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

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Feb 24, 2010 at 12:24:14PM +0800, Dave Chinner wrote:
> On Wed, Feb 24, 2010 at 02:29:34PM +1100, 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
> > 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....
> 
> FWIW, if you mount the client with "-o rsize=32k" or the server only
> supports rsize <= 32k then this will probably hurt throughput a lot
> because then readahead will be capped at 64k instead of 480k....

That's why I take the max of 2*rsize and system default readahead size
(which will be enlarged to 512K):

-       server->backing_dev_info.ra_pages = server->rpages * NFS_MAX_READAHEAD;
+       server->backing_dev_info.ra_pages = max_t(unsigned long,
+                                             default_backing_dev_info.ra_pages,
+                                             2 * server->rpages);

Thanks,
Fengguang