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From Jaikiran Pai <jai.forums2...@gmail.com>
Subject High CPU usage on broker - Why is the default heart beat interval set too low (3 seconds)?
Date Tue, 26 Apr 2016 08:16:45 GMT
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 
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?


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