Paul Chandler created CASSANDRA-19785:
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Summary: Possible memory leak in BTree.FastBuilder
Key: CASSANDRA-19785
URL: https://issues.apache.org/jira/browse/CASSANDRA-19785
Project: Cassandra
Issue Type: Bug
Reporter: Paul Chandler
Attachments: image-2024-07-19-08-44-56-714.png,
image-2024-07-19-08-45-17-289.png, image-2024-07-19-08-45-33-933.png,
image-2024-07-19-08-45-50-383.png, image-2024-07-19-08-46-06-919.png,
image-2024-07-19-08-46-42-979.png, image-2024-07-19-08-46-56-594.png,
image-2024-07-19-08-47-19-517.png, image-2024-07-19-08-47-34-582.png
We are having a problem with the heap growing in size, This is a large cluster
> 1,000 nodes across a large number of dc’s.
Each node has a 32GB heap, and the amount used continues to grow until it
reaches 30GB, it then struggles with multiple Full GC pauses, as can be seen
here:
!image-2024-07-19-08-44-56-714.png!
We took 2 heap dumps on one node a few days after it was restarted, and the
heap had grown by 2.7GB
9{^}th{^} July
!image-2024-07-19-08-45-17-289.png!
11{^}th{^} July
!image-2024-07-19-08-45-33-933.png!
This can be seen as mainly an increase of memory used by FastThreadLocalThread,
increasing from 5.92GB to 8.53GB
!image-2024-07-19-08-45-50-383.png!
!image-2024-07-19-08-46-06-919.png!
Looking deeper into this it can be seen that the growing heap is contained
within the threads for the MutationStage, Native-transport-Requests, ReadStage
etc. We would expect the memory used within these threads to be short lived,
and not grow as time goes on. We recently increased the size of theses
threadpools, and that has increased the size of the problem.
Top memory usage for FastThreadLocalThread
9{^}th{^} July
!image-2024-07-19-08-46-42-979.png!
11{^}th{^} July
!image-2024-07-19-08-46-56-594.png!
This has led us to investigate whether there could be a memory leak, and we
have found the following issues within the retained references in
BTree.FastBuilder objects. The issue appears to stem from the reset() method,
which does not properly clear all buffers. We are not really sure how the
BTree.FastBuilder works, but this this is our analysis of where a leak might
occur.
Specifically:
Leaf Buffer Not Being Cleared:
When leaf().count is 0, the statement Arrays.fill(leaf().buffer, 0,
leaf().count, null); does not clear the buffer because the end index is 0. This
leaves the buffer with references to potentially large objects, preventing
garbage collection and increasing heap usage.
Branch inUse Property:
If the inUse property of the branch is set to false elsewhere in the code, the
while loop while (branch != null && branch.inUse) does not execute, resulting
in uncleared branch buffers and retained references.
This is based on the following observations:
Heap Dumps: Analysis of heap dumps shows that leaf().count is often 0, and
as a result, the buffer is not being cleared, leading to high heap utilization.
!image-2024-07-19-08-47-19-517.png!
Remote Debugging: Debugging sessions indicate that the drain() method sets
count to 0, and the inUse flag for the parent branch is set to false,
preventing the while loop in reset() from clearing the branch buffers.
!image-2024-07-19-08-47-34-582.png!
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