Michael Griggs created IGNITE-4828:
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Summary: Improve the distribution of keys within partitions
Key: IGNITE-4828
URL: https://issues.apache.org/jira/browse/IGNITE-4828
Project: Ignite
Issue Type: Improvement
Affects Versions: 1.9
Reporter: Michael Griggs
Fix For: 2.0
An issue has been found when inserting several million string keys in to a
cache. Each string key was approximately 22-characters in length. When I
exported the partition counts (via GG Visor) I was able to see an unusual
periodicity in the number of keys allocated to partitions. I charted this in
Excel (1).
After further investigation, it appears that there is a relationship
between the number of keys being inserted, the number of partitions
assigned to the cache and amount of apparent periodicity: a small number
ofpartitions will cause periodicity to appear with a lower number of keys.
The {{RendezvousAffinityFunction#partition}} function performs a simple
calculation of key hashcode modulo partition-count:
{{U.safeAbs(key.hashCode() % parts)}}
Digging further I was led to the fact that this is how the Java HashMap
*used* to behave (2), but was upgraded around Java 1.4 to perform the
following:
{{key.hashCode() & (parts - 1)}}
which performs more efficiently. It was then updated further to do the
following:
{{(h = key.hashCode()) ^ (h >>> 16);}}
with the bit-shift performed to
bq. incorporate impact of the highest bits that would otherwise never be used
in index calculations because of table bounds
When using this function, rather than our
{{RendezvousAffinityFunction#partition}} implementation, I also saw a
significant decrease in the periodicity and a better distribution of keys
amongst partitions (3).
(1): https://i.imgur.com/0FtCZ2A.png
(2):
https://www.quora.com/Why-does-Java-use-a-mediocre-hashCode-implementation-for-strings
(3): https://i.imgur.com/8ZuCSA3.png
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