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From Kyusik Chung <kyu...@discovereads.com>
Subject Re: Continuously increasing RAM usage
Date Mon, 31 May 2010 21:01:26 GMT
Hmm...heap usage slowly growing over time...the picture doesnt look like the problems we were
running into, and I dont think they should be related to mmap-ing your data (since that is
not counted in heap mem usage).

Kyusik Chung

On May 29, 2010, at 10:16 AM, James Golick wrote:

> Well, it's been a few days on 0.6.2 and the new jvm and the behaviour looks to be about
the same:
> 
> http://skitch.com/jamesgolick/df46f/munin-fetlife.com-cassandra0.fetlife.com-cassandra-memory
> 
> There's only one cache turned on, and it's a row cache, but the sizes of the rows are
identical and it's been full since an hour after I rebooted the nodes, so it's not that.
> 
> On Thu, May 27, 2010 at 1:00 PM, Kyusik Chung <kyusik@discovereads.com> wrote:
> > I tried setting the IO mode to standard, but it seemed to be a little slower
> > and couldn't get the machine to come back online with adequate read
> > performance, so I set it back. I'll have to write a solid cache warming
> > script if I'm going to try that again.
> 
> What cache are you talking about?  Did you turn on row caching?
> 
> When we turned on row caching, repeat hits to the same rows was fast, of course, but
we didnt (given our data access patterns) see significant differences compared to mmap-ing
the data.  And once we hit the limit of our row cache, out-of-cache hits were pretty costly
(dont have hard numbers, but I recall it being worse than having mmap page in/out).
> 
> Is your client making random reads of more rows than will fit in RAM on your box?  We
found that in that scenario, after cassandra has used up all of the free memory on the box,
using mmap was slightly worse than using standard data access.
> 
> We happened to be lucky that our real world data access is limited to a small subset
of rows in any given time period, so mmap works great for us.  I guess the best thing to do
is to try to figure out how to make a cassandra node only need to service requests for data
that can fit into memory in a given time period.  More nodes, a lower replication factor,
more memory, I guess...
> 
> Im definitely waiting to hear how things change with 0.6.2.
> 
> Kyusik Chung
> 


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