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https://issues.apache.org/jira/browse/ZOOKEEPER-1505?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13526935#comment-13526935
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Alexander Shraer commented on ZOOKEEPER-1505:
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I think that I don't fully understand how the watch scenario you mention
violates the guaranteed semantics. Could you please explain ?
What's important is the serialization order of the leader (not follower). The
read of B could just as well have been received by the follower before the
write of client A, or even A and B could be connected to different followers.
Reordering of B's read with A's write can't be allowed only if B has a prior
write that the leader scheduled after the write of A - in this case the read of
B must see the write of A, because the local program order of B and the
leader's serialization of the writes require this.
Thanks,
Alex
> Multi-thread CommitProcessor
> ----------------------------
>
> Key: ZOOKEEPER-1505
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1505
> Project: ZooKeeper
> Issue Type: Improvement
> Components: server
> Affects Versions: 3.4.3, 3.4.4, 3.5.0
> Reporter: Jay Shrauner
> Assignee: Jay Shrauner
> Labels: performance, scaling
> Fix For: 3.5.0
>
> Attachments: ZOOKEEPER-1505.patch, ZOOKEEPER-1505.patch,
> ZOOKEEPER-1505.patch, 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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