From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with SMTP id BF1176B0047 for ; Tue, 23 Feb 2010 23:18:26 -0500 (EST) Date: Wed, 24 Feb 2010 12:18:22 +0800 From: Wu Fengguang Subject: Re: [RFC] nfs: use 2*rsize readahead size Message-ID: <20100224041822.GB27459@localhost> References: <20100224024100.GA17048@localhost> <20100224032934.GF16175@discord.disaster> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100224032934.GF16175@discord.disaster> Sender: owner-linux-mm@kvack.org To: Dave Chinner Cc: Trond Myklebust , "linux-nfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , Linux Memory Management List , LKML List-ID: On Wed, Feb 24, 2010 at 11:29:34AM +0800, Dave Chinner wrote: > On Wed, Feb 24, 2010 at 10:41:01AM +0800, Wu Fengguang wrote: > > With default rsize=512k and NFS_MAX_READAHEAD=15, the current NFS > > readahead size 512k*15=7680k is too large than necessary for typical > > clients. > > > > On a e1000e--e1000e connection, I got the following numbers > > > > readahead size throughput > > 16k 35.5 MB/s > > 32k 54.3 MB/s > > 64k 64.1 MB/s > > 128k 70.5 MB/s > > 256k 74.6 MB/s > > rsize ==> 512k 77.4 MB/s > > 1024k 85.5 MB/s > > 2048k 86.8 MB/s > > 4096k 87.9 MB/s > > 8192k 89.0 MB/s > > 16384k 87.7 MB/s > > > > So it seems that readahead_size=2*rsize (ie. keep two RPC requests in flight) > > can already get near full NFS bandwidth. > > > > The test script is: > > > > #!/bin/sh > > > > file=/mnt/sparse > > BDI=0:15 > > > > for rasize in 16 32 64 128 256 512 1024 2048 4096 8192 16384 > > do > > echo 3 > /proc/sys/vm/drop_caches > > echo $rasize > /sys/devices/virtual/bdi/$BDI/read_ahead_kb > > echo readahead_size=${rasize}k > > dd if=$file of=/dev/null bs=4k count=1024000 > > done > > That's doing a cached read out of the server cache, right? You It does not involve disk IO at least. (The sparse file dataset is larger than server cache.) > might find the results are different if the server has to read the > file from disk. I would expect reads from the server cache not > to require much readahead as there is no IO latency on the server > side for the readahead to hide.... Sure the result will be different when disk IO is involved. In this case I would expect the server admin to setup the optimal readahead size for the disk(s). It sounds silly to have client_readahead_size > server_readahead_size Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org