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From vd <>
Subject Re: Cassandra Write Performance, CPU usage
Date Fri, 11 Jun 2010 02:38:24 GMT
Hi Rishi

The writes in Cassandra are not directly written to the Disk, they are
stored in memory and later on flushed to the disk. May be thats why you are
not getting much out of iostat. Cant say about high cpu usage.
Vineet Daniel

Let your email find you....

On Fri, Jun 11, 2010 at 6:12 AM, 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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