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From "Vijay (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7438) Serializing Row cache alternative (Fully off heap)
Date Wed, 26 Nov 2014 00:16:14 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7438?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14225453#comment-14225453
] 

Vijay commented on CASSANDRA-7438:
----------------------------------

{quote}
Segments are hash buckets correct? 
{quote}
Yes and the way memcached and lruc is does the rehashing is based on this algorithm, and hence
yes... That was the argument earlier about JNI based solution. (Also another reason I was
talking about of configurable hash expansion capability in my previous comment)
{code}
    unsigned long current_size = cas_incr(stats.hash_items, 1);
    if (current_size > (hashsize(hashpower) * 3) / 2) {
        assoc_start_expand();
    }
{code}
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.

{quote}
The task submitted to the executor doesn't check whether another rehash is required it just
does it.
{quote}
Until you complete a rehash you don't know if you need to hash again or not... Am i missing
something?

> 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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