Date: Mon, 23 Sep 2002 23:36:38 +0300 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: MET <met@uberstats.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Time-Sharing ? Message-ID: <20020923203638.GE74987@hades.hell.gr> In-Reply-To: <000c01c26323$13ab16e0$6401a8c0@SURVIVAL> References: <000c01c26323$13ab16e0$6401a8c0@SURVIVAL>
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On 2002-09-23 13:02, MET <met@uberstats.com> wrote:
> I see this phrase everywhere. What exactly is meant by 'time-sharing'
> in Unix?
When many programs are run, on a single machine, there's eventually
some limit that they will hit as trying to run "at the same time".
There's only one CPU for instance[1]. Time-sharing refers to a
technique that many operating systems use to give the impression that
programs "run at the same time".
A simple diagram might help you understand this better. If there are
three programs that are "running" on a multi-processing operating
system (one that allows the execution of many `processes',
i.e. programs, at once), and time advances from left to right... the
following diagram shows the fragments of time that each program
occupies the cpu of the machine.
A |---XXX-----XXX----------XXX-----------XXX--------------XXX--|
B |-------XXX---------XXX-----------XXX-------XXX-----XXX------|
C |--------------XXX-----------XXX----------------XXX----------|
If you project this to a single line, replacing XXX with the name of
the program you get something like:
T |---AAA-BBB-AAACCC--BBB--AAA-CCC--BBB--AAA--BBB-AAA-BBB-AAA--|
The three programs "share" the time of the machine's cpu, and the
operating system takes care of periodically running each one for short
periods of time.
I hope this helps a bit,
Giorgos.
[1] Well, SMP machines make this statement less true, but that's
besides the point. Let's stick to a simple example.
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