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From Jonathan Shook <>
Subject Re: Cassandra Write Performance, CPU usage
Date Fri, 11 Jun 2010 04:33:12 GMT

I am not yet knowledgeable enough to answer your question in more
detail. I would like to know more about the specifics as well.
There are counters you can use via JMX to show logical events, but
this will not always translate to good baseline information that you
can use in scaling estimates.
I would like to see a good analysis that characterizes the scaling
factors of different parts of the system, both from load
characterization and from an algorithmic perspective.

This is a common area of inquiry. Maybe we should start

On Thu, Jun 10, 2010 at 11:05 PM, Rishi Bhardwaj <> wrote:
> Hi Jonathan
> Thanks for such an informative reply. My application may end up doing such
> continuous bulk writes to Cassandra and thus I was interested in such a
> performance case. I was wondering as to what are all the CPU overheads for
> each row/column written to Cassandra? You mentioned updating of bloom
> filters, would that be the main CPU overhead, there may even be copying of
> data happening? I want to investigate about all the factors in play here and
> if there is a possibility for improvement. Is it possible to profile
> cassandra and see what maybe the bottleneck here. The auxiliary I/O you had
> mentioned for the Bloom filters, wouldn't that occur with the I/O for the
> SSTable, in which case the extra I/O for the bloom filter gets piggybacked
> with the SSTable I/O? I guess I don't understand the Cassandra internals too
> well but wanted to see how much can Cassandra achieve for continuous bulk
> writes.
> Has anyone done any bulk write experiments with Cassandra? Is Cassandra
> performance always expected to be bottlenecked by CPU when doing continuous
> bulk writes?
> Thanks for all the help,
> Rishi
> ________________________________
> From: Jonathan Shook <>
> To:
> Sent: Thu, June 10, 2010 7:39:24 PM
> Subject: Re: Cassandra Write Performance, CPU usage
> You are testing Cassandra in a way that it was not designed to be used.
> Bandwidth to disk is not a meaningful example for nearly anything
> except for filesystem benchmarking and things very nearly the same as
> filesystem benchmarking.
> Unless the usage patterns of your application match your test data,
> there is not a good reason to expect a strong correlation between this
> test and actual performance.
> Cassandra is not simply shuffling data through IO when you write.
> There are calculations that have to be done as writes filter their way
> through various stages of processing. The point of this is to minimize
> the overall effort Cassandra has to make in order to retrieve the data
> again. One example would be bloom filters. Each column that is written
> requires bloom filter processing and potentially auxiliary IO. Some of
> these steps are allowed to happen in the background, but if you try,
> you can cause them to stack up on top of the available CPU and memory
> resources.
> In such a case (continuous bulk writes), you are causing all of these
> costs to be taken in more of a synchronous (not delayed) fashion. You
> are not allowing the background processing that helps reduce client
> blocking (by deferring some processing) to do its magic.
> On Thu, Jun 10, 2010 at 7:42 PM, Rishi Bhardwaj <>
> wrote:
>> Hi
>> I am investigating Cassandra write performance and see very heavy CPU
>> usage
>> from Cassandra. I have a single node Cassandra instance running on a dual
>> core (2.66 Ghz Intel ) Ubuntu 9.10 server. The writes to Cassandra are
>> being
>> generated from the same server using BatchMutate(). The client makes
>> exactly
>> one RPC call at a time to Cassandra. Each BatchMutate() RPC contains 2 MB
>> of
>> data and once it is acknowledged by Cassandra, the next RPC is done.
>> Cassandra has two separate disks, one for commitlog with a sequential b/w
>> of
>> 130MBps and the other a solid state disk for data with b/w of 90MBps.
>> Tuning
>> various parameters, I observe that I am able to attain a maximum write
>> performance of about 45 to 50 MBps from Cassandra. I see that the
>> Cassandra
>> java process consistently uses 100% to 150% of CPU resources (as shown by
>> top) during the entire write operation. Also, iostat clearly shows that
>> the
>> max disk bandwidth is not reached anytime during the write operation,
>> every
>> now and then the i/o activity on "commitlog" disk and the data disk spike
>> but it is never consistently maintained by cassandra close to their
>> peak. I
>> would imagine that the CPU is probably the bottleneck here. Does anyone
>> have
>> any idea why Cassandra beats the heck out of the CPU here? Any suggestions
>> on how to go about finding the exact bottleneck here?
>> Some more information about the writes: I have 2 column families, the data
>> though is mostly written in one column family with column sizes of around
>> 32k and each row having around 256 or 512 columns. I would really
>> appreciate
>> any help here.
>> Thanks,
>> Rishi

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