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

[Ingo Molnar <mingo _at_ xxxxxxx>]

* Con Kolivas <kernel _at_ 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.)

	Ingo
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