Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

[Wu Fengguang <fengguang.wu@intel.com>]

On Fri, Oct 09, 2009 at 09:49:50PM +0800, Martin Schwidefsky wrote:
> On Fri, 9 Oct 2009 14:29:52 +0200
> Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> > 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.
> 
> The patch from Christian fixes a performance regression in the latest
> distributions for s390. So we would opt for a larger value, 512KB seems
> to be a good one. I have no idea what that will do to the embedded
> space which is why Christian choose to make it configurable. Clearly
> the better solution would be some sort of system control that can be
> modified at runtime. 

So how about doing two patches together?

- lift default readahead size to around 512KB
- add some readahead logic to better support the thrashing case

Thanks,
Fengguang

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