https://bz.apache.org/bugzilla/show_bug.cgi?id=63802
Bug ID: 63802
Summary: epoll spin detection is missing
Product: Tomcat 8
Version: 8.5.42
Hardware: PC
OS: Mac OS X 10.1
Status: NEW
Severity: critical
Priority: P2
Component: Catalina
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ----
There is a long lived bug either in the JDK or even in Linux epoll
implementation that makes it possible for the select() call to return
immediately with 0 SelectionKey to be processed. In this case, if you call back
the select() funtion immediately, you'll get a 100% CPU usage.
A workaround has been implemented in Apache MINA, in Netty, in Grizzly, but I
don't see such a workaround implemented in Tomcat.
The idea is to avoid calling back select() if the previous call has returned 0,
after a few iteration. In this case, a new Selector is created, all the
channels registered in the old selector are registered in the new selector, and
the old selector is ditched.
You can have a look at Grizzly code, line 501 :
https://github.com/javaee/grizzly/blob/master/modules/grizzly/src/main/java/org/glassfish/grizzly/nio/SelectorRunner.java
Or Apache MINA, line 609 and following :
https://github.com/apache/mina/blob/2.1.X/mina-core/src/main/java/org/apache/mina/core/polling/AbstractPollingIoProcessor.java
Or in Netty, line 849 and following :
https://github.com/netty/netty/blob/4.1/transport/src/main/java/io/netty/channel/nio/NioEventLoop.java
This workaround is critical for those three projects to properly work on Linux
(the problem does not exist on Windows or Mac OSX, this is the reason Grizzly
has added a flag to activate it or not).
FTR, I'm currently being hit by such a random CPU 100% peak on a project I'm
working on. A thread dump shows that the thread consuming the CPU is the one
doing the infinite select() loop :
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:93)
at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
at org.apache.tomcat.util.net.NioEndpoint$Poller.run(NioEndpoint.java:825)
at java.lang.Thread.run(Thread.java:748)
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