Date: Fri, 28 Sep 2007 11:49:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Chakri n <chakriin5@gmail.com>,
       linux-pm <linux-pm@lists.linux-foundation.org>,
       lkml <linux-kernel@vger.kernel.org>, nfs@lists.sourceforge.net,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: A unresponsive file system can hang all I/O in the system on
 linux-2.6.23-rc6 (dirty_thresh problem?)
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On Fri, 28 Sep 2007 13:00:53 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:
> 
> > Actually we perhaps could address this at the VFS level in another way. 
> > Processes which are writing to the dead NFS server will eventually block in
> > balance_dirty_pages() once they've exceeded the memory limits and will
> > remain blocked until the server wakes up - that's the behaviour we want.
> > 
> > What we _don't_ want to happen is for other processes which are writing to
> > other, non-dead devices to get collaterally blocked.  We have patches which
> > might fix that queued for 2.6.24.  Peter?
> 
> Do these patches also cause the memory reclaimers to steer clear of
> devices that are congested (and stop waiting on a congested device if
> they see that it remains congested for a long period of time)? Most of
> the collateral blocking I see tends to happen in memory allocation...
> 

No, they don't attempt to do that, but I suspect they put in place
infrastructure which could be used to improve direct-reclaimer latency.  In
the throttle_vm_writeout() path, at least.

Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
sysrq-w five or ten times will probably be enough to determine this)

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