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From "Alex Rudyy (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (QPID-7918) [Broker-J] Respond to low heap memory condition by throttling producers.
Date Mon, 10 Dec 2018 15:36:00 GMT

    [ https://issues.apache.org/jira/browse/QPID-7918?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16714937#comment-16714937

Alex Rudyy commented on QPID-7918:

I played with idea of listening for notifications from  MemoryPoolMXBean and GC MBean Full
GC events to get the heap usage and enforce flow control if required, but, I could not make
it working reliably. The approach worked more or less OK with java8 and CMS GC but it did
not work that well with java 11 and G1 GC. With G1 and java11, the full GC notifications are
not issued reliably on running out of heap memory. As result, the flow control was not enforced
and broker was crashing with OOM errors.

I think, that better approach would be to estimate the heap usage by queue entries by implementing
special methods on queues like {{estimateHeapUsage}}. Thus, every queue can estimate the consumed
heap size. I looked into this approach as well but it seems it is a bit hard to get accurate
estimation of heap usage, as it is requires to traverse the object graph in memory. The latter
can be achieved either by changing field accessibility or by calling some methods on related
objects to get their heap estimation.  The former results in warnings in java11 about inappropriate
use of API and the latter requires that each QueueEntry and its ServerMessage and ServerMessageMetadata
implementations would have to provide methods for estimating  of heap usage. I attached the
in-progress patch with the latter approach. The patch requires more work but it illustrates
the idea.

I am going to descope this JIRA from 7.1 and try to implement this JIRA in one of follow up

> [Broker-J] Respond to low heap memory condition by throttling producers.
> ------------------------------------------------------------------------
>                 Key: QPID-7918
>                 URL: https://issues.apache.org/jira/browse/QPID-7918
>             Project: Qpid
>          Issue Type: Improvement
>          Components: Broker-J
>            Reporter: Keith Wall
>            Priority: Major
>             Fix For: qpid-java-broker-7.1.0
> Currently, if the cumulative size of messages headers/contents cached in direct memory
exceeds a threshold, the Java Broker responds by flowing the messages to disk and evicting
them from memory.  This guards the condition of excessive *message payload* causing an out-of-memory
> However the Broker also maintains a linked list representing every queues in the JVMs
heap memory.   Each message is represented by a small node in the linked list.  The Broker's
design currently means that the entire linked list must reside in (heap) memory (even if the
message's content has been paged out by the flow to disk mechanism).  This gives rise to the
the possibility that the linked lists backing the queue could exhaust all heap and the Broker
could fail with an OOM (heap) condition. 
> The long term aim is to allow sections linked list to be paged out to eliminate this
problem, but this would necessitate a deep redesign.
> As a interim measure, it appears that the the JVM's Memory bean (MemoryPoolMXBean) could
be used to notify the Broker when after a garbage collection cycle heap memory usage is above
a threshold.  The Broker could respond to this event by throttling producer, thus allow consumes
to catch up and hopefully avoiding the failure.  The Broker would need to relinquish flow
control once memory usage had fallen again.
> Consideration would need to be given for different JVMs and different GC implementations.
Consideration must also be given for use cases where the Broker is embedded.

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