Date: Thu, 31 Mar 2005 09:30:17 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Andrew Morton <akpm@osdl.org>
Cc: trond.myklebust@fys.uio.no, rlrevell@joe-job.com,
       linux-kernel@vger.kernel.org
Subject: Re: NFS client latencies
Message-ID: <20050331073017.GA16577@elte.hu>
References: <1112192778.17365.2.camel@mindpipe> <1112194256.10634.35.camel@lade.trondhjem.org> <20050330115640.0bc38d01.akpm@osdl.org> <1112217299.10771.3.camel@lade.trondhjem.org> <1112236017.26732.4.camel@mindpipe> <20050330183957.2468dc21.akpm@osdl.org> <1112237239.26732.8.camel@mindpipe> <1112240918.10975.4.camel@lade.trondhjem.org> <20050331065942.GA14952@elte.hu> <20050330231801.129b0715.akpm@osdl.org>
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* Andrew Morton <akpm@osdl.org> wrote:

> Well.  The radix-tree approach's best-case is probably quite a lot 
> worse than the list-based approach's best-case.  It hits a lot more 
> cachelines and involves a lot more code.

The list-based approach's best-case are large continuous append writes.  
No sorting overhead, and light data structures.

i'd say this workload should be not that bad under the radix tree either 
- the gang lookup stuffs a nice vector of 16 pages into an array.

we definitely can say nothing based on the observation that a _single_ 
page took 1.9 msecs in Lee's previous measurement, while 7700 pages now 
take 6 msecs to process.

> But of course the radix-tree's worst-case will be far better than 
> list's.

the generic VM/pagecache has proven that the radix tree wins hands down 
for alot more workloads than the worst-case.

> And presumably that list-based code rarely hits the worst-case, else 
> it would have been changed by now.

that was my other point in a previous mail: most write benchmarks do 
continuous append writes, and CPU overhead easily gets lost in network 
latency.

Also, considering that a good portion of the NFS client's code is still 
running under the BKL one would assume if the BKL hurts performance it 
would have been changed by now? ;-)

	Ingo
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