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From "Peter Schuller (JIRA)" <j...@apache.org>
Subject [jira] Created: (CASSANDRA-1955) memtable flushing can block writes due to queue size limitations even though overall write throughput is below capacity
Date Sat, 08 Jan 2011 18:33:45 GMT
memtable flushing can block writes due to queue size limitations even though overall write
throughput is below capacity
-----------------------------------------------------------------------------------------------------------------------

                 Key: CASSANDRA-1955
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1955
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Peter Schuller


It seems that someone ran into this (see cassandra-user thread "Question re: the use of multiple
ColumnFamilies").

If my interpretation is correct, the queue size is set to the concurrency in the case of the
flushSorter, and set to memtable_flush_writers in the case of flushWriter in ColumnFamilyStore.

While the choice of concurrency for the two executors makes perfect sense, the queue sizing
does not. As a user, I would expect, and did expect, that for a given memtable independently
tuned (w.r.t. flushing thresholds etc), writes to the CF would not block until there is at
least one other memtable *for that CF* waiting to be flushed.

With the current behavior, if I am not misinterpreting, whether or not writes will inappropriately
block is very much dependent on not just the overall write throughput, but also the incidental
timing of memtable flushes across multiple column families.

The simplest way to mitigate (but not fix) this is probably to set the queue size to be equal
to the number of column families if that is higher than the number of CPU cores. But that
is only a mitigation because nothing prevents e.g. a large number of memtable flushes for
a small column family under temporary write load, can still block a large (possibly more important)
memtable flush for another CF. Such a shared-but-larger queue would also not prevent heap
usage spikes resulting from some a single cf with very large memtable thresholds being rapidly
written to, with a queue sized for lots of cf:s that are in practice not used. In other words,
this mitigation technique would effectively negate the backpressure mechanism in some cases
and likely lead to more people having OOM issues when saturating a CF with writes.

A more involved change is to make each CF have it's own queue through which flushes go prior
to being submitted to flushSorter, which would guarantee that at least one memtable can always
be in pending flush state for a given CF. The global queue could effectively have size 1 hard-coded
since the queue is no longer really used as if it were a queue.

The flushWriter is unaffected since it is a separate concern that is supposed to be I/O bound.
The current behavior would not be perfect if there is a huge discrepancy between memtable
flush thresholds of different memtables, but it does not seem high priority to make a change
here in practice.

So, I propose either:

(a) changing the flushSorter queue size to be max(num cores, num cfs)
(b) creating a per-cf queue

I'll volunteer to work on it as a nice bite sized change, assuming there is agreement on what
needs to be done. Given the concerns with (a), I think (b) is the right solution unless it
turns out to cause major complexity. Worth noting is that these are not performance sensitive
given the low frequency of memtable flushes, so an extra queue:ing step should not be an issue.


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