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    SubjectRe: [ofa-general] Re: [PATCH RFC] RDMA/CMA: Allocate PS_TCPportsfrom the host TCP port space.
    Felix Marti wrote:
    > Yes, the app will take the cache hits when accessing the data. However,
    > the fact remains that if there is a copy in the receive path, you
    > require and additional 3x memory BW (which is very significant at these
    > high rates and most likely the bottleneck for most current systems)...
    > and somebody always has to take the cache miss be it the copy_to_user or
    > the app.

    The cache miss is going to cost you half the memory bandwidth of a full
    copy. If the data is already in cache, then the copy is cheaper.

    However, removing the copy removes the kernel from the picture on the
    receive side, so you lose demultiplexing, asynchronism, security,
    accounting, flow-control, swapping, etc. If it's ok with you to not use
    the kernel stack, then why expect to fit in the existing infrastructure
    anyway ?

    > Yes, RDMA support is there... but we could make it better and easier to

    What do you need from the kernel for RDMA support beyond HW drivers ? A
    fast way to pin and translate user memory (ie registration). That is
    pretty much the sandbox that David referred to.

    Eventually, it would be useful to be able to track the VM space to
    implement a registration cache instead of using ugly hacks in user-space
    to hijack malloc, but this is completely independent from the net stack.

    > use. We have a problem today with port sharing and there was a proposal

    The port spaces are either totally separate and there is no issue, or
    completely identical and you should then run your connection manager in
    user-space or fix your middlewares.

    > and not for technical reasons. I believe this email threads shows in
    > detail how RDMA (a network technology) is treated as bastard child by
    > the network folks, well at least by one of them.

    I don't think it's fair. This thread actually show how pushy some RDMA
    folks are about not acknowledging that the current infrastructure is
    here for a reason, and about mistaking zero-copy and RDMA.

    This is a similar argument than the TOE discussion, and it was
    definitively a good decision to not mess up the Linux stack with TOEs.

    Patrick
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