Re: Network Stack Locking

From: Matthew Dillon (dillon_at_apollo.backplane.com)
Date: 05/25/04

  • Next message: Luigi Rizzo: "Re: Network Stack Locking" 
    Date: Tue, 25 May 2004 11:29:45 -0700 (PDT)
To: Alan Cox <alc@cs.rice.edu>

    :>:Sounds a lot like a lot of the Mach IPC optimizations, including their use
    :>:of continuations during IPC to avoid a full context switch.
    :>:
    :>:Robert N M Watson FreeBSD Core Team, TrustedBSD Projects
    :>:robert@fledge.watson.org Senior Research Scientist, McAfee Research
    :>
    :> Well, I like the performance aspects of a continuation mechanism, but
    :> I really dislike the memory overhead. Even a minimal stack is
    :> expensive when you multiply it by potentially hundreds of thousands
    :> of 'blocking' entities such as PCBs.. say, a TCP output stream.
    :> Because of this the overhead and cache pollution generated by the
    :> continuation mechanism increases as system load increases rather
    :> then decreases.
    :
    :When the explicit continuation mechanism was used, the thread's stack
    :was freed when the thread blocked and a new stack allocated when the
    :thread was restarted. Here is a URL:
    :http://citeseer.ist.psu.edu/draves91using.html. Notice the mention of
    :space reduction in the abstract. It's worth reading.
    :
    :Alan

        Ah, now I understand. The continuation is an exit from the procedure.

        This sounds very similar to an interrupt mechanism I designed about
        a decade ago. Instead of saving and restoring the interrupt
        context for the interrupt thread, the thread switch was special-cased
        to basically create a context (by pushing a procedure call on the stack)
        on switch-in and to throw it away on switch-out (which was always at
        the end of the interrupt routine). I extended the same mechanism down
        into 'userland' by creating a 'waitforever()' system call which basically
        did nothing but wait for and dispatch signal vectors (most of the programs
        were event oriented and had no main loop). The nice thing about this was
        that no context had to be saved while the program was sitting in
        waitforever(), or restored when the program returned from a signal handler.
        This more then doubled scheduler performance (which on a 10MHz 68000 was
        important).

        In many ways, the continuation mechanism and the message queue mechanism
        appear to be nearly identical. If an explicit exit from a procedure
        is required to optimize the stack with the continuation mechanism, then
        that isn't much different then moving the message to an event queue
        and returning to the message processing loop. Neither case allows
        you to save stack context or to save the current procedural stacking
        level, and both mechanisms allow you to reuse your current stack to
        handle multiple messages/continuations.

                                            -Matt
                                            Matthew Dillon
                                            <dillon@backplane.com>
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