Re: [PATCH linux-2.6 04/04] brsem: convert cpucontrol to brsem

[Nick Piggin]

Nathan Lynch wrote:

> Nick Piggin wrote:
>
> > Note that I happen to also think the idea (brsems) have merit, and
> > that cpucontrol may be one of the places where a sane implementation
> > would actually be useful... but at least when you're introducing
> > this kind of complexity anywhere, you *really* need to be able to
> > back it up with numbers.
>
> The only performance-related complaint with cpu hotplug of which I'm
> aware -- that taking a cpu down on a large system can be painfully
> slow -- resides in the "write side" of the code, which is not the case
> that the brsem implementation optimizes.  I think this patch would
> make that case even worse.  So I don't think it's appropriate to use a
> brsem for cpu hotplug, especially without trying rwsem first.

I'm not sure that a brsem would make a noticable difference.

It isn't that cpu hotplug semaphore is a performance problem
now, but that it isn't being used in as many cases as it could
be due to its unscalable nature. For example, a while back I
wanted to use it in the fork() path in the scheduler but
couldn't.

Anyway, as I said, you need to be able to back it up with
numbers ;)

Nick


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