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From Ran Tavory <ran...@gmail.com>
Subject Re: Error reporting Key cache hit rate with cfstats or with JMX
Date Wed, 26 May 2010 20:54:15 GMT
If I disable row cache the numbers look good - key cache hit rate is > 0, so
it seems to be related to row cache.

Interestingly, after running for a really long time and with both row and
keys caches I do start to see Key cache hit rate > 0 but the numbers are so
small that it doesn't make sense.
I have capacity for 10M keys and 10M rows, the number of cached keys is ~5M
and very similarly the number of cached rows is also ~5M, however the hit
rates are very different, 0.7 for rows and 0.006 for keys. I'd expect the
keys hit rate to be identical since none of them reached the limit yet.

                Key cache capacity: 10000000
                Key cache size: 5044097
                Key cache hit rate: 0.0062089764058896576
                Row cache capacity: 10000000
                Row cache size: 5057231
                Row cache hit rate: 0.7361241352465543



On Tue, May 25, 2010 at 3:43 PM, Jonathan Ellis <jbellis@gmail.com> wrote:

> What happens if you disable row cache?
>
> On Tue, May 25, 2010 at 4:53 AM, Ran Tavory <rantav@gmail.com> wrote:
> > It seems there's an error reporting the Key cache hit rate. The value is
> > always 0.0 and I have a feeling it's incorrect. This is seen both by
> using
> > notetool cfstats as well as accessing JMX directly
> >
> (org.apache.cassandra.db:type=Caches,keyspace=outbrain_kvdb,cache=KvAdsKeyCache
> > RecentHitRate)
> >       <ColumnFamily CompareWith="BytesType" Name="KvAds"
> >                     RowsCached="10000000"
> >                     KeysCached="10000000"/>
> >                 Column Family: KvAds
> >                 SSTable count: 7
> >                 Space used (live): 1288942061
> >                 Space used (total): 1559831566
> >                 Memtable Columns Count: 73698
> >                 Memtable Data Size: 17121092
> >                 Memtable Switch Count: 33
> >                 Read Count: 3614433
> >                 Read Latency: 0.068 ms.
> >                 Write Count: 3503269
> >                 Write Latency: 0.024 ms.
> >                 Pending Tasks: 0
> >                 Key cache capacity: 10000000
> >                 Key cache size: 619624
> >                 Key cache hit rate: 0.0
> >                 Row cache capacity: 10000000
> >                 Row cache size: 447154
> >                 Row cache hit rate: 0.8460295730014572
> >                 Compacted row minimum size: 387
> >                 Compacted row maximum size: 31430
> >                 Compacted row mean size: 631
> > The Row cache hit rate looks good, 0.8 but Key cache hit rate always
> seems
> > to be 0.0 while the number of unique keys stays about 619624 for quite a
> > while.
> > Is it a real caching problem or just a reporting glitch?
>
>
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of Riptano, the source for professional Cassandra support
> http://riptano.com
>

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