Re: Feature request for md5sum

[Pádraig Brady]

On 19/02/10 22:41, Pádraig Brady wrote:
> Note the linux nfs client should be doing adaptive
> read ahead to abstract you from this latency issue.
> Check the rsize nfs setting as that may limit the
> size of read ahead done.

I just noticed the following message which
shows some of the current Linux knobs in this area:

-------- Original Message --------
Subject: [RFC] nfs: use 2*rsize readahead size
Date: Wed, 24 Feb 2010 10:41:01 +0800
From: Wu Fengguang <address@hidden>
To: Trond Myklebust <address@hidden>
CC: address@hidden, address@hidden,     Linux Memory Management List 
<address@hidden>,    LKML <address@hidden>

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