*BSD News Article 21383


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Newsgroups: comp.os.386bsd.questions
Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.Hawaii.Edu!ames!saimiri.primate.wisc.edu!news.doit.wisc.edu!psl.wisc.edu!128.104.200.15!ram
From: ram@xor.epi.wisc.edu (Ram Bhamidipaty)
Subject: Re: VLB IDE cache controller
In-Reply-To: rand@cs.UND.NoDak.Edu's message of 22 Sep 1993 16:18:13 -0500
Message-ID: <RAM.93Sep23132553@xor.epi.wisc.edu>
Sender: news@pslu1.psl.wisc.edu (USENET News System)
Organization: Physical Sciences Lab, UW-Madison
References: <27mf1g$ajt@news.u.washington.edu> <RAM.93Sep21145412@xor.epi.wisc.edu>
	<RAND.93Sep22161811@agassiz.cas.und.NoDak.Edu>
Date: 23 Sep 1993 18:25:53 GMT
Lines: 45

>>>>> "Douglas" == Douglas K. Rand <rand@cs.UND.NoDak.Edu> writes:

>>> I just upgraded my old ISA IDE controller to a VLB cache version.
>>> However, after some thorough test, I notice that the cache
>>> controller doesn't seem to improve the I/O performance at all.

Ram> I would expect almost no benefit from a caching controller, since
Ram> the cache system in 386bsd is already doing that job.  [...] but
Ram> an IDE controller can only handle two disks, which can be handled
Ram> by the ISA bus.

Douglas> I've got a ... VLB caching controller with 4 Mbytes
Douglas> controller (20 Mbytes main memory) and I see a signfigant
Douglas> difference with the cache. Here are some simple performance numbers
Douglas> for just the linking phase of building a new kernel:

Douglas>                         First time      Second time
Douglas> cache disabled:	16.41 secs	16.21 secs
Douglas> cache enabled:		 9.20 secs	 3.28 secs

Point taken. But I can't help but wonder what would happen if (a) the
4meg was taken from the controller and put in main memory [if it was
possible to "move" the memory] and (b) the kernel buffers were increased
to 4meg. Not that I know how to do this.

The kernel has all the information that the controller does, so in theory
it should be possible to have the kernel duplicate and performance increase
that one can gain from a caching controller.

I have a vague recollection of someone saying that the kernel will use
all available main memory for disk buffering if no other process is
using it...is this true? Or is there some limit on how much buffer
space the kernel uses for disk io. If the first statement is true
then there might be some in-efficiency in the kernel buffer caching
algorithms.  If one needs to configure the kernel disk io buffers when
building the kernel then it would be an interesting experiment to see
how close the two performance figures come when the kernel is given an
equal amount of cache memory.

-Ram
--
--------------
Ram Bhamidipaty                     use this ->ram@epidat.epi.wisc.edu
Department of Preventive Medicine   or this->ram%epidat.decnet@macc.wisc.edu
University of Wisconsin, Madison    Ignore the header. It might be wrong.