[Vm-dev] re: Reducing the activity of the image

Ben Coman btc at openinworld.com
Sun Aug 2 08:06:16 UTC 2015

On Fri, Mar 6, 2015 at 10:47 PM, Craig Latta <craig at netjam.org> wrote:

> Hi all--
>      Apologies, my newsreader's thread database got trashed, and I
> missed the responses to my previous message until now.
>      John McIntosh writes:
> > Craig so how does using pthread_cond_timedwait affect socket
> > processing?
>      It makes it actually work well. :)  This was the whole point of
> using pthread_cond_timedwait. Please read the manpage at [1]. It waits
> until either a condition is met (hence the "cond") or a timeout elapses.
>      In the Flow virtual machine plugin, I have a
> synchronizedSignalSemaphoreWithIndex function that calls the usual
> signalSemaphoreWithIndex provided by the virtual machine, and also sets
> the activity condition that the relinquish primitive cares about. The
> host threads which service external I/O requests from primitives use
> synchronizedSignalSemaphoreWithIndex when signalling the semaphores on
> which Smalltalk-level code is waiting. This includes not only the
> semaphores for reading and writing sockets, but also those for
> activities with other external resources entirely, like MIDI ports.
>      So you get a generalized scheme which is not tied to the arcana of
> any particular kind of external resource, and it works the same way on
> any platform which supports the POSIX API (which now is all the Unix-ish
> ones). This has seemed the obvious way to go for over ten years now.
>      Until I implemented this scheme, remote messaging throughput (and
> MIDI throughput) was horrible. Believe me, I tried all the other schemes
> that everyone has mentioned in the Squeak community and its descendants
> since 1996, and none of them were anything better than deeply embarrassing.
>      From the Flow plugin, check out flow.c[2], which implements
> synchronizedSignalSemaphoreWithIndex, the activity condition, and the
> relinquish primitive, and ip.c[3] which creates host threads to do
> background work for external resource primitives and uses
> synchronizedSignalSemaphoreWithIndex to coordinate with the
> Smalltalk-level code and the relinquish primitive.
>      It's so frustrating and weird that we're still talking about this
> in 2015.
> > The promise of nanosleep was to wake up if an interrupt arrived say
> > on a socket (Mind I never actually confirmed this the case, complete
> > hearsay...)
>      Right, nanosleep promises this and doesn't deliver on MacOS, so I
> say forget it. pthread_cond_timedwait works as advertised on MacOS and
> Linux (all distros).
>      Eliot writes:
> > +1.  What [John] said.
>      ...except John admitted himself that he hadn't verified his
> suggestion, and you both assumed for some reason that I didn't have the
> same goals in mind.
> > The problem with pthread_cond_timed_wait, or any other merely
> > delaying call...
>      But pthread_cond_timedwait is *not* a "merely delaying call". It
> does exactly what we want (wait until *either* a condition is met or a
> timeout elapses), and it actually works, and the code is the same across
> POSIX platforms.
>      What you go on to say is based on a false premise.
> > ...is that, unless all file descriptors have been set up to send
> > signals on read/writability and unless the blocking call is
> > interruptible, the call may block for as long as it is asked, not
> > until that or the read/writeability of the file descriptor.
>      In the scheme I described above, we can do what we need without
> using formal Unix signals at all (happily avoiding that whole can of
> worms). The notion of interruptible blocking calls is a red herring
> generally. All the blocking calls in Flow happen in host threads which
> are decoupled from any function call a Smalltalk primitive would make.
> > IMO a better solution here is to a) use epoll or its equivalent
> > kqueue; these are like select but the state of which selectors to
> > examine is kept in kernel space, so the set-up overhead is vastly
> > reduced, and b) wait for no longer than the next scheduled delay if
> > one is in progres.
>      I claim they are not better solutions, because they don't work for
> all kinds of external resources (e.g., MIDI ports). Also, I found that
> "waiting for no longer than the next scheduled delay" is often still far
> too long, when there is external resource activity before that time comes.
> > Of course, the VM can do both of these things, and then there's no
> > need for a background [Smalltalk] process at all.  Instead, when the
> > VM scheduler finds there's nothing to run it calls epoll or kqueue
> > with either an infinite timeout (if no delay is in progress) or the
> > time until the next delay expiration.
>      This would still leave us with poor performance when using new
> kinds of external resources that don't use selectors. (That is, the
> external resource access would perform poorly; I'm sure the main virtual
> machine would scream right along, blissfully oblivious to it all. :)
> > It strikes me that the VM can have a flag that makes it behave like
> > this so that e.g. some time in the Spur release cycle we can set the
> > flag, nuke the background process and get on with our lives.
>      If the only external resources in our lives were selector-using
> ones, I might agree.
>      thanks,
> -C
> [1] http://linux.die.net/man/3/pthread_cond_timedwait
> [2] https://github.com/ccrraaiigg/flow/blob/master/flow.c
> [3] https://github.com/ccrraaiigg/flow/blob/master/ip.c
(Sorry for this late response. I discovered it sitting in my Draft folder.)

Finding this an interesting topic, I googled around to learn more and
bumped into a few things maybe of random interest for some.

* Condition variables performance of boost, Win32, and the C++11 standard

* pthread_cond_timedwait behaving differently on different platforms

* pthread-win32 pthread_cond_timedwait is SLOW?

* Fast Event Processing in SDL (since Pharo is getting SDL)

cheers -ben
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