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

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Date:	Wed, 17 Jan 2007 08:50:14 -0500
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] nfs: fix congestion control

On Wed, 2007-01-17 at 09:49 +0100, Peter Zijlstra wrote: 
> > They are certainly _not_ dirty pages. They are pages that have been
> > written to the server but are not yet guaranteed to have hit the disk
> > (they were only written to the server's page cache). We don't care if
> > they are paged in or swapped out on the local client.
> > 
> > \All the COMMIT does, is to ask the server to write the data from its
> > page cache onto disk. Once that has been done, we can release the pages.
> > If the commit fails, then we iterate through the whole writepage()
> > process again. The commit itself does, however, not even look at the
> > page data.
> 
> Thou art correct from an NFS point of view, however for the VM they are
> (still) just dirty pages and we need shed them.
> 
> You talk of swapping them out, they are filecache pages not swapcache
> pages. The writepage() process needs to complete and that entails
> committing them.

My point is that we can and should collect as many of the little buggers
as we can and treat them with ONE commit call. We don't look at the
data, we don't lock the pages, we don't care what the VM is doing with
them. Throttling is not only unnecessary, it is actually a bad idea
since it slows up the rate at which we can free up the pages.

Trond

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