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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Error reporting Key cache hit rate with cfstats or with JMX
Date Thu, 27 May 2010 13:51:33 GMT
Essentially, yes.

On Wed, May 26, 2010 at 11:25 PM, Ran Tavory <rantav@gmail.com> wrote:
> so the row cache contains both rows and keys and if I have large enough row
> cache (in particular if row cache size equals key cache size) then it's just
> wasteful to keep another key cache and I should eliminate the key-cache,
> correct?
>
> On Thu, May 27, 2010 at 1:21 AM, Jonathan Ellis <jbellis@gmail.com> wrote:
>>
>> It sure sounds like you're seeing the "my row cache contains the
>> entire hot data set, so the key cache only gets the cold reads"
>> effect.
>>
>> On Wed, May 26, 2010 at 2:54 PM, Ran Tavory <rantav@gmail.com> wrote:
>> > 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
>> >
>> >
>>
>>
>>
>> --
>> Jonathan Ellis
>> Project Chair, Apache Cassandra
>> co-founder of Riptano, the source for professional Cassandra support
>> http://riptano.com
>
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of Riptano, the source for professional Cassandra support
http://riptano.com

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