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From Liquan Pei <liquan...@gmail.com>
Subject Re: High CPU usage on broker - Why is the default heart beat interval set too low (3 seconds)?
Date Tue, 26 Apr 2016 08:40:57 GMT
Hi Jaikiran,

Thanks for the email and the detailed analysis. One reason for setting the
heartbeat interval to a lower value is for faster failure detection. On
every received heartbeat, the coordinator starts (or resets) a timer. If no
heartbeat is received when the timer expires, the coordinator marks the
member dead and signals the rest of the group that they should rejoin so
that partitions can be reassigned.

I think the trade off here is the CPU usage and how fast you want to detect
the consumer failure. Faster failure detection makes the partitions
assigned to dead consumers to assign to other consumers.


On Tue, Apr 26, 2016 at 1:16 AM, Jaikiran Pai <jai.forums2013@gmail.com>

> We have been investigating an unreasonably high CPU usage of the Kafka
> process when there's no _real_ activity going on between the consumers and
> the broker. We had this issue in 0.8.x days and is exactly the same as
> what's being tracked in this JIRA
> https://issues.apache.org/jira/browse/KAFKA-493. We now use (both
> client libraries, new consumer APIs and the broker). However, we still see
> some CPU usage which looks a bit on the higher side when there's no real
> message production or consumption going on. Just connecting around 10-20
> consumers on different topics of a single broker Kafka instance shows up
> this issue.
> All our debugging so far points to the Processor thread on the broker side
> which has a high CPU usage. There are N such Processor threads, which
> always are in the RUNNABLE state doing this:
> "kafka-network-thread-0-PLAINTEXT-0" #21 prio=5 os_prio=0
> tid=0x00007f1858c4a800 nid=0xc81 runnable [0x00007f18106cb000]
>    java.lang.Thread.State: RUNNABLE
>     at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)
>     at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:269)
>     at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:79)
>     at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
>     - locked <0x00000006c0046128> (a sun.nio.ch.Util$2)
>     - locked <0x00000006c0046118> (a java.util.Collections$UnmodifiableSet)
>     - locked <0x00000006c0046068> (a sun.nio.ch.EPollSelectorImpl)
>     at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
>     at org.apache.kafka.common.network.Selector.select(Selector.java:425)
>     at org.apache.kafka.common.network.Selector.poll(Selector.java:254)
>     at kafka.network.Processor.run(SocketServer.scala:413)
>     at java.lang.Thread.run(Thread.java:745)
> From what we have narrowed down so far, this thread in itself isn't a
> "culprit", since when they are no consumers connected, the CPU isn't high.
> However when a consumer connects to this and just waits for messages, these
> threads start playing a role in the high CPU usage. Our debugging shows
> that each of these X number of consumers that connect to the broker keep
> doing 2 things when they are "idle":
> 1) A delayed operation every Y seconds which does the auto commit of
> offsets.
> 2) Sending heartbeats every 3 seconds to the broker
> We disabled auto commits of offsets since that's the semantic we wanted.
> So #1 isn't really an issue. However, #2 is. It looks like the default
> heartbeat interval is 3 seconds which is too low, IMO. This translates to a
> network socket operation every 3 seconds which then has to be processed by
> the broker side Processor thread. If there's just a single consumer, this
> doesn't make much of a difference. As soon as you add more consumers, the
> Processor on the broker side has to be start processing each of these
> incoming heartbeats which become too frequent. Even though the interval is
> 3 seconds, the incoming heartbeats to the broker can be much more frequent
> when more consumers are involved since the 3 second interval is just per
> consumer. So in practice there can be a heartbeat coming every second or
> few milli seconds from the X consumers to this broker which can contribute
> to this high CPU usage when the system is practically idle.
> So coming to the real question - why is the default heart beat interval so
> low - 3 seconds? We increased it to 29 seconds (just 1 second less than the
> session timeout) per consumer (via consumer configs) and in addition to
> disabling auto commit, these changes have improved noticeably the CPU usage.
> Ideally, what would be a better value for the heart beat interval that
> doesn't unnecessary flood these messages and cause the broker to continuous
> process them?
> -Jaikiran

Liquan Pei
Software Engineer, Confluent Inc

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