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    SubjectRe: questions on NAPI processing latency and dropped network packets
    Ray Lee wrote:
    > On Jan 10, 2008 9:24 AM, Chris Friesen <cfriesen@nortel.com> wrote:

    >>After a recent userspace app change, we've started seeing packets being
    >>dropped by the ethernet hardware (e1000, NAPI is enabled). The
    >>error/dropped/fifo counts are going up in ethtool:

    > Can you reproduce it with a simple userspace cpu hog? (Two, really,
    > one per cpu.)
    > Can you reproduce it with the newer e1000?

    Hmm...good questions and I haven't checked either. The first one is
    relatively straightforward. The second is a bit trickier...last time I
    tried the latest e1000 driver the card wouldn't boot (we use netboot).

    > Can you reproduce it with git head?

    Unfortunately, I don't think I'll be able to try this. We require
    kernel mods for our userspace to run, and I doubt I'd be able to get the
    time to port all the changes forward to git head.

    > If the answer to the first one is yes, the last no, then bisect until
    > you get a kernel that doesn't show the problem. Backport the fix,
    > unless the fix happens to be CFS. However, I suspect that your
    > userpace app is just starving the system from time to time.

    It's conceivable that userspace is starving the kernel, but we have do
    about 45% idle on one cpu, and 7-10% idle on the other.

    We also have an odd situation where on an initial test run after bootup
    we have 18-24% idle on cpu1, but resetting the test tool drops that to
    the 7-10% I mentioned above.

    Based on profiling and instrumentation it seems like the cost of
    sctp_endpoint_lookup_assoc() more than triples, which means that the
    amount of time that bottom halves are disabled in that function also
    triples.


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