I think it would be a good exercise to know what the CPU bottleneck is on the
write path. The fact that Cassandra optimizes disk I/O for writes would only go
so far if the CPU becomes a big bottleneck on continuous writes. I am fairly
new to Java ecosystem performance profiling but I would give it a try and see
if I can pinpoint the problem area here. I am also thinking about making
concurrent writes to cassandra instead of only one write at a time. This would
probably make Cassandra beat the hell out of all CPU resources and confirm that
Cassandra is CPU bound on continuous writes.
Again, I would love to hear from Cassandra experts here and see what they think
of this. Are Cassandra continuous bulk writes expected to be bottlenecked by
CPU? If this is definitely the case and thats what it seems right now, then it
would be a good thing to look at the algorithms in the write path.
Thanks,
Rishi
________________________________
From: Mike Malone <m...@simplegeo.com>
To: user@cassandra.apache.org
Sent: Fri, June 11, 2010 9:20:06 AM
Subject: Re: Cassandra Write Performance, CPU usage
Jonathan, while I agree with you re: this being an unusual load for the system,
it is interesting that he's found at least one use-case where Cassandra is
CPU-bound, not IO-bound. I'd definitely be interested in learning what his
critical path is and seeing if there's some low-hanging fruit that may improve
performance overall. I have also noticed very high CPU usage during high write
loads and have wondered whether write speed and throughput could be improved by
improving some of the algorithms along that path.
I'm nowhere near being an expert on the whole Java ecosystem, but I've had good
luck with the `jvisualvm` tool that comes with Java SE 6. It's a nice
lightweight CPU and memory profiling tool that can attach to a running process
like Cassandra and dump stats in real time.
Mike
On Thu, Jun 10, 2010 at 7:39 PM, Jonathan Shook <jsh...@gmail.com> wrote:
>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 <khichri...@yahoo.com> 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
>>>
>>>
>
