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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8833) Stop opening compaction results early
Date Thu, 19 Feb 2015 16:08:11 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8833?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14327678#comment-14327678
] 

Benedict commented on CASSANDRA-8833:
-------------------------------------

Well, it certainly has turned out to be more complicated than expected. In particular "shadowed"
is something I would have liked to avoid.

The mistake I made when implementing this, apart from not realising LCS required no overlaps,
was to not expend a great deal of effort making it _more_ robust than the existing solution.
Many of the problems encountered here have exhibited in the prior framework, or in other new
pieces that are unrelated, just less frequently. Cleanup of compaction has been buggy since
time immemorial; bloom filters, index summaries and entire sstables have been left allocated
indefinitely until very recent fixes. Index summary redistribution has had all of the same
DataTracker problems. So an argument could be made that this is actually helping us out, by
exercising these codepaths more aggressively. After a major overhaul, there was an oversight
(CASSANDRA-8764) which is probably to be expected, and is the impetus for this ticket. This
overhaul has fixed numerous problems prior as well as new.

Removing this will introduce its own risks (every line touched is a new bug in waiting), and
doesn't eliminate as much complexity as might be desired. SSTableReader lifecycles are still
approximately as complex due to index summary redistribution. Bloom filter resizing (CASSANDRA-6633)
will have the same requirement. The same goes for shared resources.

Reference counting has been broken for a long time, and we are now in a safer position than
we have ever been (although improvements are still needed, such as CASSANDRA-8568), and the
latest bugs we are finding in 2.0 are known not to affect 2.1. We wouldn't be there without
this as an impetus.

So, many of the changes will not be rolled back, because they were necessary in themselves.
They were just less pressing. There would still be a significant removal of code, though.

As to the improvement: the compaction "cliff" can be pretty significant if you're compacting
huge files. Possibly introducing several minutes (or longer) of seriously degraded performance.
If you compact tens of gigabytes you may see your entire page cache wiped out. Even with SSDs
that's going to be painful for a period. Without SSDs it could be tens of minutes.

Other than that, I'll let everyone else reach a conclusion about this.

> Stop opening compaction results early
> -------------------------------------
>
>                 Key: CASSANDRA-8833
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8833
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Marcus Eriksson
>             Fix For: 2.1.4
>
>
> We should simplify the code base by not doing early opening of compaction results. It
makes it very hard to reason about sstable life cycles since they can be in many different
states, "opened early", "starts moved", "shadowed", "final", instead of as before, basically
just one (tmp files are not really 'live' yet so I don't count those). The ref counting of
shared resources between sstables in these different states is also hard to reason about.
This has caused quite a few issues since we released 2.1
> I think it all boils down to a performance vs code complexity issue, is opening compaction
results early really 'worth it' wrt the performance gain? The results in CASSANDRA-6916 sure
look like the benefits are big enough, but the difference should not be as big for people
on SSDs (which most people who care about latencies are)
> WDYT [~benedict] [~jbellis] [~iamaleksey] [~JoshuaMcKenzie]?



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