Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_pages

From: Jamie Lokier
Date: Tue Sep 01 2009 - 19:56:54 EST

Jamie Lokier wrote:
> Christoph Hellwig wrote:
> > On Tue, Sep 01, 2009 at 08:38:55PM +0200, Peter Zijlstra wrote:
> > > Do we really need a tunable for this?
> >
> > It will make increasing it in the field a lot easier. And having deal
> > with really large systems I have the fear that there are I/O topologies
> > outhere for which every "reasonable" value is too low.
> >
> > > I guess we need a limit to avoid it writing out everything, but can't we
> > > have something automagic?
> >
> > Some automatic adjustment would be nice. But finding the right auto
> > tuning will be an interesting exercise.
> I have embedded systems with 32MB RAM and no MMU, where I deliberately
> make the equivalent of max_writeback_pages *smaller* to limit the
> number of dirty pages causing fragmentation and preventing allocation
> of high-order pages... Write performance is less important than being
> able to allocate contiguous memory for reads.
> They are still using 2.4 kernels, but the principle still applies.
> maybe even more on 2.6 which is more prone to fragmentation on small
> no-MMU devices.

Sorry, I must get more sleep. I confused max_writeback_pages with the
limit on dirty pages in the system, which is completely different.

So please ignore my previous mail.

-- Jamie
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