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https://issues.apache.org/jira/browse/KAFKA-747?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Guozhang Wang updated KAFKA-747:
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Fix Version/s: (was: 0.8.2)
0.9.0
> RequestChannel re-design
> ------------------------
>
> Key: KAFKA-747
> URL: https://issues.apache.org/jira/browse/KAFKA-747
> Project: Kafka
> Issue Type: New Feature
> Components: network
> Reporter: Jay Kreps
> Assignee: Neha Narkhede
> Fix For: 0.9.0
>
>
> We have had some discussion around how to handle queuing requests. There are
> two competing concerns:
> 1. We need to maintain request order on a per-socket basis.
> 2. We want to be able to balance load flexibly over a pool of threads so that
> if one thread blocks on I/O request processing continues.
> Two Approaches We Have Considered
> 1. Have a global queue of unprocessed requests. All I/O threads read requests
> off this global queue and process them. To avoid re-ordering have the network
> layer only read one request at a time.
> 2. Have a queue per I/O thread and have the network threads statically map
> sockets to I/O thread request queues.
> Problems With These Approaches
> In the first case you are not able to get any per-producer parallelism. That
> is you can't read the next request while the current one is being handled.
> This seems like it would not be a big deal, but preliminary benchmarks show
> that it might be.
> In the second case there are two problems. The first is that when an I/O
> thread gets blocked all request processing for sockets attached to that I/O
> thread will grind to a halt. If you have 10,000 connections, and 10 I/O
> threads, then each blockage will stop 1,000 producers. If there is one topic
> that has long synchronous flush times enabled (or is experiencing fsync
> locking) this will cause big latency blips for all producers using that I/O
> thread. The next problem is around backpressure and memory management. Say we
> use BlockingQueues to feed the I/O threads. And say that one I/O thread
> stalls. It's request queue will fill up and it will then block ALL network
> threads, since they will block on inserting into that queue, even though the
> other I/O threads are unused and have empty queues.
> A Proposed Better Solution
> The problem with the first solution is that we are not pipelining requests.
> The problem with the second approach is that we are too constrained in moving
> work from one I/O thread to another.
> Instead we should have a single request queue-like structure, but internally
> enforce the condition that requests are not re-ordered.
> Here are the details. We retain RequestChannel but refactor its internals.
> Internally we replace the blocking queue with a linked list. We also keep an
> in-flight-keys array with one entry per I/O thread. When removing a work item
> from the list we can't just take the first thing. Instead we need to walk the
> list and look for something with a request key not in the in-flight-keys
> array. When a response is sent, we remove that key from the in-flight array.
> This guarantees that requests for a socket with key K are ordered, but that
> processing for K can only block requests made by K.
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