it gets cycled around all 4 CPU's in a
regular pattern I'd say there's very likely to be a problem.

Peter
----------------------------------
E-Mail: Peter Waltenberg <peterw@surf.dascom.com>
Date: 09-Aug-99
Time: 12:49:43

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