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From: mike@prg.ox.ac.uk (Mike Spivey)
Newsgroups: comp.os.minix
Subject: Bigger buffer cache for PC MINIX
Message-ID: <MIKE.91Apr2131128@client25.prg.ox.ac.uk>
Date: 2 Apr 91 12:11:28 GMT
Sender: news@prg.ox.ac.uk
Distribution: comp
Organization: Oxford University Computing Laboratory, UK
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Some time ago, I suggested enlarging the buffer cache of PC MINIX by
putting part of it outside the FS address space.  I've now done this,
and will post some patches if there's enough interest -- send mail if
you'd like them.  If nothing else, a bigger cache reduces the need for
more concurrency in the file system.

Here are some rough timings for different MINIX configurations.
Neither cache nor RAM disk affects the `user' or `system' times much,
since the machine mostly runs the idle process while the disk i/o
happens.  So I've just shown the elapsed times.

	KERNEL REBUILD (root on hard disk)

	big cache			6:59
	/tmp on ram disk		7:21
	bare (no ram disk)		7:42

	Total user+sys was about 6 mins for all three.

	FACTOR TEST (see code below)

	root and /bin on ram disk	27 sec
	big cache			33 sec
	/bin on ram disk		42 sec
	bare (no ram or big cache)	59 sec

The code (with all programs in /bin):

	a=2
	while test $a -le 100
	do echo -n $a "= "
	   factor $a
	   a=`expr $a + 1`
	done			

The cache seems to be a big win because (a) you don't have to choose
what binaries to put on it and (b) you don't run out of space till the
hard disk is full -- the performance goes down more gradually.  It
works especially well when you run lots of little programs together,
in true UNIX style.

Some suggested that these mods would make the FS intolerably slow,
because it would always be having to trap to the kernel to copy
buffers around, but I haven't found that to be the case.  With a
little care, one can organise things so that directory blocks, etc.,
are put in buffers inside the FS address space, and ordinary data
blocks are outside the address space, so the almost the only copying
is to and from user space -- copying that is done anyway.

-- Mike Spivey