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From "Patrick van Beem" <>
Subject Re: RE : Multi-threading
Date Tue, 16 Dec 2008 12:20:06 GMT
Hello Carl,

> What Axis does well is freeing resources (once we figure out how to set
> everything up right!) so I am a little confused as to where exactly the
> limitations are.  You say the callback system provided is not good in
> terms of freeing resources, but have you tried freeing your resources
> from another function which itself waits for the callback to occur?
> (either error callback or success callback)  I think this is the way
> Axis was designed with as implied by Dimuthu: wait in a loop in your
> main thread while the callbacks are outstanding, do no cleanup in the
> callback itself, let that thread exit completely and after it is done,
> then from your main thread detect that the callback ocurred and do the
> cleanup there.  

Correct. But I think the design is missing one thing. If I allocate the stub and env and then
do an async call, I'm not allowed to free those two resources in the callback, because they're
used by the axis framework. But if I signal the main thread from the callback, to free the
resources, the callback might be switched out directly after this signal, and the main thread
might free the resources before the callback ended and the axis framework used them. As you
indicate, the only safe way is to wait until the thread is finished. But the axis framework
does not provide an api to find out which thread is processing you request. And it shouldn't,
because the thread mechanism is an implementation detail of the axis framework. Future versions
might re-use the thread or even use no threading at all for asynchronous calls. So the only
safe way to free resources is for the axis framework to signal the caller that the resources
are no longer needed. A (second?) callback is the most used (elegant) way to do this. Right
now, the framework does not provide a safe way of freeing resources in async calls.

> My reason for responding though is really to comment on this phrase:
> "Threads are a rather expensive resource to use for just waiting on an
> IO completion".  It may be my lack of understanding, but I am pretty
> sure that -- at least in the win32 tcp/ip stack -- once your thread goes
> into asynchronous communication on a socket, you do not see it again
> until there is some result.  This means if there is a timeout your
> thread is inactive for a long time.  

Correct. So if I've got a couple of hundred outstanding calls, they all consume precious memory.
In our case, this is a lot of memory, since we have a heavy server applications with a greedy
memory allocation strategy per thread (for performance) and a rather large default stacks.
Of course, both can be optimized for the 'just waiting on io-completion'-threads...
CPU-wise, it's no problem.

> How can one thread wait on more
> than one asychronous communication?  I admit this would be a far better
> solution, however from my understanding of winsock2 it is not possible.

With the fd_set in winsock and the select() function, you can wait at a maximum of 64 (current
implementation) sockets at once. With I/O Completion Ports you can use one thread for an infinite
number of ports (though a pool of threads might be a good idea if the number of sockets grows
large). This is also used by the well known boost (C++) library. Mechanisms like these would
be a much better implementation. But I think they don't fit well in the modular (transportation)
design of axis, since they require knowledge about the lower level transportation on a higher
> Seen this way, one thread per socket communication is maybe expensive in
> resources, but it is the only way to ensure your main thread continues
> to operate in a timely fashion.

But prone to explode with a log of async calls. As a 'workaround' I've now my own static-sized
thread pool that perform synchronous calls. If there are more async calls then threads in
the pool, they're queued.

Thank you for your input.


Patrick van Beem
Sr. Software engineer
T +31 (0) 73 691 07 39
F +31 (0) 73 691 07 54
M +31 (0) 06 15 01 65 83

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