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From Lucas Wang <lucasatu...@gmail.com>
Subject Re: [VOTE] KIP-291: Have separate queues for control requests and data requests
Date Tue, 03 Jul 2018 00:17:03 GMT
Hi Ted,

1. I'm neutral on making the poll timeout parameter configurable.
Mainly because as a config, it could be confusing for operators who try to
choose a value for it.

To understand the implication of this value better,
let's use TO to represent the timeout value under discussion,
M to denote the processing time of data requests,
and N to be the number of io threads.

- If the data request queue is empty and there is no incoming data requests,
  all io threads should be blocked on the data request queue, and
  the average delay for a controller request is (TO / 2) / N, and the worst
case delay is TO.
- If all IO threads are busy processing data requests, then the average
latency for a controller request is M / N.
- In the worst case, a controller request can just miss the train, and IO
threads get blocked on data request queue
  for TO, at the end of which they all receive a new incoming data request,
the latency for the
  controller request can be TO + M.

Given the intricacies, what do you think about choosing a relatively
meaningful value and stick with it,
rather than exposing it as a config?

2. Sorry for losing the format of the table, I've attached it below as a
picture


Regards,
Lucas

On Fri, Jun 29, 2018 at 5:28 PM, Ted Yu <yuzhihong@gmail.com> wrote:

