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From: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Message-Id: <199603121806.TAA20924@labinfo.iet.unipi.it>
Subject: Re: non-blocking read ?
To: leisner@sdsp.mc.xerox.com (Marty Leisner)
Date: Tue, 12 Mar 1996 19:06:55 +0100 (MET)
Cc: msmith@atrad.adelaide.edu.au, questions@freebsd.org
In-Reply-To: <9603121743.AA08403@gnu.mc.xerox.com> from "Marty Leisner" at Mar 12, 96 09:43:09 am
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> >That's my problem... some programs (noticeably the "harvest" cache)
> >work as a single-threaded process which serves concurrent
> >requests by trying to avoid blocking calls. It appears to do a nice
> >job, but for some things (say reads/seeks etc. from disk) it appears
> >that there is no way to avoid delays -- at least to my knowledge.
> 
> How long a "delay" are you having?

It's "just" a design problem. If one tries to avoid blocking
operations then it would be a shame to be forced to used them.

[with current disks, a read issued right after an lseek might get a
delay in the 10..100ms range; something that you want to avoid if you
have something else to do. Forking a process to absorb the delay
might help but is expensive]

Someone suggested doing async IO and handling SIGIO (I suppose this
refers to doing

	i=O_NONBLOCK;
	fcntl(handle, F_SETFL, &i)

But I have a doubt: if, afterwards, you do

	lseek(handle, ...)

	result=read(handle, buf, size);

The lseek might be non blocking -- no problem with that.
The read is non-blocking because of the previous call to fcntl().

But how much data will be available when I get the SIGIO (or select
will return that I/O is possible) ? The amount I requested (assuming it
is available), or the system idea of a block, or what ?

	Thanks
	Luigi
====================================================================
Luigi Rizzo                     Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it       Universita' di Pisa
tel: +39-50-568533              via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522              http://www.iet.unipi.it/~luigi/
====================================================================