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SubjectRe: O(1) scheduler & interactivity improvements

Helge Hafting wrote:

> How about _removing_ the io-wait bonus for waiting on pipes then?
> If you wait for disk io, someone else gets to use
> the cpu for their work. So you get a boost for
> giving up your share of time, waiting
> for that slow device.
> But if you wait for a pipe, you wait for some other
> cpu hog to do the first part of _your_ work.
> I.e. nobody else benefitted from your waiting,
> so you don't get any boost either.
> This solves the problem of someone artifically
> dividing up a job, using token passing
> to get unfair priority.
> This can be fine-tuned a bit: We may want the pipe-waiter
> to get a _little_ bonus at times, but that has to be
> subtracted from whatever bonus the process at the
> other end of the pipe has. I.e. no new bonus
> created, just shift some the existing bonus around.
> The "other end" may, after all, have gained legitimate
> bonus from waiting on the disk/network/paging/os, and passing
> some of that on to "clients" might make sense.

In other words:

Don't give pipe-waiting any kind of boost or penalty, but do balance the
interactivity points between entities at each end of the pipe.

So, if you're waiting on a pipe, but the other end is a CPU hog, then
since you don't get a boost (pipe wait), you don't give a boost to the
CPU hog, but since he's a CPU hog, he DOES share negative points with
you, lowering your priority.

Conversely, if a process is waiting on real I/O (disk, user input,
etc.), then it gets an interactivity boost that it can share with other
processes it's connected to via pipe.

Since most X clients only do pipe waiting on the server, then it's the X
server that gets the interactivity boost by waiting on user input, which
it can share with clients.

And since there is no effect from pipe wait, you can still judge a
process as interactive or not based on what it does when it's NOT
waiting on a pipe -- if it becomes a CPU hog THEN, you deduct points, etc.

Here's an interesting question: Would you often have a situation where
a process at one end of a pipe is a CPU hog, and the process at the
other end is interactive? Is that a problem?

If you're always adding or subtracting points, that situation could be
less than optimal, but you'll never get to the point where the
interactive process is believed to be a cpu hog or vice versa. The
points each process would be assessed at a faster rate than the sharing
between processes.

Do you want to always share points, or do you share only when something
wakes up from a pipe wait?

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