Re: Server - tasking and long lived connections

[Thierry Lelegard <thierry.lelegard@canal-plus.fr>]

We have applications that we successfully tested with up to 6000 tasks.
That was on OpenVMS with the DEC Ada compiler. We now use GNAT on
VMS/UNIX/WNT. We haven't yet made such load testing but several hundredths
of tasks run fine.

On most operating systems, though, this requires some tuning (memory
management, I/O, etc).

I think that the trap in this kind of applications (servers with one
or more tasks per client connection) is the fate of the tasks when
the client disconnects. If you let the tasks die and recreate new
tasks for new client connections, you will most certainly face some
slow memory leak. We have seen that, although the stack itself is
deallocated when a task dies, there aer a few bytes which remain
allocated (some kind of TCB I suppose).

So, the trick is to never let the service tasks die. When a client
disconnects, just queue the service task for future usage by a new
client connection. When a new connection comes in, reuse an idle
service task. Create a new one only when there is no available
idle task.

Of course, if you connection are long lived (several hours) and your
application is relatively short lived (a few weeks), this may not
be an issue. But if you want your application to potentially run
forever, you should take care.

In the (long) past, I have seen a Minitel server (French ancestor
of the WWW) crashing every 3 months, due to virtual memory exhaustion,
just because the Ada tasks died when the clients disconnected.

-Thierry
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Thierry Lelegard, "The Jazzing Troll", Email: thierry.lelegard@canal-plus.fr
CANAL+ Technologies, 34 place Raoul Dautry, 75906 Paris Cedex 15, France
Tel: +33 1 71 71 54 30   Fax: +33 1 71 71 52 08   Mobile: +33 6 03 00 65 75
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