From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Subject: [TUHS] Synchronous vs Asynchronous IO in Unix
Date: Sun, 20 Sep 2015 08:39:33 -0400 (EDT) [thread overview]
Message-ID: <20150920123933.8869D18C0EB@mercury.lcs.mit.edu> (raw)
> From: Peter Jeremy <peter at rulingia.com>
> Why were the original read(2) and write(2) system calls written to
> offer synchronous I/O only?
A very interesting question (to me, particularly, see below). I don't think
any of the Unix papers answer this question?
> It's relatively easy to create synchronous I/O functions given
> asynchronous I/O primitives but it's impossible to do the opposite.
Indeed, and I've seen operating systems (e.g. a real-time PDP-11 OS I worked
with a lot called MOS) that did that.
I actually did add asynchronous I/O to V6 UNIX, for use with very early
Internet networking software being done at MIT (in a user process). Actually,
it wasn't just asynchronous, it was _raw_ asynchronous I/O! (The networking
device was DMA, and the s/w did DMA directly into the user process' memory.)
The code also allowed more than one outstanding I/O request, too. (So the
input could be re-enabled on the device ASAP, without having to wake up a
process, have it run, do a new read call, etc.)
We didn't redo the whole Unix I/O system, to support/use asyn I/O throughout,
though; I just kind of warted it onto the side. (IIRC, it notified the user
process via a signal that the I/O had completed; the user software then had
to do an sgtty() call to get the transfer status, size, etc.)
Anyway, back to the original topic: I don't want to speculate (although I
could :-); perhaps someone who was around 'back then' can offer some insight?
If not, time for speculation! :-)
Noel
next reply other threads:[~2015-09-20 12:39 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-20 12:39 Noel Chiappa [this message]
2015-09-20 14:32 ` Larry McVoy
2015-09-20 14:41 ` Larry McVoy
-- strict thread matches above, loose matches on Subject: below --
2015-09-20 4:38 Peter Jeremy
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