Details
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Improvement
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Status: Resolved
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Major
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Resolution: Not A Problem
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None
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None
Description
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:
16 readahead threads * 200 deep copied children * 99% cache miss rate * 20 files * 252KB per reader = ~16GB of memory
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.
