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From Leif Hedstrom <zw...@apache.org>
Subject Re: why does cache hit rate tail off?
Date Wed, 14 Jul 2010 14:38:05 GMT

On Jul 14, 2010, at 7:27 AM, Rob Maidment <Rob.Maidment@clearswift.com> wrote:

> Hello
> 
>  
> 
> I’m measuring trafficserver performance using the web polygraph tool:
> 
> http://www.web-polygraph.org/
> 
>  
> 
> Performance is generally impressive, but I’m trying to understand the strange results
I’m seeing for cache hit rate.  The test is a short duration (20 minutes) and initially
the hit rate measured by polygraph exactly matches the expected (offered) rate.  However 3
minutes into the test the hit rate starts to drop below the expected rate and continues to
tail off for the duration of the test.  Can anyone explain why this might be?
> 

That doesn't sound good. Is the cache churning by chance? How large is the RAM cache and disk
cache?  Is there significant disk I/O or contention on the disks?
>  
> 
> I’m testing on a 1 Gbps network – does trafficserver somehow work out it’s quicker
to get responses from the server than from its local cache??  Is this tuneable at all?
> 

No. But there are algorithms for promoting and evicting object from disk cache to RAM cache,
for evicting out of RAM cache, and of course for evicting out of disk cache. The default RAM
cache eviction algorithm was recently changed, maybe you can try the old (LRU) policy as well?

One other thing to look for is to make sure the traffic_server process isn't crashing / restarting.
This might happen fairly transparently, but would increase cache misses, since while restarting
and initializing the caches, ATS will forward every request to the backend. If the restarts
happens frequent it might not even get a chance to sync the directories to disk.

Cheers!

- Leif
>  
> 
> regards
> 
> Rob
> 
>  
> 
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