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    SubjectRe: CONFIG_PREEMPT and server workloads
    On Thu, Mar 18, 2004 at 10:10:06PM -0800, Andrew Morton wrote:
    > The worst-case latency is during umount, fs/inode.c:invalidate_list() when
    > the filesystem has a zillion inodes in icache. Measured 250 milliseconds
    > on a 256MB 2.7GHz P4 here. OK, so don't do that.
    >
    > The unavoidable worst case is in the RCU callbacks for dcache shrinkage -
    > I've seen 25 millisecond holdoffs on the above machine during filesystem
    > stresstests when RCU is freeing a huge number of dentries in softirq
    > context.

    What filesystem stresstest was that ?

    >
    > This if Hard To Fix. Dipankar spent quite some time looking into it and
    > had patches, but I lost track of where they're at.

    And I am still working on this on a larger scope/scale. Yes, I have
    a patch that hands over the rcu callbacks to a per-cpu kernel thread
    reducing the softirq time. However this is not really a solution to
    the overall problem, IMO. I am collecting some instrumentation
    data to understand softirq/rcu behavior during heavy loads and
    ways to counter long running softirqs.

    Latency isn't the only issue. DoS on route cache is another
    issue that needs to be addressed. I have been experimenting
    with Robert Olsson's router test and should have some more results
    out soon.

    Thanks
    Dipankar
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