Messages in this thread | | | Date | Sun, 11 Oct 2009 09:10:06 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH] mm: make VM_MAX_READAHEAD configurable |
| |
Hi Martin,
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). [snip] > > 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.
May I ask for more details about your performance regression and why it is related to readahead size? (we didn't change VM_MAX_READAHEAD..)
Thanks, Fengguang
| |