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From Konstantin Knauf <konstantin.kn...@tngtech.com>
Subject Re: RocksDB Statebackend
Date Thu, 14 Apr 2016 00:56:05 GMT
Hi Aljoscha,

thanks for your answers. I am currently not in the office, so I can not
run any further analysis until Monday. Just some quick answers to your
questions.

We are using the partitioned state abstraction, most of the state should
correspond to buffered events in windows. Parallelism is 9. In terms of
stateful operators we basically just have a KafkaSource, a custom
stateful trigger as well as a RollingSink. Overall in this test scenario
the state is very limited (see size of state using FsStateBackend).

I will get back to you once, I have done some more experiments, which
will be in the course of next week.

Cheers,

Konstantin


On 12.04.2016 18:41, Aljoscha Krettek wrote:
> Hi,
> I'm going to try and respond to each point:
> 
> 1. This seems strange, could you give some background on parallelism,
> number of operators with state and so on? Also, I'm assuming you are
> using the partitioned state abstraction, i.e. getState(), correct?
> 
> 2. your observations are pretty much correct. The reason why RocksDB is
> slower is that the FsStateBackend basically stores the state in a Java
> HashMap and writes the contents to HDFS when checkpointing. RocksDB
> stores data in on-disk files and goes to them for every state access (of
> course there are caches, but generally it is like this). I'm actually
> impressed that it is still this fast in comparison.
> 
> 3. see 1. (I think for now)
> 
> 4. The checkpointing time is the time from the JobManager deciding to
> start a checkpoint until all tasks have confirmed that checkpoint. I
> have seen this before and I think it results from back pressure. The
> problem is that the checkpoint messages that we sent through the
> topology are sitting at the sources because they are also back pressured
> by the slow processing of normal records. You should be able to see the
> actual checkpointing times (both synchronous and asynchronous) in the
> log files of the task managers, they should be very much lower.
> 
> I can go into details, I'm just writing this quickly before calling it a
> day. :-)
> 
> Cheers,
> Aljoscha
> 
> On Tue, 12 Apr 2016 at 18:21 Konstantin Knauf
> <konstantin.knauf@tngtech.com <mailto:konstantin.knauf@tngtech.com>> wrote:
> 
>     Hi everyone,
> 
>     my experience with RocksDBStatebackend have left me a little bit
>     confused. Maybe you guys can confirm that my epxierence is the expected
>     behaviour ;):
> 
>     I have run a "performancetest" twice, once with FsStateBackend and once
>     RocksDBStatebackend in comparison. In this particular test the state
>     saved is generally not large (in a production scenario it will be
>     larger.)
> 
>     These are my observations:
> 
>     1. Minimal Checkpoint Size (no records) with RocksDB was 33MB compared
>     to <<1MB with the FSStatebackend.
> 
>     2. Throughput dropped from 28k/s -> 18k/s on a small cluster.
> 
>     3. Checkpoint sizes as reported in the Dashboard was ca. 1MB for
>     FsStatebackend but >100MB for RocksDbStatebackend. I hope the difference
>     gets smaller for very large state. Can you confirm?
> 
>     4. Checkpointing Times as reported in the Dashboard were 26secs for
>     RocksDB during the test and <1 second for FsStatebackend. Does the
>     reported time correspond to the sync. + asynchronous part of the
>     checkpointing in case of RocksDB? Is there any way to tell how long the
>     synchronous part takes?
> 
>     Form these first observations RocksDB does seem to bring a large
>     overhead for state < 1GB, I guess? Is this expected?
> 
>     Cheers,
> 
>     Konstantin
> 

-- 
Konstantin Knauf * konstantin.knauf@tngtech.com * +49-174-3413182
TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring
Geschäftsführer: Henrik Klagges, Christoph Stock, Dr. Robert Dahlke
Sitz: Unterföhring * Amtsgericht München * HRB 135082

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