Messages in this thread | | | Date | Fri, 9 Oct 2009 14:29:52 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] mm: make VM_MAX_READAHEAD configurable |
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On Fri, Oct 09 2009, Peter Zijlstra wrote: > 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.
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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