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[DAT] (requires LDraw-compatible viewer)
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Subject:
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defer() ??
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 19 Mar 1998 02:57:01 GMT
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Original-From:
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Will Bain <willbain@NOMORESPAMcs.umt.edu>
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Viewed:
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1427 times
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At 07:13 PM 3/18/98 -0600, Sudeshna wrote:
> Hi !
>
> Does anybody know how the defer() function works ? Also, what are its
> applications ?
The defer function is used in multithreaded programs when one process
doesn't require all the processor cycles that it would ordinarily be allotted.
Typically, if you have multiple processes (threads, tasks, whatever) running
at the same time, the system ordinarily cycles through them one at a time,
giving each one a few clock cycles (I believe the default is five). If you
have one thread that doesn't require many cycles, and you know that it can
afford to wait 'til its next turn in the sequence before it needs to do more
work, then you defer that thread, and in that way it politely relinquishes
the rest of its turn to whichever process comes next. It can be thought of
as a way to lower the priority of a particular process; a similar effect
could be obtained by specifying fewer than the default number of cycles to a
process when you create it (see the HB Tech. Reference for the exact syntax
of process creation).
Here's an example: Say you have three processes--A, B & C--and you create B
such that it gets 10 cycles per turn, and C such that it gets 20 cycles per
turn. Process A gets the default 5 cycles per turn. Now suppose that the
code in process B performs a test, and if the test comes up negative (which
happens, say, about half the time) the process defers itself after only five
cycles, and if it comes up positive, it defers after nine cycles. In that
case, you might get the following allotment of CPU cycles:
A B C
---- ---- ----
5 9 20
5 5 20
5 5 20
5 9 20
5 9 20
5 5 20
So the result in this example is that process B never actually uses its
whole ten cycles each time it has its turn. Now this is an artificial
example in that most processes will probably require more than nine cycles
to do a useful test and accomplish anything, but by making B defer, the
other processes don't have to wait quite so long, which can make the program
as a whole run more efficiently.
Hope that answers your question, and if I'm wrong about any of this, I hope
someone else posts a correction. Good luck.
--Will
, ,
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Wendy Parson, If you can't say anything good about someone,
Will Bain, sit right here by me.
& Tatoosh --Alice Roosevelt Longworth
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