From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
Date: Mon, 21 Aug 2000 08:59:32 +0000
From: "Douglas A. Gwyn" <DAGwyn@null.net>
Message-ID: <399F246E.251B1062@null.net>
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References: <200008182206.SAA19560@cse.psu.edu>, <20000819083447.J4695@cackle.proxima.alt.za>
Subject: Re: [9fans] cooperative threads
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Lucio De Re wrote:
> It is quite amusing to recollect Edsger Dijkstra's horror when asked
> to program a very early pre-emptive computer.  He is reported to have
> needed serious convincing before he accepted the principle of
> _interrupts_ :-)

With good reason.  In the early days, before people like Dijkstra
and Hoare had developed reasonable models for concurrency, it was
hard to control the concurrency in preemptive (interrupt)
environments.  For example, the CDC 1700 operating system had
several forms of I/O, the one necessary for double-buffering
involving a "completion routine" that would be invoked when the
I/O request had been satisfied.  It was possible to write correct
applications under such a scheme, but it was much easier to write
ones with race conditions etc.

I think threads are popular for two main reasons:
(1) They are an easy way to exploit multiple processing units.
(2) They are much lighter weight than communicating full processes.