cache hit ratio

[Tres Seaver]

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Daniel Beckham wrote:
> When someone says "I have a memcached hit ratio of 90%" they are talking 
> about a ratio (http://en.wikipedia.org/wiki/Ratio) as it relates to the 
> total amount of hits+misses: (get_misses / (get_hits+get_misses)) * 
> 100.  e.g.  If get_hits returns 900 and get_misses returns 100 we can 
> calculate the percentage of hits as related to the whole.  
> (900/(100+900)) * 100 = 90% of the memcached requests were hits.  
> Incidentally, this is a ratio of 9:1.

I've found that working with the "hits / hits + misses" ratio is
counter-intuitive for people:  for instance, the difference between 90%
and 95% hit ration is *double* the load to the backing store.
Therefore, when scaling a caching architecture, I nearly always talk
about the "miss ratio" (10%, in the case above).


Tres.
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Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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