Re: Kernel locking & the G5 Quad
Re: Kernel locking & the G5 Quad
- Subject: Re: Kernel locking & the G5 Quad
- From: Glen Beane <email@hidden>
- Date: Thu, 20 Oct 2005 11:51:13 -0400 (EDT)
Tiger introduced finer grained locking
On Thu, 20 Oct 2005, Charles Gray wrote:
> Hi,
> I noticed this morning that Apple have now released a quad processor G5
> Power Mac. I'm curious if anyone has any details of how this interacts
> with the funnel locking in the BSD part of darwin.
>
> The BSDCon 2002 paper "Advanced Synchronization in Mac OS X: Extending
> Unix to SMP and Real-Time" (Gerbarg) describes thw two BSD funnels for
> files and networking. I also remember reading (though it would seem not
> in this paper, and I right now can't find where) some other document
> from apple which stated that adding more than two processors to a darwin
> system had little benefit due to funnel contention and cross CPU
> transfers for VFS and networking.
>
> As far as I've seen funnels still exist in the latest kernels I've seen
> at opendarwin.org. I'm just trying to sus out what's going on here,
> since the apple web page lists definite performance benefits from the
> quad CPU system. Eg...
>
> * Has the VFS & networking slowly been getting finer grained locking &
> been made re-entrant over the last few years so there is less contention
> on kernel locking?
>
> * Do apps now have less of a dependence on the BSD part of the kernel so
> the funnels are no longer held as frequently?
>
> * Do the modern CPUs have much better IPI performance so the overheads
> of the lock contention aren't so bad?
>
> * Or, quite simply, are the extra processors only of real benefit if you
> have a very parallel task that won't hit VFS or the network (eg.
> rendering, simulation, whatever), and the scheduler has been tweaked to
> ensure a minimum level of performance (ie. not below that of a dual).
>
> Anyway, just very confused :)
>
> Chuck
>
>
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