Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!amdahl!uunet!mcvax!hp4nl!kunivv1!wn2!janhen
From: janhen@wn2.sci.kun.nl (Jan Hendrikx)
Newsgroups: comp.sys.cbm
Subject: Re: SEQ file access speedup
Keywords: Interleave, fast SEQ access
Message-ID: <338@wn2.sci.kun.nl>
Date: 18 Feb 89 20:21:19 GMT
References: <89Feb10.182100est.2732@godzilla.eecg.toronto.edu> <89Feb18.010253est.2384@godzilla.eecg.toronto.edu>
Organization: University of Nijmegen, The Netherlands.
Lines: 53

In article <89Feb18.010253est.2384@godzilla.eecg.toronto.edu>, leblanc@eecg.toronto.edu (Marcel LeBlanc) writes:
> I don't remember under what situations the DOS will do read-ahead, but the
> point was that since the DOS follows the interleave chain, it won't send the
> file any faster than the interleave allows, no matter what sort of interface
> you are using. 

That was not the point that I was trying to make. What you say is of
course true. I just wanted to inform the person who thought the DOS
does no read-ahead at all.

> Since you brought it up, under what situations will the DOS do read-ahead?

As far as I remember without my references around, (I don't have a 64
anymore for over two years now), the algorithm is about the following:

If a file is opened for sequential reading, an 'active' buffer is
allocated, which is filled from the disk. Active means that the next
byte that can be requested from the computer, must come from that
buffer.  Also, an 'inactive' buffer is allocated, if possible. That
buffer is filled in from disk asyncronously. When the active buffer is
emtied by read requests from the computer, buffers are switched from
active to inactive and vv. Of course, if the inactive buffer has not
yet finished reading from disk, the computer must wait.

There is a routine which takes a buffer number and sets a pointer to
that buffer somewhere. It maintais a Least Recently Used stack, based
on the requests it gets.

Whenever a buffer is needed, but no unused buffer is available, an
inactive buffer is found based on the LRU stack. The idea is that the
buffer that is not used for the longest time, will not likely be used
again soon. So it is 'safe' to use it for something else.

The routine that switches buffers knows that inactive buffers may
disappear. If they do, it does something reasonable. I am not sure if
it just reads the next block into the one buffer that is left, or if it
first tries to get a new second buffer. In any case, the buffer that
was read-ahead must be re-read from the disk.

Relative files have two active buffers: one for a side-sector, and one
for actual data. I am not sure if it also reads-ahead.

Some other things I would need to look up are whether files open for
writing also have an inactive (write-behind) buffer, and whether
(trying to) allocate an inactive buffer may cause another inactive
buffer to be discarded.

As you can see, the operating system in the drive is considerably more
complex than that in the computer...

> Marcel A. LeBlanc	  | University of Toronto -- Toronto, Canada
> leblanc@eecg.toronto.edu  | also: LMS Technologies Ltd, Fredericton, NB, Canada
-Olaf Seibert