[
https://issues.apache.org/jira/browse/CASSANDRA-1876?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12994459#comment-12994459
]
Jonathan Ellis commented on CASSANDRA-1876:
-------------------------------------------
A while ago I said:
bq. ideally we would parallelize within a single sstable (breaking out the
deserialize / merge / write stages) but this is Hard.
It's hard, but for a lot of users (anyone where a single CF holds the bulk of
the data) this is the only kind of optimization that will make a difference.
There are five stages: read, deserialize, merge, serialize, and write. We
probably want to continue doing read+deserialize and serialize+write together,
or you waste a lot copying to/from buffers.
So, what I would suggest is: one thread per input sstable doing read +
deserialize (a row at a time). One thread merging corresponding rows from each
input sstable. One thread doing serialize + writing the output. This should
give us between 2x and 3x speedup (depending how much doing the merge on
another thread than write saves us).
This will require roughly 2x the memory, to allow the reader threads to work
ahead of the merge stage. (I.e. for each input sstable you will have up to one
row in a queue waiting to be merged, and the reader thread working on the
next.) Seems quite reasonable on that front.
Multithreaded compaction should be either on or off. It doesn't make sense to
try to do things halfway (by doing the reads with a
threadpool whose size you can grow/shrink, for instance): we still have
compaction threads tuned to low priority, by default, so the impact on the rest
of the system won't be very different. Nor do we expect to have so many input
sstables that we lose a lot in context switching between reader threads. (If
this is a concern, we already have a tunable to limit the number of sstables
merged at a time in a single CF.)
IMO it's acceptable to punt completely on rows that are larger than memory, and
fall back to the old non-parallel code there. I don't see any sane way to
parallelize large-row compactions.
> Allow minor Parallel Compaction
> -------------------------------
>
> Key: CASSANDRA-1876
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1876
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Germán Kondolf
> Priority: Minor
> Attachments: 1876-reformatted.txt, compactionPatch-V2.txt,
> compactionPatch-V3.txt
>
>
> Hi,
> According to the dev's list discussion (1) I've patched the CompactionManager
> to allow parallel compaction.
> Mainly it splits the sstables to compact in the desired buckets, configured
> by a new parameter: compaction_parallelism with the current default of "1".
> Then, it just submits the units of work to a new executor and waits for the
> finalization.
> The patch was created in the trunk, so I don't know the exact affected
> version, I assume that is 0.8.
> I'll try to apply this patch to 0.6.X also for my current production
> installation, and then reattach it.
> (1) http://markmail.org/thread/cldnqfh3s3nufnke
--
This message is automatically generated by JIRA.
-
For more information on JIRA, see: http://www.atlassian.com/software/jira
