Re: [RFC PATCH] Fix Readahead stalling by plugged device queues

[Greg KH <greg@kroah.com>]

On Thu, Mar 11, 2010 at 09:29:13PM +0800, Wu Fengguang wrote:
> On Thu, Mar 11, 2010 at 05:58:08PM +0800, Christian Ehrhardt wrote:
> > Wu Fengguang wrote:
> > > On Wed, Mar 10, 2010 at 10:31:46PM +0800, Christian Ehrhardt wrote:
> > >>
> > >> Wu Fengguang wrote:
> > >> [...]
> > >>> Christian, did you notice this commit for 2.6.33?
> > >>>
> > >>> commit 65a80b4c61f5b5f6eb0f5669c8fb120893bfb388
> > >> [...]
> > >>
> > >> I didn't see that particular one, due to the fact that whatever the 
> > >> result is it needs to work .32
> > >>
> > >> Anyway I'll test it tomorrow and if that already accepted one fixes my 
> > >> issue as well I'll recommend distros older than 2.6.33 picking that one 
> > >> up in their on top patches.
> > > 
> > > OK, thanks!
> > 
> > That patch fixes my issue completely and is as we discussed less 
> > aggressive which is fine - thanks for pointing it out - Now I have 
> > something already upstream accepted to fix the issue, thats much better!
> 
> That's great news, it works beyond my expectation.. :)
> 
> > >>> It should at least improve performance between .32 and .33, because
> > >>> once two readahead requests are merged into one single IO request,
> > >>> the PageUptodate() will be true at next readahead, and hence
> > >>> blk_run_backing_dev() get called to break out of the suboptimal
> > >>> situation.
> > >> As you saw from my blktrace thats already the case without that patch.
> > >> Once the second readahead comes in and merged it gets unplugged in 
> > >> 2.6.32 too - but still that is bad behavior as it denies my things like 
> > >> 68% throughput improvement :-).
> > > 
> > > I mean, when readahead windows A and B are submitted in one IO --
> > > let's call it AB -- commit 65a80b4c61 will explicitly unplug on doing
> > > readahead C.  While in your trace, the unplug appears on AB.
> > > 
> > > The 68% improvement is very impressive. Wondering if commit 65a80b4c61
> > > (the _conditional_ unplug) can achieve the same level of improvement :)
> > 
> > Yep it can !
> > We can post update the patch description to bigger numbers :-)
> 
> Andrew/Greg, shall we push the patch to .32 stable?

I've now queued it up.

thanks,

greg k-h

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