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    SubjectRe: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3
    On 2/21/07, Ingo Molnar <> wrote:
    > threadlets (and syslets) are parallel contexts and they behave so -
    > queuing and execution semantics are then ontop of that, implemented
    > either by glibc, or implemented by the application. There is no
    > 'pipeline' of requests imposed - the structure of pending requests is
    > totally free-form. For example in threadlet-test.c i've in essence
    > implemented a 'set of requests' with the submission site only interested
    > in whether all requests are done or not - but any stricter (or even
    > looser) semantics and ordering can be used too.

    In short, you have a dataflow model with infinite parallelism,
    implemented using threads of control mapped willy-nilly onto the
    underlying hardware. This has not proven to be a successful model in
    the past.

    > in terms of AIO, the best queueing model is i think what the kernel uses
    > internally: freely ordered, with barrier support. (That is equivalent to
    > a "queue of sets", where the queue are the barriers, and the sets are
    > the requests within barriers. If there is no barrier pending then
    > there's just one large freely-ordered set of requests.)

    That's a big part of why Linux scales poorly for workloads that
    involve a large volume of in-flight I/O transactions. Unless you
    essentially lock one application thread to each CPU core, with a
    complete understanding of its cache sharing and latency relationships
    to all the other cores, and do your own userspace I/O scheduling and
    dispatching state machine -- which is what all industrial-strength
    databases and other sorts of transaction engines currently do -- you
    get the same old best-effort context-thrashing scheduler we've had
    since Solaris 2.0.

    Let's do something genuinely better this time, OK?

    - Michael
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