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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-11327) Maintain a histogram of times when writes are blocked due to no available memory
Date Thu, 10 Mar 2016 17:23:40 GMT


Benedict commented on CASSANDRA-11327:

Perhaps you should outline precisely the algorithm you propose, since there's a whole class
of similar algorithms and it would narrow the discussion?

But the statement that you are reducing the total memory available for memtables must by definition
increase latency for those writes that would have been fully accommodated by the full buffer
capacity (and no longer can due to artificial reduction).  The only way this does not affect
latency is when the cluster is overloaded - which admittedly all of our performance tests
induce, despite this being completely not what Cassandra is designed for.

Memtables are there to smooth out the natural variance in the message arrival distribution.
 A properly tuned cluster would ensure that overload occurs only some SLA frequency, say 3
sigma chance.  By reducing their size, transient overload becomes more frequent, and SLAs
are not met or the cluster capacity must be increased.  Now, a Cassandra cluster simply _cannot_
cope with sustained overload, no matter what we do here; LSMTs seal our fate very rapidly
in that situation.  So I don't personally see the rationale for making transient overload
(Cassandra's strong suit) worse, in exchange for a really temporary reprieve on sustained

bq. I wasn't aware the partially off heap and off heap memtables were able to reclaim memory
incrementally during flushing.

They aren't, but the patch I linked introduced this against a pre-2.1 branch.  It wasn't exactly
trivial to do, though (it introduced a constrained pauseless compacting GC), and it is probably
better to wait until TPC to think about reattempting this.

> Maintain a histogram of times when writes are blocked due to no available memory
> --------------------------------------------------------------------------------
>                 Key: CASSANDRA-11327
>                 URL:
>             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)|]
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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