Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote:
> From: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
> 
> 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.


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