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    SubjectRe: [ANN] Userspace M-on-N threading model implementation. Alpha release.
    On Sun, 4 Feb 2007, Jakub Jelinek wrote:

    > On Sun, Feb 04, 2007 at 03:12:32PM -0500, Bill Davidsen wrote:
    > > Arjan van de Ven wrote:
    > > >>Because user threading can avoid context switches, there will always be
    > > >>cases where it will outperform o/s threads for hardware reasons.
    > > >
    > > >actually.. switching from one "real" thread to another in Linux is not
    > > >an actual context switch in the hardware sense... at least this part of
    > > >your argument seems to be incorrect ;)
    > > >
    > > How does that work? Switching between kernel threads requires going into
    > > the kernel, user level thread switches are all done in user mode.
    > >
    > > Do you have some way to change o/s threads w/o going into the kernel?
    >
    > But going into kernel is not very expensive on Linux.
    >
    > On the other side, the overhead you need to add for every single syscall
    > that might block for the M:N threads and the associated complications
    > which make it far harder to conform to POSIX IMHO far outweight the costs
    > of going into the kernel for a context switch.

    Agreed, definitely. A libpcl (using swapcontext(3)) cobench is about 50
    times faster than an context switch measured by lmbench (although I have
    serious doubts about about the ability of lat_ctx to measure it - but
    that's another story) on an Opteron 254. One may say "Wow! Really?!?".
    The point is, who cares. We are talking about differences between
    super-fast (~2us) and ultra-fast (~0.04us).
    The time (and code) that you'll have to drop in the syscall path to handle
    M:N is very likely to make you lose way more of what you gain by avoiding
    an OS context switch (a "soft" context switch you still have to do it).
    Either use N:N (requires locking, but spread over multiple CPUs) or 1:N
    (I/O driven state machines or coroutines - no locking, once-CPU bound).



    - Davide


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