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From: bjornl@tds.kth.se (Bj|rn Lisper)
Newsgroups: comp.parallel
Subject: Re: optimistic computation: what and wh
Message-ID: <3600@hubcap.UUCP>
Date: 21 Nov 88 00:07:12 GMT
Sender: fpst@hubcap.UUCP
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Approved: parallel@hubcap.clemson.edu


In article <3508@hubcap.UUCP> wilson@uicbert.eecs.uic.edu writes:
....
>Optimistic computation is the premature computation of things that
>may or may not turn out to be useful.  This can increase parallelism
>by allowing you to execute things without waiting to be *sure* you're
>computing the right thing. ....

When reading this posting I came to think of a funny example of optimistic
parallelism. The example is Swedish teller machines:

Most teller machines work this way: first they ask you for your code. Then
they ask you for the amount of withdrawal. Then they count the number of
bills to give you. Finally, if the above steps are successfully passed, they
give you the money. This is a strictly serial execution pattern.

A Swedish teller machine instead does it this way: first it asks you not for
your code, but for the amount of withdrawal. As soon as you provide the
answer it will start counting bills. THEN it asks you for your code. While
you're typing the code it will simultaneously count bills. This is
optimistic parallelism, since there is a possibility that you will fail to
provide the proper code. When the bills are counted and you have provided
the right code, you will get your money. If, for some reason, you would fail
to type the correct code, the machine will just drop the money in a box
instead of giving it to you.

This way of doing things makes Swedish teller machines faster, since the
most time-consuming part of the cycle is counting bills and therefore this
process should be started as soon as possible. Most users do succeed to
provide the proper code. Therefore the failures are few and optimistic
parallelism works very well.

Just a funny example of everyday optimistic parallelism....

Bjorn Lisper