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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8518) Cassandra Query Request Size Estimator
Date Thu, 15 Jan 2015 00:09:35 GMT


Benedict commented on CASSANDRA-8518:

This is one of the two methods I proposed, and I'm comfortable aiming for the global threshold.
Per-request thresholds are also a possibility, and seem reasonable also. Whether or not we
_throttle_ or simply discard some in-flight queries on exceeding our limit is another matter
though. I would prefer to go the route of discarding some random in-flight queries, as this
brings the system back to full health immediately, instead of letting it crawl along until
the blockage clears.

> Cassandra Query Request Size Estimator
> --------------------------------------
>                 Key: CASSANDRA-8518
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Cheng Ren
> We have been suffering from cassandra node crash due to out of memory for a long time.
The heap dump from the recent crash shows there are 22 native transport request threads each
of which consumes 3.3% of heap size, taking more than 70% in total.  
> Heap dump:
> !|height=400,width=600!
> Expanded view of one thread:
> !|height=400,width=600!
> The cassandra we are using now (2.0.4) utilized MemoryAwareThreadPoolExecutor as the
request executor and provided a default request size estimator which constantly returns 1,
meaning it limits only the number of requests being pushed to the pool. To have more fine-grained
control on handling requests and better protect our node from OOM issue, we propose implementing
a more precise estimator. 
> Here is our two cents:
> For update/delete/insert request: Size could be estimated by adding size of all class
members together.
> For scan query, the major part of the request is response, which can be estimated from
the history data. For example if we receive a scan query on a column family for a certain
token range, we keep track of its response size used as the estimated response size for later
scan query on the same cf. 
> For future requests on the same cf, response size could be calculated by token range*recorded
size/ recorded token range. The request size should be estimated as (query size + estimated
response size).
> We believe what we're proposing here can be useful for other people in the Cassandra
community as well. Would you mind providing us feedbacks? Please let us know if you have any
concerns or suggestions regarding this proposal.
> Thanks,
> Cheng

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