Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!aplcen!samsung!think!mintaka!mit-eddie!uw-beaver!ubc-cs!jalbert
From: jalbert@cs.ubc.ca (Francois Jalbert)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Program Execution Probe
Summary: RAM disk versus disk caching
Message-ID: <5966@ubc-cs.UUCP>
Date: 13 Dec 89 02:48:11 GMT
Sender: news@cs.ubc.ca
Reply-To: jalbert@cs.ubc.ca (Francois Jalbert)
Organization: UBC Department of Computer Science, Vancouver, B.C., Canada
Lines: 56

Recently, I mentioned I was looking for a way of determining how much time 
is spent doing disk operations in a typical run of PC-TeX. By comparing
runs in slow and turbo modes, I was able to do just that. I estimated I 
would get a 50% increase in efficiency on medium size jobs with a 4Mb RAM 
disk. This brought along a few additional comments. Once again, I thank 
heartily everybody who sent mail and suggestions. Obviously, this is a 
topic of interest. 

I was surprised to see several people independently suggest I should go
for a disk cache approach. Typically, 1 Mb was recommended. So many people
can't be wrong! I have been thinking about that today. A few comments 
came to mind:

A) Disk caching is obviously slower than simply having everything on a
   RAM disk, unless everything fits in the cache. Then perhaps, some
   overhead from DOS might be avoided. Therefore, the estimated 50% 
   improvement seems like an upper bound no matter what approach is
   selected. It would be interesting to have somebody with a big memory
   expansion board try to run some application in slow and turbo mode from
   the RAM disk and see if the formula I worked out before estimates 
   reasonably the speed improvement achievable with a RAM disk (and cache).
   (See Article 36592 of comp.sys.ibm.pc:)

B) My typical work cycle involves reading PC-Write 3.03, then PC-TeX, then 
   the preloaded lplain format, then a bunch of small files, and then my PTI
   previewer. We have the following very approximate sizes:

        PC-Write   250Kb <--
        PC-TeX     250Kb | |
        lplain.fmt 250Kb | ^ Read Cycle
        misc       250Kb | |
        PTI viewer 250Kb -->

C) I have the impression such a cycle in a 1Mb cache would cause the next 
   application involved in my cycle to be flushed out of the cache by the
   loading of my current application. I would need at least a 1.5Mb cache.

D) Furthermore, these are just the files routinely read. I assume the ones
   written also flush data out of the cache. And these can be quite big,
   usually a few 100's of Kb.  So a 2Mb seems to be the minimum called for.

E) However, additional RAM will speed up dramatically the loading of these
   applications. No matter what I do, having PC-Write available in any
   situation within 0.5 sec is a plus.

To make a short story long, The RAM disk allows me to tailor performance
optimally with the use of setup batch files for each application. Disk
caching is perhaps less optimal given a specific application, but is very 
flexible as it works for me all the time. I wonder if there is a driver out 
there that can be switched between disk-cache and RAM-disk mode. That would 
be the best of both worlds. 

I am still thinking (and overheating...).

                                                   Francois
                                                       '