[lkml]   [2010]   [Feb]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC] nfs: use 2*rsize readahead size
On Wed, Feb 24, 2010 at 07:18:26PM +0800, Akshat Aranya wrote:
> On Wed, Feb 24, 2010 at 12:22 AM, Dave Chinner <> wrote:
> >
> >> It sounds silly to have
> >>
> >>         client_readahead_size > server_readahead_size
> >
> > I don't think it is  - the client readahead has to take into account
> > the network latency as well as the server latency. e.g. a network
> > with a high bandwidth but high latency is going to need much more
> > client side readahead than a high bandwidth, low latency network to
> > get the same throughput. Hence it is not uncommon to see larger
> > readahead windows on network clients than for local disk access.
> >
> > Also, the NFS server may not even be able to detect sequential IO
> > patterns because of the combined access patterns from the clients,
> > and so the only effective readahead might be what the clients
> > issue....
> >
> In my experiments, I have observed that the server-side readahead
> shuts off rather quickly even with a single client because the client
> readahead causes multiple pending read RPCs on the server which are
> then serviced in random order and the pattern observed by the
> underlying file system is non-sequential. In our file system, we had
> to override what the VFS thought was a random workload and continue to
> do readahead anyway.

What's the server side kernel version, plus client/server side
readahead size? I'd expect the context readahead to handle it well.

With the patchset in <>, you can
actually see the readahead details:

# echo 1 > /debug/tracing/events/readahead/enable
# cp test-file /dev/null
# cat /debug/tracing/trace # trimmed output
readahead-initial(dev=0:15, ino=100177, req=0+2, ra=0+4-2, async=0) = 4
readahead-subsequent(dev=0:15, ino=100177, req=2+2, ra=4+8-8, async=1) = 8
readahead-subsequent(dev=0:15, ino=100177, req=4+2, ra=12+16-16, async=1) = 16
readahead-subsequent(dev=0:15, ino=100177, req=12+2, ra=28+32-32, async=1) = 32
readahead-subsequent(dev=0:15, ino=100177, req=28+2, ra=60+60-60, async=1) = 24
readahead-subsequent(dev=0:15, ino=100177, req=60+2, ra=120+60-60, async=1) = 0

And I've actually verified the NFS case with the help of such traces
long ago. When client_readahead_size <= server_readahead_size, the
readahead requests may look a bit random at first, and then will
quickly turn into a perfect series of sequential context readaheads.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2010-02-25 13:41    [W:0.152 / U:0.804 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site