Re: Likley stupid question on "throttle_vm_writeout"

[Martin Knoblauch]

----- Original Message ----

> From: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Martin Knoblauch <spamtrap@xxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> 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).

 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? :-)

 Somehow I have the impression that NFS writeout is able to absolutely dominate the dirty pages to an extent that the system is unusable.

Cheers
Martin

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