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@xxxxxxxxxxxxx
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