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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Row caching + Wide row column family == almost crashed?
Date Sun, 02 Dec 2012 22:07:51 GMT
Row cache has to store the entire row. It is a very bad option for wide
rows.

On Sunday, December 2, 2012, Mike <mtheroux2@yahoo.com> wrote:
> Hello,
>
> We recently hit an issue within our Cassandra based application.  We
> have a relatively new Column Family with some very wide rows (10's of
> thousands of columns, or more in some cases).  During a periodic
> activity, we the range of columns to retrieve various pieces of
> information, a segment at a time.
>
> We do these same queries frequently at various stages of the process,
> and I thought the application could see a performance benefit from row
> caching.  We have a small row cache (100MB per node) already enabled,
> and I enabled row caching on the new column family.
>
> The results were very negative.  When performing range queries with a
> limit of 200 results, for a small minority of the rows in the new column
> family, performance plummeted.  CPU utilization on the Cassandra node
> went through the roof, and it started chewing up memory.  Some queries
> to this column family hung completely.
>
> According to the logs, we started getting frequent GCInspector
> messages.  Cassandra started flushing the largest mem_tables due to
> hitting the "flush_largest_memtables_at" of 75%, and scaling back the
> key/row caches.  However, to Cassandra's credit, it did not die with an
> OutOfMemory error.  Its measures to emergency measures to conserve
> memory worked, and the cluster stayed up and running.  No real errors
> showed in the logs, except for Messages getting drop, which I believe
> was caused by what was going on with CPU and memory.
>
> Disabling row caching on this new column family has resolved the issue
> for now, but, is there something fundamental about row caching that I am
> missing?
>
> We are running Cassandra 1.1.2 with a 6 node cluster, with a replication
> factor of 3.
>
> Thanks,
> -Mike
>
>

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