Re: dynamic-hz

[linux-os]

On Mon, 13 Dec 2004, Nish Aravamudan wrote:

> On Mon, 13 Dec 2004 03:25:21 -0800, Andrew Morton <akpm@xxxxxxxx> wrote:
> > Andrea Arcangeli <andrea@xxxxxxx> wrote:
> > > The patch only does HZ at dynamic time. But of course it's absolutely
> > >  trivial to define it at compile time, it's probably a 3 liner on top of
> > >  my current patch ;). However personally I don't think the three liner
> > >  will worth the few seconds more spent configuring the kernel ;).
> >
> > We still have 1000-odd places which do things like
> >
> >         schedule_timeout(HZ/10);
>
> Yes, yes, we do :) I replaced far more than I ever thought I could...
> There are a few issues I have with the remaining schedule_timeout()
> calls which I think fit ok with this thread... I'd especially like
> your input, Andrew, as you end up getting most of my patches from KJ.
>
> Many drivers use
>
> set_current_state(TASK_{UN,}INTERRUPTIBLE);
> schedule_timeout(1); // or some other small value < 10
>
> This may or may not hide a dependency on a particular HZ value. If the
> code is somewhat old, perhaps the author intended the task to sleep
> for 1 jiffy when HZ was equal to 100. That meants that they ended up
> sleeping for 10 ms. If the code is new, the author intends that the
> task sleeps for 1 ms (HZ==1000). The question is, what should the
> replacement be?
>
> If they really meant to use schedule_timeout(1) in the sense of
> highest resolution delay possible (the latter above), then they
> probably should just call schedule() directly. schedule_timeout(1)
> simply sets up a timer to fire off after 1 jiffy & then calls
> schedule() itself. The overhead of setting up a timer and the
> execution of schedule() itself probably means that the timer will go
> off in the middle of the schedule() call or very shortly thereafter (I
> think). In which case, it makes more sense to use schedule()
> directly...
>
> If they meant to schedule a delay of 10ms, then msleep() should be
> used in those cases. msleep() will also resolve the issues with 0-time
> timeouts because of rounding, as it adds 1 to the converted parameter.
>
> Obviously, changing more and more sleeps to msecs & secs will really
> help make the changing of HZ more transparent. And specifying the time
> in real time units just seems so much clearer to me.
>
> What do people think?
>
> -Nish

I found that if you use schedule() directly then the sleeping
task appears to be spinning in "system" in `top`. If you use
schedule_timeout(0), it works the same, but doesn't appear
to be eating CPU cycles as shown by `top`. Many common
drivers need to have the timeout interruptible, but wait
<forever if necessary> for a particular event. They need
to get the CPU back fairly often to check again for the
event. They need the equavalent of user-mode sched_yield().
sys_sched_yield() did't seem to work correctly, last time
I tried.

Maybe somebody could make a sched_yield() for the kernel.
That would improve a lot of drivers.



Cheers,
Dick Johnson
Penguin : Linux version 2.6.9 on an i686 machine (5537.79 BogoMips).
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