Date: Wed, 30 Mar 2005 11:56:40 -0800
From: Andrew Morton <akpm@osdl.org>
To: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: rlrevell@joe-job.com, mingo@elte.hu, linux-kernel@vger.kernel.org
Subject: Re: NFS client latencies
Message-Id: <20050330115640.0bc38d01.akpm@osdl.org>
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Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
>
> on den 30.03.2005 Klokka 09:26 (-0500) skreiv Lee Revell:
> > On Tue, 2005-03-29 at 18:18 -0500, Trond Myklebust wrote:
> > > ty den 29.03.2005 Klokka 18:04 (-0500) skreiv Lee Revell:
> > > > I am seeing long latencies in the NFS client code.  Attached is a ~1.9
> > > > ms latency trace.
> > > 
> > > What kind of workload are you using to produce these numbers?
> > > 
> > 
> > Here is the other long latency I am seeing in the NFS client.  I posted
> > this before, but did not cc: the correct people.
> > 
> > It looks like nfs_wait_on_requests is doing thousands of
> > radix_tree_gang_lookups while holding some lock.
> 
> That's normal and cannot be avoided: when writing, we have to look for
> the existence of old nfs_page requests. The reason is that if one does
> exist, we must either coalesce our new dirty area into it or if we
> can't, we must flush the old request out to the server.

One could use the radix-tree tagging stuff so that the gang lookup only
looks up pages which are !NFS_WBACK_BUSY.
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