Caching disk controllers and 386 multiprocessor

[John Pettitt]

>From article <134 at unifax.UUCP>, by sl at unifax.UUCP (Stuart Lynne):
+ What I would like to see is a comparison of a caching disk controller (say a
+ DPT with 2MB of RAM) and a normal controller (say a WD1006) *plus* the same
+ amount of memory allocated to block buffers.
+ 
+ I get the impression from the articles I read that the reviewer simply puts
+ in the caching controller steps back and marvels at the improved response
+ :-) 
+ 
+ It seems obvious that adding 2MB of buffers is going to help somewhat. 
+ But where is the best place to put the memory, on the controller or in the 
+ kernel?

>From the tests I ran the DPT wins over adding 2 MB of buffers to /xenix
(don't know about `real' unix).  The improvment gained by having more 
buffers in /xenix was not great.  I think this was mostly due to the 
cacheing code being rather old and not that well written (putting on
flame proof suit :-).   The basic problem seemed to be the cache write
back code:  When lots of buffers are added to xenix it fills almost all of
them with dirty data then spends several seconds writing them all back and the
real time response goes out the window.

The DPT is far better balanced in this respect and so gives a mush more
even throughput.
+ 
+ I presume it reduces to a comparison of the efficencies of the two block 
+ bufferring algorithms and the time to transfer between kernel memory and the
+ controller memory.
+ 
+ One wild idea would be to implement a controller with dual ported memory and
+ have it interact with the kernel to implement a shared buffer cache. Unix
+ just ask's for a block and it auto-magically appears in the appropriate
+ buffer after some period of time. Maybe someone will do this on a 32bit EISA
+ card!

Arrrrrrrgh,   and how do you hope to track the changes in xenix ?
+ -- 
+ Stuart.Lynne at wimsey.bc.ca uunet!van-bc!sl 604-937-7532(voice) 604-939-4768(fax)
-- 
John Pettitt, Specialix, Giggs Hill Rd, Thames Ditton, Surrey, U.K., KT7 0TR
{backbone}!ukc!slxsys!jpp     jpp at slxinc.uucp     jpp at slxsys.specialix.co.uk
Tel: +44-1-941-2564       Fax: +44-1-941-4098         Telex: 918110 SPECIX G
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