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    SubjectRe: read() on relayfs channel returns premature 0
    Kingsley Cheung writes:
    > On Wed, Mar 23, 2005 at 09:29:12AM -0600, Tom Zanussi wrote:
    > > writes:
    > > >
    > > > Now I understand that this is not the latest release of relayfs (there
    > > > are the redux patches, which I have yet to try). Nonetheless I'd like


    > I've tested it on 2.6.10 with the pagg and relayfs patches from
    > and
    > read() gives me a zero still once about a page of data has been read.

    Thanks - I've tried it out and haven't immediately been able to
    reproduce the problem yet - I'll do some more pounding on it though
    when I get the chance.

    BTW, I just want to point out that there aren't any problems with
    read() in the version of relayfs included in the -mm tree (i.e. the
    'redux' version), since of course it doesn't support read().

    I went ahead and did a quick port of your stuff to the new version of
    relayfs, which you can find here:

    There's a README in the tarball with some notes on building and
    running it. It includes both the kernel and user-side code, which makes
    use of the relay-apps code and documentation found here (I've included
    the necessary files in the RelayfsModule-new tarball so you don't have
    to actually get this):

    Hopefully the new version will still be useful for what you're trying
    to do, but it does differ in a couple important ways from the old
    version, and points up the fact that the new relayfs really is now
    much more specialized for high-volume applications -

    - your old version used 'packet' mode with read(). The new relayfs
    only supports 'bulk' mode with mmap(), which means it's not really
    useful for notification of single events. I'm not sure how important
    that is for your application - if you're mainly interested in
    collecting data, you can certainly use it even for low-volume
    applications, but the events will be sent to user space in 'blocks'.
    You can modify the user space app to process blocks of events as they
    come in, and play around with buffer sizes to get them more often, but
    it's not quite the same thing.

    - your old version used a single buffer, while the new relayfs only
    supports per-cpu buffers, which means you'd have to sort out the
    events in user space and impose some ordering using timestamps for
    instance if your data doesn't already have a natural ordering (BTW,
    the new relayfs doesn't have an option any longer to do automatic
    timestamping either).

    I'll continue maintaining the old relayfs for existing users (so
    thanks for the patch and the test code) so if the new version doesn't
    fit your needs, you'll still have the old version to fall back on.



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