Peter Wemm wrote:
> IMHO, it is probably worth just putting the halts in and be done with it.
> Firing off an IPI to wake up cpus is the correct fix though. The catch is
> that is expensive so you have to know when to do it and when not to. ie:
> keep some sort of idle/busy state and have the add-run-queue checks notice
> that they added a new runnable process (vs previously idle) and notice that
> there is an idle cpu that could pick it up and fire an IPI in its
> direction. Only experimentation will tell us if this is worth it.
Yeah.
I looked at doing the IPI thing back in 1996 with Jack Vogel's
original FreeBSD SMP code from October of 1995, and it turned
into an incredible mess really quickly.
I know a lot more now than I did then, but I'd still hesitate to
do it, unless I was being paid to bash my head against the wall
until the wall broke. 8-(.
The one comment I'd make about "just putting the halts in" is
that it becomes a progressively worse trade-off the more CPUs
there are in the machine.
Of course, it's not like anyone is building a lot of large CPU
count systems, like back in the Sequent 32 CPU machine days,
but I know of at least two 8 CPU boxes that might lose based on
adding in the HLT.
The real problem is that when you have a crapload of CPUs, you
generally have them for a reason, which involves shared state,
which means multiple sleepers on shared resources. So you may
be missing the wakeup on 6 processes on a shared wait. That
can really add up, and it's not going to show up in a heterogenous
load, like you get from a "make world". For all we know, a
homogeneous load could show negative effects at 4 or even 2
CPUs. 8-(.
Maybe it could be made into a compile time option? My preference
would be to default it "off", to encourage someone to bash their
head on "the right fix" (with a comment to that effect, so that
someone doesn't "fix it"; have to have learned something from the
ATA write caching default state toggle war...).
-- Terry
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