[jira] [Created] (FLINK-12785) RocksDB savepoint recovery can use a lot of unmanaged memory

["Mike Kaplinskiy (JIRA)" <ji...@apache.org>]

Mike Kaplinskiy created FLINK-12785:
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             Summary: 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


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 managed 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 jon. 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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