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From Alessandro Pieri <siri...@gmail.com>
Subject Re: Latency overhead on Cassandra cluster deployed on multiple AZs (AWS)
Date Tue, 12 Apr 2016 16:52:14 GMT
Hi Jack,

As mentioned before I've used m3.xlarge instance types together with two
ephemeral disks in raid 0 and, according to Amazon, they have "high"
network performance.

I ran many tests starting with a brand-new cluster every time and I got
consistent results.

I believe there's something that I cannot explain yet with the client used
by cassandra-stress to connect to the nodes, I'd like to understand why
there is such a big difference:

Multi-AZ, CL=ONE, "--nodes node1,node2,node3,node4,node5,node6" -> 95th
percentile: 38.14ms
Multi-AZ, CL=ONE, "--nodes node1" -> 95th percentile: 5.9ms

Hope you can help to figure it out.

Cheers,
Alessandro




On Tue, Apr 12, 2016 at 5:43 PM, Jack Krupansky <jack.krupansky@gmail.com>
wrote:

> Which instance type are you using? Some may be throttled for EBS access,
> so you could bump into a rate limit, and who knows what AWS will do at that
> point.
>
> -- Jack Krupansky
>
> On Tue, Apr 12, 2016 at 6:02 AM, Alessandro Pieri <alessandro@getstream.io
> > wrote:
>
>> Thanks Chris for your reply.
>>
>> I ran the tests 3 times for 20 minutes/each and I monitored the network
>> latency in the meanwhile, it was very low (even the 99th percentile).
>>
>> I didn't notice any cpu spike caused by the GC but, as you pointed out, I
>> will look into the GC log, just to be sure.
>>
>> In order to avoid the problem you mentioned with EBS and to keep the
>> deviation under control I used two ephemeral disks in raid 0.
>>
>> I think the odd results come from the way cassandra-stress deals with
>> multiple nodes. As soon as possible I will go through the Java code to get
>> some more detail.
>>
>> If you have something else in your mind please let me know, your comments
>> were really appreciated.
>>
>> Cheers,
>> Alessandro
>>
>>
>> On Mon, Apr 11, 2016 at 4:15 PM, Chris Lohfink <clohfink85@gmail.com>
>> wrote:
>>
>>> Where do you get the ~1ms latency between AZs? Comparing a short term
>>> average to a 99th percentile isn't very fair.
>>>
>>> "Over the last month, the median is 2.09 ms, 90th percentile is
>>> 20ms, 99th percentile is 47ms." - per
>>> https://www.quora.com/What-are-typical-ping-times-between-different-EC2-availability-zones-within-the-same-region
>>>
>>> Are you using EBS? That would further impact latency on reads and GCs
>>> will always cause hiccups in the 99th+.
>>>
>>> Chris
>>>
>>>
>>> On Mon, Apr 11, 2016 at 7:57 AM, Alessandro Pieri <sirio7g@gmail.com>
>>> wrote:
>>>
>>>> Hi everyone,
>>>>
>>>> Last week I ran some tests to estimate the latency overhead introduces
>>>> in a Cassandra cluster by a multi availability zones setup on AWS EC2.
>>>>
>>>> I started a Cassandra cluster of 6 nodes deployed on 3 different AZs (2
>>>> nodes/AZ).
>>>>
>>>> Then, I used cassandra-stress to create an INSERT (write) test of 20M
>>>> entries with a replication factor = 3, right after, I ran cassandra-stress
>>>> again to READ 10M entries.
>>>>
>>>> Well, I got the following unexpected result:
>>>>
>>>> Single-AZ, CL=ONE -> median/95th percentile/99th percentile:
>>>> 1.06ms/7.41ms/55.81ms
>>>> Multi-AZ, CL=ONE -> median/95th percentile/99th percentile:
>>>> 1.16ms/38.14ms/47.75ms
>>>>
>>>> Basically, switching to the multi-AZ setup the latency increased of
>>>> ~30ms. That's too much considering the the average network latency between
>>>> AZs on AWS is ~1ms.
>>>>
>>>> Since I couldn't find anything to explain those results, I decided to
>>>> run the cassandra-stress specifying only a single node entry (i.e. "--nodes
>>>> node1" instead of "--nodes node1,node2,node3,node4,node5,node6") and
>>>> surprisingly the latency went back to 5.9 ms.
>>>>
>>>> Trying to recap:
>>>>
>>>> Multi-AZ, CL=ONE, "--nodes node1,node2,node3,node4,node5,node6" -> 95th
>>>> percentile: 38.14ms
>>>> Multi-AZ, CL=ONE, "--nodes node1" -> 95th percentile: 5.9ms
>>>>
>>>> For the sake of completeness I've ran a further test using a
>>>> consistency level = LOCAL_QUORUM and the test did not show any large
>>>> variance with using a single node or multiple ones.
>>>>
>>>> Do you guys know what could be the reason?
>>>>
>>>> The test were executed on a m3.xlarge (network optimized) using the
>>>> DataStax AMI 2.6.3 running Cassandra v2.0.15.
>>>>
>>>> Thank you in advance for your help.
>>>>
>>>> Cheers,
>>>> Alessandro
>>>>
>>>
>>>
>>
>>
>> --
>> *Alessandro Pieri*
>> *Software Architect @ Stream.io Inc*
>> e-Mail: alessandro@getstream.io - twitter: sirio7g
>> <http://twitter.com/sirio7g>
>>
>>
>

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