linux-kernel - Re: [PATCH] nfs: fix congestion control

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Message-Id: <1169001692.22935.84.camel@twins>
Date:	Wed, 17 Jan 2007 03:41:32 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] nfs: fix congestion control

On Tue, 2007-01-16 at 17:27 -0500, Trond Myklebust wrote:
> On Tue, 2007-01-16 at 23:08 +0100, Peter Zijlstra wrote:
> > Subject: nfs: fix congestion control
> > 
> > The current NFS client congestion logic is severely broken, it marks the
> > backing device congested during each nfs_writepages() call and implements
> > its own waitqueue.
> > 
> > Replace this by a more regular congestion implementation that puts a cap
> > on the number of active writeback pages and uses the bdi congestion waitqueue.
> > 
> > NFSv[34] commit pages are allowed to go unchecked as long as we are under 
> > the dirty page limit and not in direct reclaim.

> 
> What on earth is the point of adding congestion control to COMMIT?
> Strongly NACKed.

They are dirty pages, how are we getting rid of them when we reached the
dirty limit?

> Why 16MB of on-the-wire data? Why not 32, or 128, or ...

Andrew always promotes a fixed number for congestion control, I pulled
one from a dark place. I have no problem with a more dynamic solution.

> Solaris already allows you to send 2MB of write data in a single RPC
> request, and the RPC engine has for some time allowed you to tune the
> number of simultaneous RPC requests you have on the wire: Chuck has
> already shown that read/write performance is greatly improved by upping
> that value to 64 or more in the case of RPC over TCP. Why are we then
> suddenly telling people that they are limited to 8 simultaneous writes?

min(max RPC size * max concurrent RPC reqs, dirty threshold) then?



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