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https://issues.apache.org/jira/browse/CASSANDRA-7438?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14225478#comment-14225478
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Ariel Weisberg edited comment on CASSANDRA-7438 at 11/26/14 12:29 AM:
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bq. if we don't like the constant overhead of the cache in heap and If you are
talking about CAS which we already do for ref counting, as mentioned before we
need an alternative strategy for global locks for rebalance if we go with lock
less strategy.
Just take what you have and do it off heap. You don't need to change anything
about how locking is done, just put the segments off heap so each segment would
be a 4-byte lock field and an 8 byte pointer to the first entry. I am not clear
on the alignment requirements for 4 or 8 byte CAS.
bq. Until you complete a rehash you don't know if you need to hash again or
not... Am i missing something?
https://github.com/Vijay2win/lruc/blob/master/src/main/java/com/lruc/unsafe/UnsafeConcurrentMap.java#L38
The check on line 38 races with the assignment on line 39. N threads could do
the check and think a rehash is necessary. Each would submit a rehash task and
the table size would be doubled N times instead of 1 time.
was (Author: aweisberg):
bq. if we don't like the constant overhead of the cache in heap and If you are
talking about CAS which we already do for ref counting, as mentioned before we
need an alternative strategy for global locks for rebalance if we go with lock
less strategy.
Just take what you have and do it off heap. You don't need to change anything
about how locking is done, just put the segments off heap so each segment would
be a 4-byte lock field and an 8 byte pointer to the first entry.
bq. Until you complete a rehash you don't know if you need to hash again or
not... Am i missing something?
https://github.com/Vijay2win/lruc/blob/master/src/main/java/com/lruc/unsafe/UnsafeConcurrentMap.java#L38
The check on line 38 races with the assignment on line 39. N threads could do
the check and think a rehash is necessary. Each would submit a rehash task and
the table size would be doubled N times instead of 1 time.
> Serializing Row cache alternative (Fully off heap)
> --------------------------------------------------
>
> Key: CASSANDRA-7438
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7438
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Environment: Linux
> Reporter: Vijay
> Assignee: Vijay
> Labels: performance
> Fix For: 3.0
>
> Attachments: 0001-CASSANDRA-7438.patch
>
>
> Currently SerializingCache is partially off heap, keys are still stored in
> JVM heap as BB,
> * There is a higher GC costs for a reasonably big cache.
> * Some users have used the row cache efficiently in production for better
> results, but this requires careful tunning.
> * Overhead in Memory for the cache entries are relatively high.
> So the proposal for this ticket is to move the LRU cache logic completely off
> heap and use JNI to interact with cache. We might want to ensure that the new
> implementation match the existing API's (ICache), and the implementation
> needs to have safe memory access, low overhead in memory and less memcpy's
> (As much as possible).
> We might also want to make this cache configurable.
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