Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cwjcc!hal!nic.MR.NET!umn-cs!berlin!grg
From: grg@berlin.acss.umn.edu (George Gonzalez)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Apple HD SC 80 does not support Asynch i/o.
Summary: ASync I/O not too useful on the Mac right now...
Keywords: Hall of Shame entry?
Message-ID: <271@berlin.acss.umn.edu>
Date: 3 Jan 89 16:24:17 GMT
References: <11605@dartvax.Dartmouth.EDU>
Organization: U of M MicroGroup, Minneapolis
Lines: 27


Asynchronous I/O is a wonderful idea if you have a true pre-emptive multi-
tasking system.  The system can start an I/O request, and run another task
while waiting for the I/O to complete.  This means tasks get more CPU time;
less time is spent waiting for I/O.  If its done right, the tasks need not know
or worry about the I/O being done asyncronously.  Just wonderful.

But the current Mac O/S can't make good, general use of async I/O right now.
In general, there isn't another task that can be run while waiting for the I/O.
Even under Multi-Finder, the system can't preeemtively switch tasks when
doing I/O.  So async disk I/O is mostly pointless right now.

One exception: if you are a clever programmer, you *can* write a program
that starts an async disk operation and then goes and does something useful
while waiting for the I/O.  But this is a special case that speeds up just your
special program.  It doesnt help the rest of us sods waiting for the Finder
to copy a file.  It doesnt help the 1000+ other applications out there either.

The current Mac O/S puts the async I/O burden on the programmer.  This is
a bit less than perfect, as async I/O code is somewhat tricky.  I'd hate
to see 1000 different applications implementing async I/O, all in slightly
different, and possibly buggy ways.  Ideally, they would have built async I/O
into the system so we wouldnt have to discuss it now.  Maybe a future version
of the system will do just that.  I suspect A/UX does it right now.

So, let's not worry to much about some disks drives/drivers not supporting
async I/O right now. Its not too useful for most of us at this time.