linux-kernel - Re: [RFC] nfs: use 2*rsize readahead size

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Date:	Wed, 24 Feb 2010 18:39:40 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Trond Myklebust <Trond.Myklebust@...app.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] nfs: use 2*rsize readahead size

On Wed, Feb 24, 2010 at 02:12:47PM +0800, Wu Fengguang wrote:
> On Wed, Feb 24, 2010 at 01:22:15PM +0800, Dave Chinner wrote:
> > What I'm trying to say is that while I agree with your premise that
> > a 7.8MB readahead window is probably far larger than was ever
> > intended, I disagree with your methodology and environment for
> > selecting a better default value.  The default readahead value needs
> > to work well in as many situations as possible, not just in perfect
> > 1:1 client/server environment.
> 
> Good points. It's imprudent to change a default value based on one
> single benchmark. Need to collect more data, which may take time..

Agreed - better to spend time now to get it right...

> > > It sounds silly to have
> > > 
> > >         client_readahead_size > server_readahead_size
> > 
> > I don't think it is  - the client readahead has to take into account
> > the network latency as well as the server latency. e.g. a network
> > with a high bandwidth but high latency is going to need much more
> > client side readahead than a high bandwidth, low latency network to
> > get the same throughput. Hence it is not uncommon to see larger
> > readahead windows on network clients than for local disk access.
> 
> Hmm I wonder if I can simulate a high-bandwidth high-latency network
> with e1000's RxIntDelay/TxIntDelay parameters..

I think netem is the blessed method of emulating different network
behaviours. There's a howto+faq for setting it up here:

http://www.linuxfoundation.org/collaborate/workgroups/networking/netem

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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