Re: Likley stupid question on "throttle_vm_writeout"

[Martin Knoblauch <spamtrap@knobisoft.de>]

----- Original Message ----

> From: Wu Fengguang <fengguang.wu@intel.com>
> To: Martin Knoblauch <spamtrap@knobisoft.de>
> Cc: Peter Zijlstra <peterz@infradead.org>; "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>; "Myklebust, Trond" <Trond.Myklebust@netapp.com>; Peter Staubach <staubach@redhat.com>; linux-fsdevel@vger.kernel.org
> Sent: Tue, November 10, 2009 2:08:18 PM
> Subject: Re: Likley stupid question on "throttle_vm_writeout"
> 
> On Tue, Nov 10, 2009 at 08:01:47PM +0800, Martin Knoblauch wrote:
> > ----- Original Message ----
> > 
> > > From: Wu Fengguang 
> > > To: Peter Zijlstra 
> > > Cc: Martin Knoblauch ; linux-kernel@vger.kernel.org
> > > Sent: Tue, November 10, 2009 3:08:58 AM
> > > Subject: Re: Likley stupid question on "throttle_vm_writeout"
> > > 
> > > On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote:
> > > > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote:
> > > > > Hi, (please CC me on replies)
> > > > > 
> > > > >  I have a likely stupid question on the function "throttle_vm_writeout". 
> 
> > > Looking at the code I find:
> > > > > 
> > > > >                 if (global_page_state(NR_UNSTABLE_NFS) +
> > > > >                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
> > > > >                                 break;
> > > > >                 congestion_wait(WRITE, HZ/10);
> > > > > 
> > > > > Shouldn't the NR_FILE_DIRTY pages be considered as well?
> > > > 
> > > > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-)
> > > > 
> > > > The intent of throttle_vm_writeout() is to limit the total pages in
> > > > writeout and to wait for them to go-away.
> > > 
> > > Like this:
> > > 
> > >         vmscan fast => large NR_WRITEBACK => throttle vmscan based on it
> > > 
> > > > Everybody hates the function, nobody managed to actually come up with
> > > > anything better.
> > > 
> > > btw, here is another reason to limit NR_WRITEBACK: I saw many
> > > throttle_vm_writeout() waits if there is no wait queue to limit
> > > NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK
> > > is _not_ caused by fast vmscan..
> > > 
> > 
> >  That is exactely what made me look again into the code. My observation is 
> that when doing something like:
> > 
> > dd if=/dev/zero of=fast-local-disk bs=1M count=15000
> > 
> > most of the "dirty" pages are in NR_FILE_DIRTY with some relatively small 
> amount (10% or so) in NR_WRITEBACK. If I do:
> > 
> > dd if=/dev/zero of=some-nfs-mount bs=1M count=15000
> > 
> > NR_WRITEBACK almost immediatelly goes up to dirty_ratio, with
> > NR_UNSTABLE_NFS small. Over time NR_UNSTABLE_NFS grows, but is
> > always lower than NR_WRITEBACK (maybe 40/60).
> 
> This is interesting, though I don't see explicit NFS code to limit
> NR_UNSTABLE_NFS. Maybe there are some implicit rules.
> 
> >  But don't ask what happens if I do both in parallel.... The local
> >  IO really slows to a crawl and sometimes the system just becomes
> >  very unresponsive. Have we heard that before? :-)
> 
> You may be the first reporter as far as I can tell :)
>

 Oh come on :-) I (and others) have reported bad writeout behaviour since years. But maybe not in the combination of local and NFS I/O.
 
> >  Somehow I have the impression that NFS writeout is able to
> >  absolutely dominate the dirty pages to an extent that the system is
> >  unusable.
> 
> This is why I want to limit NR_WRITEBACK for NFS:
> 
>         [PATCH] NFS: introduce writeback wait queue
>         http://lkml.org/lkml/2009/10/3/198
> 

 Thanks. I will have a look. Is 2.6.32.x OK for testing?

Cheers
Martin