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From Jason Brelloch <jb.bc....@gmail.com>
Subject Re: Tuning RocksDB
Date Wed, 03 May 2017 17:10:25 GMT
So looking through the logs I found these lines (repeated same test again
with a rocksDB backend, took 5m55s):

2017-05-03 12:52:24,131 INFO
 org.apache.flink.runtime.checkpoint.CheckpointCoordinator     - Triggering
checkpoint 2 @ 1493830344131
2017-05-03 12:52:24,132 INFO  org.apache.flink.core.fs.FileSystem
                - Created new CloseableRegistry
org.apache.flink.core.fs.SafetyNetCloseableRegistry@56ff02da for Async
calls on Source: CIC Json Event Source -> Map -> Filter (1/1)
2017-05-03 12:52:24,132 INFO  org.apache.flink.core.fs.FileSystem
                - Created new CloseableRegistry
org.apache.flink.core.fs.SafetyNetCloseableRegistry@116cba7c for Async
calls on Source: Custom Source -> Filter -> Map -> CIC Control Source (1/1)
2017-05-03 12:52:24,134 INFO
 org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend  -
Asynchronous RocksDB snapshot (File Stream Factory @
file:/home/jason/flink/checkpoint/b10bfbd4911c2d37dc2d610f4951c28a,
synchronous part) in thread Thread[Sequence Function -> Sink: Unnamed
(1/1),5,Flink Task Threads] took 0 ms.
2017-05-03 12:52:24,142 INFO  org.apache.flink.core.fs.FileSystem
                - Ensuring all FileSystem streams are closed for Async
calls on Source: CIC Json Event Source -> Map -> Filter (1/1)
2017-05-03 12:52:24,142 INFO  org.apache.flink.core.fs.FileSystem
                - Ensuring all FileSystem streams are closed for Async
calls on Source: Custom Source -> Filter -> Map -> CIC Control Source (1/1)
2017-05-03 12:58:19,167 INFO
 org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend  -
Asynchronous RocksDB snapshot (File Stream Factory @
file:/home/jason/flink/checkpoint/b10bfbd4911c2d37dc2d610f4951c28a,
asynchronous part) in thread Thread[pool-4-thread-2,5,Flink Task Threads]
took 355032 ms.
2017-05-03 12:58:19,170 INFO
 org.apache.flink.runtime.checkpoint.CheckpointCoordinator     - Completed
checkpoint 2 (27543402 bytes in 355037 ms).

Which I think means the async save to rocksDB took up most of time, and not
the serializing.  We had some serialization slowdown when we were using an
ever growing ValueState object, but switching to a ListState seems to have
resolved that, so I am not sure that that is the issue.

On Wed, May 3, 2017 at 12:05 PM, Stefan Richter <s.richter@data-artisans.com
> wrote:

> Sorry, just saw that your question was actually mainly about
> checkpointing, but it can still be related to my previous answer. I assume
> the checkpointing time is the time that is reported in the web interface?
> This would be the end-to-end runtime of the checkpoint which does not
> really tell you how much time is spend on writing the state itself, but you
> can find this exact detail in the logging; you can grep for lines that
> start with "Asynchronous RocksDB snapshot“. The background is that
> end-to-end also includes the time the checkpoint barrier needs to travel to
> the operator. If there is a lot of backpressure and a lot of network
> buffers, this can take a while. Still, the reason for the backpressure
> could still be in the way you access RocksDB, as it seems you are
> de/serializing every time you update an ever-growing value under a single
> key. I can see that accesses under this conditions could become very slow
> eventually, but could remain fast on the FSBackend for the reason from my
> first answer.
>
> Am 03.05.2017 um 17:54 schrieb Stefan Richter <s.richter@data-artisans.com
> >:
>
> Hi,
>
> typically, I would expect that the bottleneck with the RocksDB backend is
> not RocksDB itself, but your TypeSerializers. I suggest to first run a
> profiler/sampling attached to the process and check if the problematic
> methods are in serialization or the actual accesses to RocksDB. The RocksDB
> backend has to go through de/serialize roundtrips on every single state
> access, while the FSBackend works on heap objects immediately. For
> checkpoints, the RocksDB backend can write bytes directly whereas the
> FSBackend has to use the serializers to get from objects to bytes, so their
> actions w.r.t. how serializers are used are kind of inverted between
> operation and checkpointing. For Flink 1.3 we also will introduce
> incremental checkpoints on RocksDB that piggyback on the SST files. Flink
> 1.2 is writing checkpoints and savepoints fully and in a custom format.
>
> Best,
> Stefan
>
> Am 03.05.2017 um 16:46 schrieb Jason Brelloch <jb.bc.flk@gmail.com>:
>
> Hey all,
>
> I am looking for some advice on tuning rocksDB for better performance in
> Flink 1.2.  I created a pretty simple job with a single kafka source and
> one flatmap function that just stores 50000 events in a single key of
> managed keyed state and then drops everything else, to test checkpoint
> performance.  Using a basic FsStateBackend configured as:
>
> val backend = new FsStateBackend("file:///home/jason/flink/checkpoint")
> env.setStateBackend(backend)
>
> With about 30MB of state we see the checkpoints completing in 151ms.
> Using a RocksDBStateBackend configured as:
>
> val backend = new RocksDBStateBackend("file:///home/jason/flink/checkpoint
> ")
> backend.setDbStoragePath("file:///home/jason/flink/rocksdb")
> backend.setPredefinedOptions(PredefinedOptions.FLASH_SSD_OPTIMIZED)
> env.setStateBackend(backend)
>
> Running the same test the checkpoint takes 3 minutes 42 seconds.
>
> I expect it to be slower, but that seems excessive.  I am also a little
> confused as to when rocksDB and flink decide to write to disk, because
> watching the database the .sst file wasn't created until significantly
> after the checkpoint was completed, and the state had not changed.  Is
> there anything I can do to increase the speed of the checkpoints, or
> anywhere I can look to debug the issue?  (Nothing seems out of the ordinary
> in the flink logs or rocksDB logs)
>
> Thanks!
>
> --
> *Jason Brelloch* | Product Developer
> 3405 Piedmont Rd. NE, Suite 325, Atlanta, GA 30305
> <http://www.bettercloud.com/>
> Subscribe to the BetterCloud Monitor
> <https://www.bettercloud.com/monitor?utm_source=bettercloud_email&utm_medium=email_signature&utm_campaign=monitor_launch>
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-- 
*Jason Brelloch* | Product Developer
3405 Piedmont Rd. NE, Suite 325, Atlanta, GA 30305
<http://www.bettercloud.com/>
Subscribe to the BetterCloud Monitor
<https://www.bettercloud.com/monitor?utm_source=bettercloud_email&utm_medium=email_signature&utm_campaign=monitor_launch>
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