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

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Date:	Wed, 17 Jan 2007 15:29:46 +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 Wed, 2007-01-17 at 08:50 -0500, Trond Myklebust wrote:
> 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.

Ah, OK.

I was thinking that since the server needs to actually sync the page a
commit might be quite expensive (timewise), hence I didn't want to flush
too much, and interleave them with writing out some real pages to
utilise bandwidth.

But if you think I should just bulk commit I can do that.

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