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

[Wu Fengguang]

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
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