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

[Dave Chinner <david@fromorbit.com>]

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
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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com