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 <jbel...@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 <ran...@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 <jbel...@gmail.com> > wrote: > >> > >> What happens if you disable row cache? > >> > >> On Tue, May 25, 2010 at 4:53 AM, Ran Tavory <ran...@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 >
