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From: Daniel Miller (daniel.miller_at_[hidden])
Date: 2002-06-03 11:08:33

William E. Kempf wrote:

> I've got a very focused plan for Boost.Threads, which I have
> to have to get the library into a form that's usable and a good basis.
> There's some things that are missing for RTOSes that I'm aware of,
and those
> *ARE* planned today. Of course this may not be obvious since they mostly
> fall under the category I've labeled as "thread parameters" when I've
> about this on public lists. Beyond this there are only two other things
> brought to my attention that is need by RTOSes, and they are at least
> somewhat debatable:
> * Semaphores. As I've said publicly, I've not counted semaphores out
> entirely. So this criticism, though possibly valid, is not evidence that
> Boost.Threads will (or has) made the wrong decision in this regard,
> RTOS/embedded systems or no.
> * Event queues. I believe this concept to be higher level then the
scope of
> Boost.Threads. I also believe it to not be a thread specific concept (I
> know others disagree about this, so there may well be something I'm not
> understanding here). Despite this I think it an important concept,
and one
> that probably should be standardized. So I don't think it's exclusion
> should rule out Boost.Threads, RTOS/embedded systems or no.

    Ahhh, we are making some progress regarding realtime embedded systems
education. But this progress is not yet complete.

    The 2 categories which you mention above are actually 3 separate &
distinct categories in the multithreaded RTOS world (e.g., pSOS, VxWorks).

    1) semaphores (same as your semaphore category above)

    2) nonlossy/queued event delivery (of which POSIX realtime signals is
one presentation, as is pSOS events). Here delivery is via an
asynchronous call-back function invocation. For more information
regarding POSIX realtime signals, see:

    3) FIFO message-queues (of which POSIX System-V ioctl-based message
queues & ACE message-queues are distant overly-embellished cousins and
best ignored for the purpose of this discussion) (of which pSOS
message-queues & VxWorks message-queues are the best examples, except
that instead of having a 4-byte payload, payloads of arbitrary types in
C++ for which an efficient copy-ctor has been defined would be
preferable). Often for expository purposes, I refer to this
time-honored 2-to-3-decade-old idiom as (multi)producer-(multi)consumer,
emphasizing the fact that M producer threads can post/push to a FIFO
message-queue and N consumer threads can pend/pop from that
message-queue. The message-queue shall not be limited to being coupled
to any one producer thread. The message-queue shall not be limited to
being coupled to any one consumer thread. The one or more consumer
threads which pend on the message-queue do form an implicit thread-pool
whose threads are scheduled by the kernel instead of in user-space.
Here the delivery is via explicit retrieval from an interthread FIFO
queue at one or more well-known points in the control-flow.

    For the written record:
    Contrary to the mistaken belief that message-queues are applicable to
single-threaded applications, the message-queues in #3 are thoroughly
inappropriate for single-threaded applications due to their blocking
nature. If the sole thread acts as a consumer by pending on an empty
message-queue in a truly single-threaded application, there is no
producer to post/push into that queue, causing a permanent starvation.

    What is applicable to single-threaded software is not the RTOS-style
MT FIFO message-queue, but rather std::queue. The std::queue already
has all of the semantics which single-threaded applications would need:
queuing without thread-suspension. On the other hand, the RTOS-style MT
FIFO message-queue has a FIFO queuing behavior like std::queue but it
has thread-suspension which is at best to be avoided in single-threaded
software (e.g., an organization's local coding-conventions rule which
states that only the try-series member-functions are to be invoked for
single-threaded software) and at worst dangerous (lest the permanent
starvation case described above occurs).


    truly single-threaded: Software is "truly single-threaded" if and
only if the embodiment of the software is comprised solely of one thread
in one address-space.
    NOTE: Multiple single-threaded UNIX processes with shared-memory
between them and/or with interprocess thread synchronization between
them specifically do *NOT* satisfy this definition. Such processes are
a subcategory of multithreaded software requiring
interthread-*inter*addressspace analogous forms of the
more-frequently-encountered interthread-*intra*addressspace
thread-synchronization mechanisms.

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