Details
Major Description
When keeping the wiki running but idle, the server has still one CPU busy at about 100% with a servlet container process.
Stack traces show at first nothing conspicuous, but I guess it is the thread in the MentionsConsumer inner class of the org.xwiki.mentions.internal.DefaultMentionsEventExecutor as there is always at least one stack trace in state RUNNABLE for that thread.
Also when I manually brute force uninstall the extension from XWiki by editing the /var/lib/xwiki/data/extension/repository/org%2Exwiki%2Eplatform%3Axwiki-platform-mentions-default/12%2E6%2E1/org%2Exwiki%2Eplatform%3Axwiki-platform-mentions-default-12%2E6%2E1.xed the CPU load goes away.
Instead XWiki complains on startup about the missing component "Can't find descriptor for the component with type [interface org.xwiki.mentions.MentionsConfiguration] and hint [default]" which is expected and shows that uninstalling the extension worked.
I got quite a couple of stack dumps, and a typical stack trace for the "Mentions thread" looks like:
[2020-08-15 14:55:50] [info] "Mentions thread" #119 daemon prio=4 os_prio=0 cpu=1031372.07ms elapsed=1647.72s tid=0x00007fb6f865e800 nid=0x13c3 runnable [0x00007fb6c611c000] [2020-08-15 14:55:50] [info] java.lang.Thread.State: RUNNABLE [2020-08-15 14:55:50] [info] #011at org.xwiki.mentions.internal.DefaultMentionsEventExecutor$MentionsConsumer.run(DefaultMentionsEventExecutor.java:184) [2020-08-15 14:55:50] [info] #011at java.lang.Thread.run(java.base@11.0.8/Thread.java:834)
but there are also a few like:
[2020-08-15 14:58:09] [info] "Mentions thread" #119 daemon prio=4 os_prio=0 cpu=1169789.03ms elapsed=1787.09s tid=0x00007fb6f865e800 nid=0x13c3 runnable [0x00007fb6c611c000] [2020-08-15 14:58:09] [info] java.lang.Thread.State: RUNNABLE [2020-08-15 14:58:09] [info] #011at org.xwiki.mentions.internal.async.MapBasedLinkedBlockingQueue.poll(MapBasedLinkedBlockingQueue.java:252) [2020-08-15 14:58:09] [info] #011at org.xwiki.mentions.internal.DefaultMentionsEventExecutor$MentionsConsumer.consume(DefaultMentionsEventExecutor.java:195) [2020-08-15 14:58:09] [info] #011at org.xwiki.mentions.internal.DefaultMentionsEventExecutor$MentionsConsumer.run(DefaultMentionsEventExecutor.java:184) [2020-08-15 14:58:09] [info] #011at java.lang.Thread.run(java.base@11.0.8/Thread.java:834)
This looks like a "busy wait" loop to me.
I locally patched my instance to use a "blocking wait" instead:
diff --git a/xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-default/src/main/java/org/xwiki/mentions/internal/DefaultMentionsEventExecutor.java b/xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-default/src/main/java/org/xwiki/mentions/internal/DefaultMentionsEventExecutor.java index d9174c62daa..5f63b653c0c 100644 --- a/xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-default/src/main/java/org/xwiki/mentions/internal/DefaultMentionsEventExecutor.java +++ b/xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-default/src/main/java/org/xwiki/mentions/internal/DefaultMentionsEventExecutor.java @@ -192,7 +192,7 @@ public void consume() { MentionsData data = null; try { - data = DefaultMentionsEventExecutor.this.queue.poll(); + data = DefaultMentionsEventExecutor.this.queue.take(); if (data != null) { data.increaseAttempts(); if (data.isStop()) {
This made the problem go away for me, but two unit tests in the DefaultMentionsEventExecutorTest fail with this modification. I have not looked in the cause for this yet.