Jay Shrauner created ZOOKEEPER-1505:
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Summary: Multi-thread CommitProcessor
Key: ZOOKEEPER-1505
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1505
Project: ZooKeeper
Issue Type: Improvement
Components: server
Reporter: Jay Shrauner
Assignee: Jay Shrauner
Attachments: ZOOKEEPER-1505.patch
CommitProcessor has a single thread that both pulls requests off its queues and
runs all downstream processors. This is noticeably inefficient for
read-intensive workloads, which could be run concurrently. The trick is
handling write transactions. I propose multi-threading this code according to
the following two constraints
- each session must see its requests responded to in order
- all committed transactions must be handled in zxid order, across all
sessions
I believe these cover the only constraints we need to honor. In particular, I
believe we can relax the following:
- it does not matter if the read request in one session happens before or
after the write request in another session
With these constraints, I propose the following threads
- 1 primary queue servicing/work dispatching thread
- 0-N assignable worker threads, where a given session is always assigned to
the same worker thread
By assigning sessions always to the same worker thread (using a simple
sessionId mod number of worker threads), we guarantee the first
constraint--requests we push onto the thread queue are processed in order. The
way we guarantee the second constraint is we only allow a single commit
transaction to be in flight at a time--the queue servicing thread blocks while
a commit transaction is in flight, and when the transaction completes it clears
the flag.
On a 32 core machine running Linux 2.6.38, achieved best performance with 32
worker threads for a 56% +/- 5% improvement in throughput (this improvement was
measured on top of that for ZOOKEEPER-1504, not in isolation).
New classes introduced in this patch are:
WorkerService (also in ZOOKEEPER-1504): ExecutorService wrapper that makes
worker threads daemon threads and names then in an easily debuggable manner.
Supports assignable threads (as used here) and non-assignable threads (as used
by NIOServerCnxnFactory).
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