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

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

On Wed, Feb 24, 2010 at 07:18:26PM +0800, Akshat Aranya wrote:
> On Wed, Feb 24, 2010 at 12:22 AM, Dave Chinner <david@fromorbit.com> 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 <http://lkml.org/lkml/2010/2/23/376>, 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.

Thanks,
Fengguang