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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6916) Preemptive opening of compaction result
Date Tue, 08 Apr 2014 16:34:23 GMT


Benedict commented on CASSANDRA-6916:

bq. I know intuitively it feels like it would be a big win, not only for the page cache, but
also since we get to only read one sstable for the compacted keys, but still would be nice
to have some evidence.

It would also be interesting to test this patch out against a constrained memory scenario,
and see how well it behaves, as it should occupy much less memory at any given moment during
a compaction. However, I think for the moment confirming we get the expected outcome of fixing
CASSANDRA-6746 is enough in my book. I'll ask Russ to run that test once I've addressed your
points below.

# Problem with fully expired sstables in CompactionTask, we need to replace "toCompact" instead
of "actuallyCompact".
# KeyCacheTest fails (not had time to look deeper into it)
# A few comments around the replaceReader/replaceCompacted... and related in CompactionTask
would be helpful, a bit harder to grasp now that we start using the compacted sstables earlier
and then call DataTracker.replace(...) with an empty list.
# In MMappedSegmentedFile, line 199, it looks like that ternary expression is not really needed
since we add raf.length() in boundaries above?
# Preheat comment left in CompactionTask

Good points, thanks. I'll check the failing test.

bq. In SSTR.cloneWithNewStart(..), CLibrary.trySkipCache(dfile.path, 0, 0); is called when
newStart > last, is that on purpose? I guess we could drop the page cache for the whole
file in this case?

That's the idea, yes. A parameter of 0 indicates to drop the whole file.

> Preemptive opening of compaction result
> ---------------------------------------
>                 Key: CASSANDRA-6916
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>            Priority: Minor
>             Fix For: 2.1
> Related to CASSANDRA-6812, but a little simpler: when compacting, we mess quite badly
with the page cache. One thing we can do to mitigate this problem is to use the sstable we're
writing before we've finished writing it, and to drop the regions from the old sstables from
the page cache as soon as the new sstables have them (even if they're only written to the
page cache). This should minimise any page cache churn, as the old sstables must be larger
than the new sstable, and since both will be in memory, dropping the old sstables is at least
as good as dropping the new.
> The approach is quite straight-forward. Every X MB written:
> # grab flushed length of index file;
> # grab second to last index summary record, after excluding those that point to positions
after the flushed length;
> # open index file, and check that our last record doesn't occur outside of the flushed
length of the data file (pretty unlikely)
> # Open the sstable with the calculated upper bound
> Some complications:
> # must keep running copy of compression metadata for reopening with
> # we need to be able to replace an sstable with itself but a different lower bound
> # we need to drop the old page cache only when readers have finished

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