linux-kernel - Re: [PATCH] improve the performance of large sequential write NFS workloads

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Message-Id: <1261171211.1947.135.camel@serenity>
Date:	Fri, 18 Dec 2009 16:20:11 -0500
From:	Steve Rago <sar@...-labs.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, Trond.Myklebust@...app.com,
	Wu Fengguang <fengguang.wu@...el.com>,
	"jens.axboe" <jens.axboe@...cle.com>
Subject: Re: [PATCH] improve the performance of large sequential write NFS
	workloads


On Fri, 2009-12-18 at 20:41 +0100, Ingo Molnar wrote:
> * Steve Rago <sar@...-labs.com> wrote:
> 
> > > Also, I don't think this needs to have a sysctl, it should just work.
> > 
> > The sysctl is a *good thing* in that it allows the eager writeback behavior 
> > to be tuned and shut off if need be.  I can only test the changes on a 
> > finite set of systems, so better safe than sorry.
> 
> This issue has been settled many years ago and that's not what we do in the 
> Linux kernel. We prefer patches to core code where we are reasonably sure they 
> result in good behavior - and then we fix bugs in the new behavior, if any. 
> 
> (Otherwise odd sysctls would mushroom quickly and the system would become 
> untestable in practice.)
> 
> 	Ingo

I don't disagree, but "that's not what we do" hardly provides insight
into making the judgment call.  In this case, the variety of
combinations of NFS server speed, NFS client speed, transmission link
speed, client memory size, and server memory size argues for a tunable
parameter, because one value probably won't work well in all
combinations.  Making it change dynamically based on these parameters is
more complicated than these circumstances call for, IMHO.

Steve


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