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<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="Author" CONTENT="James CE Johnson">
<TITLE>ACE Tutorial 018</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 018</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>The FIFO Nature of ACE_Token</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
That's it for the code, now let's take a quick look at some output.
The first set of output is from the Token test, the second is Mutex
test. Notice how the threads are evenly utilized in the Token test.
Each thread gets to see exactly two messages. There's also an
interesting side-effect that the messages are processed in order. (You
can't rely on that, it just happend in this particular run.)
<P>
With the Mutex test, however, we see that the first thread gets no
less than 1/2 of all messages. In fact, if we didn't have the
governor in svc() it might have gotten them all!
<P>
Why does this happen?
<P>
Primarily because of time slicing. Even though each thread takes time
to do work (1 second in our test), it can still own the timeslice when
it gets back to the mutex acquire. Since the other threads are still
switched out, the current thread regets the lock and continues. On
the other hand, the ACE_Token is very careful about the order in which
the acquisition is allowed and more evenly distributes the work.
<P>
Play around with the sleep() call in svc(). You'll find that as you
decrease it, there is more chance that even the Token test will do
most of its work in one thread. You're still at the mercy of the OS
time slicing. In reality, though, it will take a moment or two for
work to be done. The end goal isn't necessarily to distribute the
work evenly over all threads but, rather, to distribute it evenly
among <i>available</i> threads. The distinction is subtle but important.
<HR>
<PRE>
(21386|1024|15:26:32.366520) Test (Token) created
1 (21386|1025|15:26:32.390340) <font color=#008888>Test::svc</font>() Entry
2 (21386|2050|15:26:32.408330) <font color=#008888>Test::svc</font>() Entry
3 (21386|3075|15:26:32.427363) <font color=#008888>Test::svc</font>() Entry
4 (21386|4100|15:26:32.447285) <font color=#008888>Test::svc</font>() Entry
5 (21386|5125|15:26:32.482479) <font color=#008888>Test::svc</font>() Entry
1 (21386|1025|15:26:32.498583) <font color=#008888>Test::svc</font>() received message #1 (Message Number 0)
2 (21386|2050|15:26:33.517770) <font color=#008888>Test::svc</font>() received message #1 (Message Number 1)
3 (21386|3075|15:26:34.537701) <font color=#008888>Test::svc</font>() received message #1 (Message Number 2)
4 (21386|4100|15:26:35.557675) <font color=#008888>Test::svc</font>() received message #1 (Message Number 3)
5 (21386|5125|15:26:36.577650) <font color=#008888>Test::svc</font>() received message #1 (Message Number 4)
1 (21386|1025|15:26:37.597689) <font color=#008888>Test::svc</font>() received message #2 (Message Number 5)
2 (21386|2050|15:26:38.607689) <font color=#008888>Test::svc</font>() received message #2 (Message Number 6)
3 (21386|3075|15:26:39.617675) <font color=#008888>Test::svc</font>() received message #2 (Message Number 7)
4 (21386|4100|15:26:40.637653) <font color=#008888>Test::svc</font>() received message #2 (Message Number 8)
5 (21386|5125|15:26:41.657637) <font color=#008888>Test::svc</font>() received message #2 (Message Number 9)
(21386|1024|15:26:42.679919) Test (Mutex) created
1 (21386|6150|15:26:42.700301) <font color=#008888>Test::svc</font>() Entry
2 (21386|7175|15:26:42.737413) <font color=#008888>Test::svc</font>() Entry
3 (21386|8200|15:26:42.754241) <font color=#008888>Test::svc</font>() Entry
4 (21386|9225|15:26:42.772122) <font color=#008888>Test::svc</font>() Entry
5 (21386|10250|15:26:42.788867) <font color=#008888>Test::svc</font>() Entry
1 (21386|6150|15:26:42.806260) <font color=#008888>Test::svc</font>() received message #1 (Message Number 0)
1 (21386|6150|15:26:43.807539) <font color=#008888>Test::svc</font>() received message #2 (Message Number 5)
1 (21386|6150|15:26:44.817569) <font color=#008888>Test::svc</font>() received message #3 (Message Number 6)
1 (21386|6150|15:26:45.827571) <font color=#008888>Test::svc</font>() received message #4 (Message Number 7)
1 (21386|6150|15:26:46.837581) <font color=#008888>Test::svc</font>() received message #5 (Message Number 8)
2 (21386|7175|15:26:47.847908) <font color=#008888>Test::svc</font>() received message #1 (Message Number 1)
2 (21386|7175|15:26:48.857555) <font color=#008888>Test::svc</font>() received message #2 (Message Number 9)
4 (21386|9225|15:26:49.867991) <font color=#008888>Test::svc</font>() received message #1 (Message Number 3)
3 (21386|8200|15:26:50.887559) <font color=#008888>Test::svc</font>() received message #1 (Message Number 2)
5 (21386|10250|15:26:51.898275) <font color=#008888>Test::svc</font>() received message #1 (Message Number 4)
</PRE>
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