> bq. which is hard coded to be 300 milliseconds
>
> Have you considered making the duration configurable ?
>
> The comparison at the end of your email seems to be copied where tabular
> form is lost.
> Do you mind posting that part again ?
>
> Thanks
>
> On Fri, Jun 29, 2018 at 4:53 PM, Lucas Wang <lucasatucla@gmail.com> wrote:
>
> > Hi Jun,
> >
> > Thanks for your comments.
> > 1. I just replied in the discussion thread about the positive change this
> > KIP can still bring
> > if implemented on the latest trunk, which includes the async ZK
> operations
> > for KAFKA-5642.
> > The evaluation is done using an integration test.
> > In production, we have not upgraded to Kafka 1.1 yet, and the code we are
> > currently running does
> > not include async ZK operations, therefore I don't have any real usage
> > result.
> >
> > 2. Thanks for bringing this up. I haven't considered this setting, and
> the
> > existing proposal in this KIP
> > would make data requests and controller requests share a memory poll of
> > size specified by the config
> > queued.max.request.bytes. The downside is that if there is memory
> pressure,
> > controller requests may be blocked
> > from being read from a socket and does not get prioritized at the socket
> > layer.
> >
> > If we have a separate bytes limit for the controller requests, I imagine
> > there would be a separate memory pool
> > dedicated to controller requests. Also it requires the processors to tell
> > connections from a controller apart
> > from connections from other brokers or clients, which would probably
> > require a dedicated port for the controller?
> > IMO, this change is mainly driven by the memory pressure, kind of an
> > orthogonal issue, and we can address it with a separate KIP
> > if desired. Please let me know what you think.
> >
> > 3. I plans to change the implementation of the method
> > receiveRequest(timeout: Long) in the RequestChannel class as follows:
> >
> > val controllerRequest = controllerRequestQueue.poll()
> > if (controllerRequest != null) {
> >   controllerRequest
> > } else {
> >   dataRequestQueue.poll(timeout, TimeUnit.MILLISECONDS)
> > }
> >
> > with this implementation, there is no need to explicitly choose a request
> > handler thread to wake up depending on
> > the types of request enqueued, and if a controller request arrives while
> > some request handler threads are blocked on an empty data request queue,
> > they will simply timeout and call the receiveRequest method again.
> >
> > In terms of performance, it means that in the worst case, for a
> controller
> > request that just missed the receiveRequest call, it can be delayed for
> as
> > long as
> > the timeout parameter, which is hard coded to be 300 milliseconds. If
> there
> > is just one request handler thread, the average delay is
> > 150 milliseconds assuming the chance of a controller request arriving at
> > any particular time is the same. With N request handler threads,
> > the average delay is 150/N milliseconds, which does not seem to be a
> > problem.
> >
> > We have considered waking up of request handler threads based on which
> > queue the request handler threads are blocked,
> > and that design was turned down because of its complexity. The design can
> > be found at here
> > <https://cwiki.apache.org/confluence/display/KAFKA/Old+
> > controller+request+queue+design>
> > .
> >
> > If you mean a general purpose priority queue such as the
> > java.util.PriorityQueue, we also have considered it and turned down the
> > design because
> > - The readily available class java.util.PriorityQueue is unbounded and
> > we'll need to implement a bounded version
> > - We would still like to have the FIFO semantics on both the controller
> > request queue and data request queue, which conceptually does not fit
> very
> > well
> > with a general purpose priority queue, e.g. we would probably need to use
> > the enqueue time to enforce FIFO semantics.
> > - A typical operation on the priority queue is O(log n), whereas the
> sample
> > implementation above gives O(1) performance regardless of the size of
> both
> > queues.
> >
> > 4. For the two APIs sendRequest and receiveRequest, since we are only
> > changing their implementation, not the API itself
> > the two metrics will support two queues and the meaning of "Idle" still
> > holds:
> >
> >
> >
> >
> >
> >
> > *Before this KIPAfter this KIPNetworkProcessorAvgIdlePercentidle =
> blocked
> > on selectnot idle includes being blocked on requestQueueidle = blocked on
> > selectnot idle includes being blocked on either controller request queue
> or
> > data request queueRequestHandlerAvgIdlePercentidle = blocked on reading
> > from requestQueue idle = taking a request from the controller request
> > queue, or blocked on reading from the data request queue*
> >
> > Regards,
> > Lucas
> >
> > On Fri, Jun 29, 2018 at 11:22 AM, Jun Rao <jun@confluent.io> wrote:
> >
> > > Hi, Lucas,
> > >
> > > Thanks for the KIP. A few comments below.
> > >
> > > 1. As Eno mentioned in the discussion thread, I am wondering how much
> of
> > > this is still an issue after KAFKA-5642. With that fix, the requests
> from
> > > the controller to the brokers are batched in all the common cases. Have
> > you
> > > deployed Kafka 1.1? What's the request queue time and the request queue
> > > size that you have observed in production?
> > >
> > > 2. For the request queue, currently we can also bound it by size
> > > through queued.max.request.bytes. Should we consider the same for the
> > > control queue?
> > >
> > > 3. Implementation wise, currently the request handler threads just
> block
> > on
> > > the request queue when the queue is empty. With two queues, it seems
> that
> > > we need to wake up a request handler thread blocked on one queue, when
> > > another queue gets a request? Have we considered just making the
> request
> > > queue a priority queue?
> > >
> > > 4. Related to 3, currently we have 2
> > > metrics  NetworkProcessorAvgIdlePercent and
> RequestHandlerAvgIdlePercent
> > > that measure the utilization of the network and the request handler
> > thread
> > > pools. They are computed by measuring the amount of time waiting on the
> > > request queue. Will these 2 metrics be extended to support 2 request
> > > queues.
> > >
> > > Jun
> > >
> > >
> > > On Mon, Jun 18, 2018 at 1:04 PM, Lucas Wang <lucasatucla@gmail.com>
> > wrote:
> > >
> > > > Hi All,
> > > >
> > > > I've addressed a couple of comments in the discussion thread for
> > KIP-291,
> > > > and
> > > > got no objections after making the changes. Therefore I would like to
> > > start
> > > > the voting thread.
> > > >
> > > > KIP:
> > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-291%
> > > > 3A+Have+separate+queues+for+control+requests+and+data+requests
> > > >
> > > > Thanks for your time!
> > > > Lucas
> > > >
> > >
> >
>

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