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https://issues.apache.org/jira/browse/FLINK-12785?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16860481#comment-16860481
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Congxian Qiu(klion26) commented on FLINK-12785:
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I think your analysis is correct, we should better add another flush strategy
based on the in-flight byte size in RocksDBWriteBatchWrapper. If you don't mind
I'll give a patch for this [~mikekap]
> RocksDB savepoint recovery can use a lot of unmanaged memory
> ------------------------------------------------------------
>
> Key: FLINK-12785
> URL: https://issues.apache.org/jira/browse/FLINK-12785
> Project: Flink
> Issue Type: Bug
> Components: Runtime / State Backends
> Reporter: Mike Kaplinskiy
> Priority: Major
>
> I'm running an application that's backfilling data from Kafka. There's
> approximately 3 years worth of data, with a lot of watermark skew (i.e. new
> partitions were created over time) and I'm using daily windows. This makes a
> lot of the windows buffer their contents before the watermark catches up to
> "release" them. In turn, this gives me a lot of in-flight windows (200-300)
> with very large state keys in rocksdb (on the order of 40-50mb per key).
> Running the pipeline tends to be mostly fine - it's not terribly fast when
> appends happen but everything works. The problem comes when doing a savepoint
> restore - specifically, the taskmanagers eat ram until the kernel kills it
> due to being out of memory. The extra memory isn't JVM heap since the memory
> usage of the process is ~4x the -Xmx value and there aren't any
> {{OutOfMemoryError}} exceptions.
> I traced the culprit of the memory growth to
> [RocksDBFullRestoreOperation.java#L212|https://github.com/apache/flink/blob/68910fa5381c8804ddbde3087a2481911ebd6d85/flink-state-backends/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/restore/RocksDBFullRestoreOperation.java#L212]
> . Specifically, while the keys/values are deserialized on the Java heap,
> {{RocksDBWriteBatchWrapper}} forwards it to RocksDB's {{WriteBatch}} which
> buffers in unmanaged memory. That's not in itself an issue, but
> {{RocksDBWriteBatchWrapper}} flushes only based on a number of records - not
> a number of bytes in-flight. Specifically, {{RocksDBWriteBatchWrapper}} will
> flush only once it has 500 records, and at 40mb per key, that's at least 20Gb
> of unmanaged memory before a flush.
> My suggestion would be to add an additional flush criteria to
> {{RocksDBWriteBatchWrapper}} - one based on {{batch.getDataSize()}} (e.g. 500
> records or 5mb buffered). This way large key writes would be immediately
> flushed to RocksDB on recovery or even writes. I applied this approach and I
> was able to complete a savepoint restore for my job. That said, I'm not
> entirely sure what else this change would impact since I'm not very familiar
> with Flink.
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