lkml.org 
[lkml]   [2009]   [Oct]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] mm: make VM_MAX_READAHEAD configurable
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.

Given the (increasing) level of disparity between different kinds of
storage devices, having _any_ default is crazy.

Would be better to make some sort of vaguely informed guess at
runtime, based upon the characteristics of the device.



\
 
 \ /
  Last update: 2009-10-09 23:47    [W:0.070 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site