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From Yuheng Du <yuheng.du.h...@gmail.com>
Subject Re: latency test
Date Fri, 04 Sep 2015 18:06:36 GMT
No problem. Thanks for your advice. I think it would be fun to explore. I
only know how to program in java though. Hope it will work.

On Fri, Sep 4, 2015 at 2:03 PM, Helleren, Erik <Erik.Helleren@cmegroup.com>
wrote:

> I thing the suggestion is to have partitions/brokers >=1, so 32 should be
> enough.
>
> As for latency tests, there isn’t a lot of code to do a latency test.  If
> you just want to measure ack time its around 100 lines.  I will try to
> push out some good latency testing code to github, but my company is
> scared of open sourcing code… so it might be a while…
> -Erik
>
>
> On 9/4/15, 12:55 PM, "Yuheng Du" <yuheng.du.hust@gmail.com> wrote:
>
> >Thanks for your reply Erik. I am running some more tests according to your
> >suggestions now and I will share with my results here. Is it necessary to
> >use a fixed number of partitions (32 partitions maybe) for my test?
> >
> >I am testing 2, 4, 8, 16 and 32 brokers scenarios, all of them are running
> >on individual physical nodes. So I think using at least 32 partitions will
> >make more sense? I have seen latencies increase as the number of
> >partitions
> >goes up in my experiments.
> >
> >To get the latency of each event data recorded, are you suggesting that I
> >rewrite my own test program (in Java perhaps) or I can just modify the
> >standard test program provided by kafka (
> >https://gist.github.com/jkreps/c7ddb4041ef62a900e6c )? I guess I need to
> >rebuild the source if I modify the standard java test program
> >ProducerPerformance provided in kafka, right? Now this standard program
> >only has average latencies and percentile latencies but no per event
> >latencies.
> >
> >Thanks.
> >
> >On Fri, Sep 4, 2015 at 1:42 PM, Helleren, Erik
> ><Erik.Helleren@cmegroup.com>
> >wrote:
> >
> >> That is an excellent question!  There are a bunch of ways to monitor
> >> jitter and see when that is happening.  Here are a few:
> >>
> >> - You could slice the histogram every few seconds, save it out with a
> >> timestamp, and then look at how they compare.  This would be mostly
> >> manual, or you can graph line charts of the percentiles over time in
> >>excel
> >> where each percentile would be a series.  If you are using HDR
> >>Histogram,
> >> you should look at how to use the Recorder class to do this coupled
> >>with a
> >> ScheduledExecutorService.
> >>
> >> - You can just save the starting timestamp of the event and the latency
> >>of
> >> each event.  If you put it into a CSV, you can just load it up into
> >>excel
> >> and graph as a XY chart.  That way you can see every point during the
> >> running of your program and you can see trends.  You want to be careful
> >> about this one, especially of writing to a file in the callback that
> >>kfaka
> >> provides.
> >>
> >> Also, I have noticed that most of the very slow observations are at
> >> startup.  But don’t trust me, trust the data and share your findings.
> >> Also, having a 99.9 percentile provides a pretty good standard for
> >>typical
> >> poor case performance.  Average is borderline useless, 50%’ile is a
> >>better
> >> typical case because that’s the number that says “half of events will be
> >> this slow or faster”, or for values that are high like 99.9%’ile, “0.1%
> >>of
> >> all events will be slower than this”.
> >> -Erik
> >>
> >> On 9/4/15, 12:05 PM, "Yuheng Du" <yuheng.du.hust@gmail.com> wrote:
> >>
> >> >Thank you Erik! That's is helpful!
> >> >
> >> >But also I see jitters of the maximum latencies when running the
> >> >experiment.
> >> >
> >> >The average end to acknowledgement latency from producer to broker is
> >> >around 5ms when using 92 producers and 4 brokers, and the 99.9
> >>percentile
> >> >latency is 58ms, but the maximum latency goes up to 1359 ms. How to
> >>locate
> >> >the source of this jitter?
> >> >
> >> >Thanks.
> >> >
> >> >On Fri, Sep 4, 2015 at 10:54 AM, Helleren, Erik
> >> ><Erik.Helleren@cmegroup.com>
> >> >wrote:
> >> >
> >> >> WellŠ not to be contrarian, but latency depends much more on the
> >>latency
> >> >> between the producer and the broker that is the leader for the
> >>partition
> >> >> you are publishing to.  At least when your brokers are not saturated
> >> >>with
> >> >> messages, and acks are set to 1.  If acks are set to ALL, latency on
> >>an
