When using ant to run windows applications in parallel, termination of one
application will not always result in a new process. 

The problem can be verified on XP and Server 2003 with simple GUI hello world
programs that pop up a window and have to be closed manually. I experienced it
with Ant 1.65 and the current ant 1.7.0 nightly build (may 28th 2007), on both a
P4 machine (with 2 virtual processors) and on a Xeon server with 4 processors.

In the following script on a 2-processor machine, sometimes (25%-50% of the
time) I have to close both HelloWorld1 and HelloWorld2 before HelloWorld3 is
startet. The problem does not depend on which program is closed, it occurs
sometimes if I close HelloWorld1 and sometimes if I close HelloWorld2. 

<parallel threadsPerProcessor="1">
            <exec executable="HelloWorld1.exe"/>
            <exec executable="HelloWorld2.exe"/>
            <exec executable="HelloWorld3.exe"/>
            <exec executable="HelloWorld4.exe"/>
</parallel>


The same problem occurs if I enter a fixed threadCount. If I run the next code
snippet, it is rare that a new application is startet if I terminate a randomly
picked process of the 4 started process, often I have to close all 4
applications before the next 4 are started.

<parallel threadCount="4">
        <exec executable="HelloWorld1.exe"/>
        <exec executable="HelloWorld2.exe"/>
        <exec executable="HelloWorld3.exe"/>
        <exec executable="HelloWorld4.exe"/>
        <exec executable="HelloWorld5.exe"/>
        <exec executable="HelloWorld6.exe"/>
        <exec executable="HelloWorld7.exe"/>
        <exec executable="HelloWorld8.exe"/>
</parallel>

In fact, monitoring our Nightly Build which compiles a large number of C++
programs in parallel on a 4-processor machine showed that with
threadsPerProcessor="1", sometimes only one process is running. This severely
prolongs out build time, because the compile time of a few projects is very long
and if ant waits for such a project then only one processor is used instead of
all 4.