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From: gillies@p.cs.uiuc.edu
Newsgroups: comp.realtime
Subject: Re: Lightweight Tasks
Message-ID: <129300004@p.cs.uiuc.edu>
Date: 23 Jul 89 15:57:00 GMT
References: <2153@gmu90x.UUCP>
Lines: 20
Nf-ID: #R:gmu90x.UUCP:2153:p.cs.uiuc.edu:129300004:000:1056
Nf-From: p.cs.uiuc.edu!gillies    Jul 23 10:57:00 1989


Lightweight is to tasking as RISC is to computer architecture.

"Lightweight tasks" became popular about 8 years ago.  It is a
buzzword that indicates the operating system can do a context switch
quickly.  Lightweight tasks came about because most UNIXes take
several milliseconds to perform a context switch.  This is *death* to
a well-structured, layered protocol implementation.  Some early
machines (like the Xerox DLION) could perform a task switch as fast as
a procedure call (in about 20 microseconds), using some hardware
support.  Other operating systems minimize the task context block so
that very little state must be changed in order to switch tasks.
Sometimes, all the lightweight tasks run together in the same address
space (In the DLion, and in MIT's IBM PC/IP network protocol package,
and in its unix V7 TCP/IP)

There is no precise definition, just as there is no precise definition
of RISC.  RISC was a knee-jerk reaction to the hideous complexity of
the VAX; Lightweight tasks were a reaction to the hideous slowness of
UNIX tasking.