Re: [PATCH] O13int for interactivity

[Mike Galbraith]

At 05:07 PM 8/12/2003 +1000, Nick Piggin wrote:


> Mike Galbraith wrote:
>
> > At 12:51 PM 8/12/2003 +1000, Nick Piggin wrote:
> >
> >
> > > Rob Landley wrote:
> > >
> > > > On Tuesday 05 August 2003 06:32, Nick Piggin wrote:
> > > >
> > > >
> > > > > But by employing the kernel's services in the shape of a blocking
> > > > > syscall, all sleeps are intentional.
> > > >
> > > >
> > > > Wrong.  Some sleeps indicate "I have run out of stuff to do right now, 
> > > > I'm going to wait for a timer or another process or something to wake 
> > > > me up with new work".
> > > >
> > > >
> > > >
> > > > Some sleeps indicate "ideally this would run on an enormous ramdisk 
> > > > attached to gigabit ethernet, but hard drives and internet connections 
> > > > are just too slow so my true CPU-hogness is hidden by the fact I'm 
> > > > running on a PC instead of a mainframe."
> > >
> > >
> > > I don't quite understand what you are getting at, but if you don't want to
> > > sleep you should be able to use a non blocking syscall. But in some cases
> > > I think there are times when you may not be able to use a non blocking call.
> > > And if a process is a CPU hog, its a CPU hog. If its not its not. Doesn't
> > > matter how it would behave on another system.
> >
> >
> > Ah, but there is something there.  Take the X and xmms's gl thread thingy 
> > I posted a while back.  (X runs long enough to expire in the presence of 
> > a couple of low priority cpu hogs.  gl thread, which is a mondo cpu hog, 
> > and normally runs and runs and runs at cpu hog priority, suddenly 
> > acquires extreme interactive priority, and X, which is normally sleepy 
> > suddenly becomes permanently runnable at cpu hog priority)  The gl thread 
> > starts sleeping because X isn't getting enough cpu to be able to get it's 
> > work done and go to sleep.  The gl thread isn't voluntarily sleeping, and 
> > X isn't voluntarily running.
> > The behavior change is forced upon both.
>
>
> It does... It is I tell ya!
>
> Look, the gl thread is probably _very_ explicitly asking to sleep. No I
> don't know how X works, but I have an idea that select is generally used
> as an event notification, right?

Oh, sure, it blocks because it asks for it... but not because it _wants_ to 
:)  It wants to create work for X fast enough to make a nice stutter free 
bit of eye-candy.

> Now the gl thread is essentially saying "wait until X finishes the work
> I've given it, or I get some other event": ie. "put me to sleep until
> this fd becomes readable".

Yes.  Voluntary or involuntary is just a matter of point of view.

> OK maybe your scenario is a big problem. Its not due to any imagined
> semantics in the way things are sleeping. Its due to the scheduler.

It's due to the scheduler to a point... only in that it doesn't recognize 
the problem and correct it (that might be pretty hard to do).  If my 
hardware were fast enough that X could get the work done in the allotted 
time, the problem wouldn't arise in the first place.  I bet it's fairly 
hard to reproduce on a really fast box.  It happens easily on my box 
because the combination of X and the gl thread need most of what my 
hardware has to offer.

        -Mike 

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