Re: RSDL-mm 0.28

[Nick Piggin]

David Schwartz wrote:
> > > There's a substantial performance hit for not yield, so we probably
> > > want to investigate alternate semantics for it. It seems reasonable
> > > for apps to say "let me not hog the CPU" without completely expiring
> > > them. Imagine you're in the front of the line (aka queue) and you
> > > spend a moment fumbling for your wallet. The polite thing to do is to
> > > let the next guy in front. But with the current sched_yield, you go
> > > all the way to the back of the line.
>
>
> > Well... are you advocating we change sched_yield semantics to a
> > gentler form? This is a cinch to implement but I know how Ingo feels
> > about this. It will only encourage more lax coding using sched_yield
> > instead of proper blocking (see huge arguments with the ldap people on
> > this one who insist it's impossible not to use yield).
>
>
> The basic point of sched_yield is to allow every other process at the same
> static priority level a chance to use the CPU before you get it back. It is
> generally an error to use sched_yield to be nice. It's nice to get your work
> done when the scheduler gives you the CPU, that's why it gave it to you.
>
> It is proper to use sched_yield as an optimization when it more efficient to
> allow another process/thread to run than you, for example, when you
> encounter a task you cannot do efficiently at that time because another
> thread holds a lock.
>
> It's also useful prior to doing something that can most efficiently be done
> without interruption. So a thread that returns from 'sched_yield' should
> ideally be given a full timeslice if possible. This may not be sensible if
> the 'sched_yield' didn't actuall yield, but then again, if nothing else
> wants to run, why not give the only task that does a full slice?
>
> In no case is much of anything guaranteed, of course. (What can you do if
> there's no other process to yield to?)
>
> Note that processes that call sched_yield should be rewarded for doing so
> just as process that block on I/O are, assuming they do in fact wind up
> giving up the CPU when they would otherwise have had it.
>
> DS
>
>
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