Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

[Jens Axboe]

On Fri, Oct 09 2009, Peter Zijlstra wrote:
> On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote:
> > From: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
> > 
> > On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default
> > and can be configured per block device queue.
> > On the other hand a lot of admins do not use it, therefore it is reasonable to
> > set a wise default.
> > 
> > This path allows to configure the value via Kconfig mechanisms and therefore
> > allow the assignment of different defaults dependent on other Kconfig symbols.
> > 
> > Using this, the patch increases the default max readahead for s390 improving
> > sequential throughput in a lot of scenarios with almost no drawbacks (only
> > theoretical workloads with a lot concurrent sequential read patterns on a very
> > low memory system suffer due to page cache trashing as expected).
> 
> Why can't this be solved in userspace?
> 
> Also, can't we simply raise this number if appropriate? Wu did some
> read-ahead trashing detection bits a long while back which should scale
> the read-ahead window back when we're low on memory, not sure that ever
> made it in, but that sounds like a better option than having different
> magic numbers for each platform.

Agree, making this a config option (and even defaulting to a different
number because of an arch setting) is crazy.

-- 
Jens Axboe

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