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    SubjectRe: [PATCH 8/25] msi: Simplify the msi irq limit policy.
    Roland Dreier <> writes:

    > > Only the s2io driver even takes advantage of this feature
    > > all other drivers have a fixed number of irqs they need and
    > > bail if they can't get them.
    > My todo list for the mthca (InfiniBand HCA) driver includes adding
    > support for more event queues. When I do that, I'll likely want to
    > try to get something on the order of number_of_cpus plus two or three
    > MSI-X vectors, and fall back to a lower number of vectors if that
    > allocation fails.

    Allowing drivers that can take advantage of large numbers of
    irqs is part of this patch. To be clear the method still supports
    allocate a bunch of irqs and then falling back.

    We have to kinds of drivers. Those that allocate a lot of irqs and
    allocate fewer if they can't get them, and those just allocate a
    couple and fail if they can't get them.

    The policy I deleted tries to be fair, and give all drivers the
    same number of irqs. What I left us with is a simple first
    come first serve policy.

    To do better you need an accurate count and you need to separate
    out the various kinds of drivers, and you need a shortage of

    Currently x86_64 hardware supports up to 244*NR_CPUS irqs. I
    don't expect we will exceed that limit any time soon even with
    a first come first serve policy.

    The worst case I can think of with your proposed irq allocation
    policy in the mthca driver is a 128 cpu machine with 128 IB cards
    in it. That would just barely fail with every driver allocating
    3 IRQs per cpu, and it could be trivially fixed by putting in
    dual core cpus :)

    So when we exceed the limit on the number of IRQs we actually
    have then it probably makes sense to see if a policy more
    aggressive than first come first serve makes sense. But until
    then it is a waste of time and we should be concentrating
    our efforts on making more IRQs usable.

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