[OpenAFS-devel] good and bad performance of memory cache

[Edward Moy]

Since I was starting to investigate memory caching, I decided to run  
iozone to really exercise the cache.  With disk caching on my  
moderately slow (400MHz) test machine, the full iozone test suite took  
nearly seven hours, with the largest files (going up to half a  
gigabyte) really dragging down performance.  So much so that iozone  
running over NFS2 actually runs 20 minutes faster than AFS.

I ran the same test with memory caching enabled (and even a larger  
cache; 80 MB memory cache versus the 30 MB disk cache).  The whole test  
suite now took nearly *twelve* hours.

In comparing results, speed is definitely faster for the memory cache  
when the file size is smaller than the cache size.  But when the file  
size is larger than the memory cache, performance is often worse (in  
some cases more that twice as bad) as the disk cache.

Anyone have any ideas about why this is so?
------------------------------------------------------------------------ 
--
Edward Moy
Apple Computer, Inc.
emoy@apple.com

(This message is from me as a reader of this list, and not a statement
from Apple.)