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https://issues.apache.org/jira/browse/ACCUMULO-4626?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15975517#comment-15975517
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Adam Fuchs commented on ACCUMULO-4626:
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Basically, the eviction thread is separate, and the work that it has to do to
evict a set of blocks relative to the work done in the iterators is small. It
is technically a race condition (at least from a performance perspective), and
the cache eviction thread wins the race. I believe the core condition needed to
trigger this is that the sum of sizes of the referenced blocks across all of
the concurrently running queries exceeds the 25% or so of the total cache that
is reserved for single-use blocks. We were able to work around it in this case
by increasing the total block cache size, but that's not necessarily always a
viable solution.
> improve cache hit rate via weak reference map
> ---------------------------------------------
>
> Key: ACCUMULO-4626
> URL: https://issues.apache.org/jira/browse/ACCUMULO-4626
> Project: Accumulo
> Issue Type: Improvement
> Components: tserver
> Reporter: Adam Fuchs
> Labels: performance, stability
> Time Spent: 10m
> Remaining Estimate: 0h
>
> When a single iterator tree references the same RFile blocks in different
> branches we sometimes get cache misses for one iterator even though the
> requested block is held in memory by another iterator. This is particularly
> important when using something like the IntersectingIterator to intersect
> many deep copies. Instead of evicting completely, keeping evicted blocks into
> a WeakReference value map can avoid re-reading blocks that are currently
> referenced by another deep copied source iterator.
> We've seen this in the field for some of Sqrrl's queries against very large
> tablets. The total memory usage for these queries can be equal to the size of
> all the iterator block reads times the number of readahead threads times the
> number of files times the number of IntersectingIterator children when cache
> miss rates are high. This might work out to something like:
> {code}
> 16 readahead threads * 200 deep copied children * 99% cache miss rate * 20
> files * 252KB per reader = ~16GB of memory
> {code}
> In most cases, evicting to a weak reference value map changes the cache miss
> rate from very high to very low and has a dramatic effect on total memory
> usage.
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