Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!pyrnj!pyrdc!uunet!munnari!otc!metro!ipso!runx!brucee
From: brucee@runx.ips.oz (Bruce Evans)
Newsgroups: comp.os.minix
Subject: Re: 1.3 checksums and bugs
Message-ID: <1792@runx.ips.oz>
Date: 16 Oct 88 02:33:55 GMT
References: <658@faui44.informatik.uni-erlangen.de>
Reply-To: brucee@runx.OZ (Bruce Evans)
Organization: RUNX  Un*x Timeshare. Sydney, Australia.
Lines: 48

Robert Regn writes:
>At last: I'm very interested to have an improved fs process, which
>doesn't block the system when waiting for floppy or disk tasks.
>If I type :
>cp image /dev/fd0 &
>ls -l		,
>I don't believe minix is a multitasking system, because the ls runs after
>the cp has finished!

Let me emphasize this problem again. Minix really is multi-tasking, but fs
kills it. What happens is that cp is very "efficient", and makes a few
large requests, while ls makes many smaller ones. The processes probably
take it in turns to get into fs, so cp finishes before ls.

If fs were cleverer, it would often be able to handle ls's requests from the
cache while cp was held up waiting for physical i/o. Of course when ls also
needs physical i/o, some delay is necessary. 

Apart from the apparently major rewrite of fs needed to solve this, a hog
program like cp may be difficult to handle because it will gobble up Minix's
tiny cache. My solution would be to junk the RAM disk and use the memory for
a cache instead. The main obstacle is that a large cache couldn't all be in
fs's 64K data space.

The way cp and ls take turns reminds me of another scheduling problem.
Suppose 2 processes are running,

 (H) a CPU Hog which makes no system calls
 (I) and Inefficient I/O bound process which does nothing much other than
     making system calls, e.g. single-char output.

The processes will take turns, resulting in (I) running at the glacial
speed of 1 I/O operation per quantum (1/10 sec or so).

This is relatively easy to fix.

>[shell program to check file system against crc's posted]

It requires a "then" and a non-comment statement in the "if" statement along
with the "#customize for your own structure".

It still fails on Minix when the sh runs out of string space. The following
one-liner demonstrates the problem:

  while :; set 1; done
 
Bruce Evans
Internet: brucee@runx.ips.oz.au    UUCP: uunet!runx.ips.oz.au!brucee