> >> >> non-saturated kafka cluster will be: Round Trip Latency from
> >>producer to
> >> >> leader for partition + Max( slowest Round Trip Latency to a replicas
> >>of
> >> >> that partition).  If a cluster is saturated with messages, we have
to
> >> >> assume that all partitions receive an equal distribution of messages
> >>to
> >> >> avoid linear algebra and queueing theory models.  I don¹t like linear
> >> >> algebra :P
> >> >>
> >> >> Since you are probably putting all your latencies into a single
> >> >>histogram
> >> >> per producer, or worse, just an average, this pattern would have been
> >> >> obscured.  Obligatory lecture about measuring latency by Gil Tene
> >> >> (https://www.youtube.com/watch?v=9MKY4KypBzg).  To verify this
> >> >>hypothesis,
> >> >> you should re-write the benchmark to plot the latencies for each
> >>write
> >> >>to
> >> >> a partition for each producer into a histogram. (HRD histogram is
> >>pretty
> >> >> good for that).  This would give you producers*partitions histograms,
> >> >> which might be unwieldy for that many producers. But wait, there is
> >> >>hope!
> >> >>
> >> >> To verify that this hypothesis holds, you just have to see that there
> >> >>is a
> >> >> significant difference between different partitions on a SINGLE
> >> >>producing
> >> >> client. So, pick one producing client at random and use the data from
> >> >> that. The easy way to do that is just plot all the partition latency
> >> >> histograms on top of each other in the same plot, that way you have
a
> >> >> pretty plot to show people.  If you don¹t want to setup plotting,
you
> >> >>can
> >> >> just compare the medians (50¹th percentile) of the partitions¹
> >> >>histograms.
> >> >>  If there is a lot of variance, your latency anomaly is explained by
> >> >> brokers 4-7 being slower than nodes 0-3!  If there isn¹t a lot of
> >> >>variance
> >> >> at 50%, look at higher percentiles.  And if higher percentiles for
> >>all
> >> >>the
> >> >> partitions look the same, this hypothesis is disproved.
> >> >>
> >> >> If you want to make a general statement about latency of writing to
> >> >>kafka,
> >> >> you can merge all the histograms into a single histogram and plot
> >>that.
> >> >>
> >> >> To Yuheng¹s credit, more brokers always results in more throughput.
> >>But
> >> >> throughput and latency are two different creatures.  Its worth noting
> >> >>that
> >> >> kafka is designed to be high throughput first and low latency second.
> >> >>And
> >> >> it does a really good job at both.
> >> >>
> >> >> Disclaimer: I might not like linear algebra, but I do like
> >>statistics.
> >> >> Let me know if there are topics that need more explanation above that
> >> >> aren¹t covered by Gil¹s lecture.
> >> >> -Erik
> >> >>
> >> >> On 9/4/15, 9:03 AM, "Yuheng Du" <yuheng.du.hust@gmail.com> wrote:
> >> >>
> >> >> >When I using 32 partitions, the 4 brokers latency becomes larger
> >>than
> >> >>the
> >> >> >8
> >> >> >brokers latency.
> >> >> >
> >> >> >So is it always true that using more brokers can give less latency
> >>when
> >> >> >the
> >> >> >number of partitions is at least the size of the brokers?
> >> >> >
> >> >> >Thanks.
> >> >> >
> >> >> >On Thu, Sep 3, 2015 at 10:45 PM, Yuheng Du
> >><yuheng.du.hust@gmail.com>
> >> >> >wrote:
> >> >> >
> >> >> >> I am running a producer latency test. When using 92 producers
in
> >>92
> >> >> >> physical node publishing to 4 brokers, the latency is slightly
> >>lower
> >> >> >>than
> >> >> >> using 8 brokers, I am using 8 partitions for the topic.
> >> >> >>
> >> >> >> I have rerun the test and it gives me the same result, the
4
> >>brokers
> >> >> >> scenario still has lower latency than the 8 brokers scenarios.
> >> >> >>
> >> >> >> It is weird because I tested 1broker, 2 brokers, 4 brokers,
8
> >> >>brokers,
> >> >> >>16
> >> >> >> brokers and 32 brokers. For the rest of the case the latency
> >> >>decreases
> >> >> >>as
> >> >> >> the number of brokers increase.
> >> >> >>
> >> >> >> 4 brokers/8 brokers is the only pair that doesn't satisfy
this
> >>rule.
> >> >> >>What
> >> >> >> could be the cause?
> >> >> >>
> >> >> >> I am using a 200 bytes message, the test let each producer
> >>publishes
> >> >> >>500k
> >> >> >> messages to a given topic. Every test run when I change the
> >>number of
> >> >> >> brokers, I use a new topic.
> >> >> >>
> >> >> >> Thanks for any advices.
> >> >> >>
> >> >>
> >> >>
> >>
> >>
>
>

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