Kihwal Lee created HDFS-8611:
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Summary: Improve the performance of retry cache eviction
Key: HDFS-8611
URL: https://issues.apache.org/jira/browse/HDFS-8611
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: Kihwal Lee
Priority: Critical
As discussed in HDFS-7609, removing expired entry from retry cache can be
costly. Following is the comment left by [~szetszwo] in HDFS-7609.
{quote}
PriorityQueue#remove is O\(n), so that definitely could be problematic. It's
odd that there would be so many collisions that this would become noticeable
though. Are any of you running a significant number of legacy applications
linked to the RPC code before introduction of the retry cache support? If that
were the case, then perhaps a huge number of calls are not supplying a call ID,
and then the NN is getting a default call ID value from protobuf decoding, thus
causing a lot of collisions.
{quote}
The priority queue can be improved using a balanced tree as stated in the java
comment in LightWeightCache. We should do it if it could fix the problem.
{code}
//LightWeightCache.java
/*
* The memory footprint for java.util.PriorityQueue is low but the
* remove(Object) method runs in linear time. We may improve it by using a
* balanced tree. However, we do not yet have a low memory footprint balanced
* tree implementation.
*/
private final PriorityQueue<Entry> queue;
{code}
BTW, the priority queue is used to evict entries according the expiration time.
All the entries (with any key, i.e. any caller ID) are stored in it.
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