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    SubjectRe: A peek at the future of storage
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    On Wednesday 12 December 2007 09:46, J. Bruce Fields wrote:
    > On Wed, Dec 12, 2007 at 08:46:18AM -0800, Daniel Phillips wrote:
    > > Incidentally, we ran our tests with 128 knfsd threads. The default
    > > of 8 threads produces miserable performance on the SSD, which gave
    > > us a good scare on our initial test run. It would be very nice to
    > > implement an algorithm to scale the knfsd thread pool
    > > automatically, in order to eliminate this class of thing that can
    > > go wrong. If somebody became inspired to take on that little
    > > project that would be great, otherwise it is in our pipeline for,
    > > hmm, Christmas delivery. (Exactly which Christmas is left
    > > unspecified.)
    >
    > People have proposed writing a daemon that just reads
    > /proc/net/rpc/nfsd periodically and uses that to adjust the number of
    > threads from userspace, probably subject to some limits in a config
    > file someplace. (Think that could do the job, or is there some reason
    > this would be easier in the kernel?)

    I didn't actually say "kernel", though that was what I was thinking,
    perhaps just out of habit. It seems to me it would be a relatively
    small change to the existing code, essentially just finishing the idea,
    without needing to be patched up by userspace.

    So how would a userspace daemon know that kernel is blocking and new
    threads are needed? In kernel this is pretty easy: when a new request
    arrives, look on the thread list and if none are available, generate a
    new one. Something special needs to be done to handle the case where
    there are no threads available because they are all piled up on a
    semaphore due to, for example, somebody unplugging the network cable
    for a remote disk. We have to avoid generating infinite threads in
    that case. Ideas?

    Daniel


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