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SubjectRe: Locking a process or thread onto a specific CPU
Steve Linton wrote:
> Is there any way to do this with the 2.2 SMP code? A colleague is
> implementing his own language for research purposes and wants, in
> effect, to schedule his own threads on the available CPUs. The obvious
> way to do this seems to be to run one "real" thread locked onto each
> CPU. I know that threads will typically stay on the same CPU they
> were on anyway, but I can imagine an ill-timed intervention of a third
> process (in the case of 2 CPUs) resulting in both his processes trying
> to run on the same CPU.

> If this is not possible at the moment, can anyone (a) comment on its
> feasibility (b) advise us on where to look to implement it or
> (c) comment on its desirability as a new feature.

(c) I am also into implementing threads. Fast ones, but flexible. Real
fast, like I really mean fast fast.

Locking tasks (= cloned processes) to CPUs isn't quite ideal, because it
means if something else runs for a while, one of your tasks pauses, and
so does the userspace thread running in that task at the time. As you
still have another CPU, it is rather silly that one thread may get
blocked indefinitely out of a pool of many. Or more than one if you're
optimising your user space scheduler.

There's another problem. Sometimes, one of your tasks blocks in the
kernel. This happens however much effort you go to with poll(),
O_NONBLOCK etc. Some things will just block anyway. Disk I/O is
especially unpredictable at this; paging to disk also comes into this
category. Ideally, you'd like to continue using that CPU for some more
of your threads. A solution often used (see nfsd) is to run more than
one task per CPU. But that's non-optimal because the number is often
more than is optimal (= 1 per CPU), and sometimes less than is optimal
(= 1 + number of blocked tasks, per CPU).

IMO, the optimal mechanism is something which lets you schedule threads
purely in userspace the way the kernel can do it for tasks. That is,
have one task at a time running on each CPU at all times (as far as your
program is concerned).

As other programs run from time to time, they interfere with this.
Also, your tasks block unpredictably in the kernel despite efforts with
O_NONBLOCK et al., if you want them to have access to the full kernel
functionality.

To work optimally in the presence of these effects, we need to know how
many tasks to have running at any time.

Unfortunately, I can't think of a simple solution for the first
desirable: keeping your pool of threads running and switching without
multiple tasks context switching on the same CPU, in the presence of
other programs using big timeslices on one or more CPUs. Locking tasks
to CPUs is clearly part of the solution, but it is not the whole
solution. Perhaps this problem is not worth worrying about.

The second thing, blocking in kernel, is a problem for NUMBER CRUNCHING
FARMS and the like. Mainly due to tasks blocking for paging I/O, but
also network I/O etc. Fortunately this is easy to fix. When one of
your task blocks, you'd like to know its worth creating another task (or
unpausing one) to take over on that CPU. Let's assume the kernel picks
the good CPU always. This isn't a few % tweak -- this is a whole
potentially idle CPU that we want to use while some slow thing blocks in
the kernel. So we'd like a mechnism to tell our program to restart
another task and schedule threads into it. No problem: a task flag to
say "if I block, wake up process X", plus user space code to say "when I
carry on, I'll stop myself if X is running, or tell process X to stop".

-- Jamie

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