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https://issues.apache.org/jira/browse/CASSANDRA-11327?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15189372#comment-15189372
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Ariel Weisberg commented on CASSANDRA-11327:
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Benedict I don't follow how it adds latency? The threads are already blocked on
the lack of memory. What it could be construed to do is reduce the total memory
available for memtables since it's faking it via memory accounting instead of
actually reclaiming memory. During saturating load all available memtable
memory will be filled pretty quickly and then it will stay that way forever.
>From the perspective of the user a sawtooth that doesn't go to zero is better
>than a sawtooth that goes to zero for extended periods. If you are saying we
>should actually reclaim the memory instead of doing it via accounting well
>yeah I agree. I wasn't aware the partially off heap and off heap memtables
>were able to reclaim memory incrementally during flushing.
> Maintain a histogram of times when writes are blocked due to no available
> memory
> --------------------------------------------------------------------------------
>
> Key: CASSANDRA-11327
> URL: https://issues.apache.org/jira/browse/CASSANDRA-11327
> Project: Cassandra
> Issue Type: New Feature
> Components: Core
> Reporter: Ariel Weisberg
>
> I have a theory that part of the reason C* is so sensitive to timeouts during
> saturating write load is that throughput is basically a sawtooth with valleys
> at zero. This is something I have observed and it gets worse as you add 2i to
> a table or do anything that decreases the throughput of flushing.
> I think the fix for this is to incrementally release memory pinned by
> memtables and 2i during flushing instead of releasing it all at once. I know
> that's not really possible, but we can fake it with memory accounting that
> tracks how close to completion flushing is and releases permits for
> additional memory. This will lead to a bit of a sawtooth in real memory
> usage, but we can account for that so the peak footprint is the same.
> I think the end result of this change will be a sawtooth, but the valley of
> the sawtooth will not be zero it will be the rate at which flushing
> progresses. Optimizing the rate at which flushing progresses and it's
> fairness with other work can then be tackled separately.
> Before we do this I think we should demonstrate that pinned memory due to
> flushing is actually the issue by getting better visibility into the
> distribution of instances of not having any memory by maintaining a histogram
> of spans of time where no memory is available and a thread is blocked.
> [MemtableAllocatr$SubPool.allocate(long)|https://github.com/apache/cassandra/blob/cassandra-3.0/src/java/org/apache/cassandra/utils/memory/MemtableAllocator.java#L186]
> should be a relatively straightforward entry point for this. The first
> thread to block can mark the start of memory starvation and the last thread
> out can mark the end. Have a periodic task that tracks the amount of time
> spent blocked per interval of time and if it is greater than some threshold
> log with more details, possibly at debug.
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