Re: [PATCH] 2.6.1 Hyperthread smart "nice" 2

[Con Kolivas <kernel@kolivas.org>]

On Tue, 3 Feb 2004 22:12, Ingo Molnar wrote:
> * Con Kolivas <kernel@kolivas.org> wrote:
> > > for lowprio tasks they are of little use, unless you modify gcc to
> > > sprinkle mwait yields all around the 'lowprio code' - not very
> > > practical i think.
> >
> > Yuck!
> >
> > Looks like the kernel is the only thing likely to be smart enough to
> > do this correctly for some time yet.
>
> no, there's no way for the kernel to do this 'correctly', without
> further hardware help. mwait is suspending the current virtual CPU a bit
> better than rep-nop did. This can be exploited for the idle loop because
> the idle loop does nothing so it can execute the rep-nop. (mwait can
> likely also be used for spinlocks but that is another issue.)
>
> user-space code that is 'low-prio' cannot be slowed down via mwait,
> without interleaving user-space instructions with mwait (or with
> rep-nop).
>
> this is a problem area that is not solved by mwait - giving priority to
> virtual CPUs should be offered by CPUs, once the number of logical cores
> increases significantly - if the interaction between those cores is
> significant. (there are SMT designs where this isnt an issue.)

Actually I was trying to say something like my patch, but done correctly. I 
agree with new instructions not helping at the moment.

Con