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Subject[RFC] nfs: use 4*rsize readahead size
On Wed, Feb 24, 2010 at 03:39:40PM +0800, Dave Chinner wrote:
> On Wed, Feb 24, 2010 at 02:12:47PM +0800, Wu Fengguang wrote:
> > On Wed, Feb 24, 2010 at 01:22:15PM +0800, Dave Chinner wrote:
> > > What I'm trying to say is that while I agree with your premise that
> > > a 7.8MB readahead window is probably far larger than was ever
> > > intended, I disagree with your methodology and environment for
> > > selecting a better default value. The default readahead value needs
> > > to work well in as many situations as possible, not just in perfect
> > > 1:1 client/server environment.
> >
> > Good points. It's imprudent to change a default value based on one
> > single benchmark. Need to collect more data, which may take time..
>
> Agreed - better to spend time now to get it right...

I collected more data with large network latency as well as rsize=32k,
and updates the readahead size accordingly to 4*rsize.

===
nfs: use 2*rsize readahead size

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
(this reads sparse file from server and involves no disk IO)

readahead size normal 1ms+1ms 5ms+5ms 10ms+10ms(*)
16k 35.5 MB/s 4.8 MB/s 2.1 MB/s 1.2 MB/s
32k 54.3 MB/s 6.7 MB/s 3.6 MB/s 2.3 MB/s
64k 64.1 MB/s 12.6 MB/s 6.5 MB/s 4.7 MB/s
128k 70.5 MB/s 20.1 MB/s 11.9 MB/s 8.7 MB/s
256k 74.6 MB/s 38.6 MB/s 21.3 MB/s 15.0 MB/s
rsize ==> 512k 77.4 MB/s 59.4 MB/s 39.8 MB/s 25.5 MB/s
1024k 85.5 MB/s 77.9 MB/s 65.7 MB/s 43.0 MB/s
2048k 86.8 MB/s 81.5 MB/s 84.1 MB/s 59.7 MB/s
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
4096k 87.9 MB/s 77.4 MB/s 56.2 MB/s 59.2 MB/s
8192k 89.0 MB/s 81.2 MB/s 78.0 MB/s 41.2 MB/s
16384k 87.7 MB/s 85.8 MB/s 62.0 MB/s 56.5 MB/s
readahead size normal 1ms+1ms 5ms+5ms 10ms+10ms(*)
16k 37.2 MB/s 6.4 MB/s 2.1 MB/s 1.2 MB/s
rsize ==> 32k 56.6 MB/s 6.8 MB/s 3.6 MB/s 2.3 MB/s
64k 66.1 MB/s 12.7 MB/s 6.6 MB/s 4.7 MB/s
128k 69.3 MB/s 22.0 MB/s 12.2 MB/s 8.9 MB/s
256k 69.6 MB/s 41.8 MB/s 20.7 MB/s 14.7 MB/s
512k 71.3 MB/s 54.1 MB/s 25.0 MB/s 16.9 MB/s
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1024k 71.5 MB/s 48.4 MB/s 26.0 MB/s 16.7 MB/s
2048k 71.7 MB/s 53.2 MB/s 25.3 MB/s 17.6 MB/s
4096k 71.5 MB/s 50.4 MB/s 25.7 MB/s 17.1 MB/s
8192k 71.1 MB/s 52.3 MB/s 26.3 MB/s 16.9 MB/s
16384k 70.2 MB/s 56.6 MB/s 27.0 MB/s 16.8 MB/s
(*) 10ms+10ms means to add delay on both client & server sides with
# /sbin/tc qdisc change dev eth0 root netem delay 10ms
The total >=20ms delay is so large for NFS, that a simple `vi some.sh`
command takes a dozen seconds. Note that the actual delay reported
by ping is larger, eg. for the 1ms+1ms case:
rtt min/avg/max/mdev = 7.361/8.325/9.710/0.837 ms


So it seems that readahead_size=4*rsize (ie. keep 4 RPC requests in
flight) is able to get near full NFS bandwidth. Reducing the mulriple
from 15 to 4 not only makes the client side readahead size more sane
(2MB by default), but also reduces the disorderness of the server side
RPC read requests, which yeilds better server side readahead behavior.

To avoid small readahead when the client mount with "-o rsize=32k" or
the server only supports rsize <= 32k, we take the max of 2*rsize and
default_backing_dev_info.ra_pages. The latter defaults to 512K, and can
be explicitly changed by user with kernel parameter "readahead=" and
runtime tunable "/sys/devices/virtual/bdi/default/read_ahead_kb" (which
takes effective for future NFS mounts).

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
CC: Dave Chinner <david@fromorbit.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/nfs/client.c | 4 +++-
fs/nfs/internal.h | 8 --------
2 files changed, 3 insertions(+), 9 deletions(-)
--- linux.orig/fs/nfs/client.c 2010-02-26 10:10:46.000000000 +0800
+++ linux/fs/nfs/client.c 2010-02-26 11:07:22.000000000 +0800
@@ -889,7 +889,9 @@ static void nfs_server_set_fsinfo(struct
server->rpages = (server->rsize + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;

server->backing_dev_info.name = "nfs";
- 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,
+ 4 * server->rpages);
server->backing_dev_info.capabilities |= BDI_CAP_ACCT_UNSTABLE;

if (server->wsize > max_rpc_payload)
--- linux.orig/fs/nfs/internal.h 2010-02-26 10:10:46.000000000 +0800
+++ linux/fs/nfs/internal.h 2010-02-26 11:07:07.000000000 +0800
@@ -10,14 +10,6 @@

struct nfs_string;

-/* Maximum number of readahead requests
- * FIXME: this should really be a sysctl so that users may tune it to suit
- * their needs. People that do NFS over a slow network, might for
- * instance want to reduce it to something closer to 1 for improved
- * interactive response.
- */
-#define NFS_MAX_READAHEAD (RPC_DEF_SLOT_TABLE - 1)
-
/*
* Determine if sessions are in use.
*/

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