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From Gábor Gévay <gga...@gmail.com>
Subject Memory manager behavior in iterative jobs
Date Sat, 30 Jan 2016 13:04:07 GMT
Hello!

We have a strangely behaving iterative Flink job: when we give it more
memory, it gets much slower (more than 10 times). The problem seems to
be mostly caused by GCs. Enabling object reuse didn’t help.

With some profiling and debugging, we traced the problem to the
operators requesting new memory segments from the memory manager at
every superstep of the iteration, and the memory manager satisfying
these requests by allocating new memory segments from the Java heap
[1], and then the old ones have to be eventually reclaimed by garbage
collections. We found the option “taskmanager.memory.preallocate”,
which mostly solved the GC problem, but we would like to understand
the situation better.

What is the reason for the default value of this setting being false?
Is there a downside to enabling this option? If the only downside is
the slower startup of the task managers, then we could have the best
of both worlds, by modifying the logic of the memory manager to use
pooling only after releases. I mean the memory manager would give the
segments back to the pool when the operators release them even when
“preallocate” is false, and then `allocatePages` would use a new
method of the memory pool, which would first check if there are
segments in the pool and calls `allocateNewSegment` or
`requestSegmentFromPool` accordingly. (Instead of the current
behaviour, which is to basically disable pooling, when the
“preallocate” setting is false.)

[1] https://github.com/apache/flink/blob/master/flink-runtime/src/main/java/org/apache/flink/runtime/memory/MemoryManager.java#L293-L307

Best,
Gábor and Márton

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