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[DOC] Recommendation when using a lot of metrics filters #61

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[DOC] Recommendation when using a lot of metrics filters #61

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3 changes: 3 additions & 0 deletions docs/index.asciidoc
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Expand Up @@ -75,6 +75,9 @@ For a `timer => { "thing" => "%{duration}" }` you will receive the following fie
The default lengths of the event rate window (1, 5, and 15 minutes)
can be configured with the `rates` option.

IMPORTANT: When using several `metrics` filters (in particular the `timer`), it might
become necessary to increase the `Xss` setting in `jvm.options` to avoid `StackOverflowException`.
Comment on lines +78 to +79
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Suggested change
IMPORTANT: When using several `metrics` filters (in particular the `timer`), it might
become necessary to increase the `Xss` setting in `jvm.options` to avoid `StackOverflowException`.
TIP: If you see `StackOverflowException` erors and are using several `metrics` filters (the `timer` in particular ), try increasing the `Xss` setting in `jvm.options`.

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Consider rewording as a remedy IF a user sees this problem rather than tempting people to fiddle with jvm settings without need?

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It's not clear to me why the StackOverflowException happens more if they use the particular timer setting. The StackOverflowException is a general problem, it's related to the way Logstash plans the execution of a pipeline.

Explaining better what I mean: suppose the pipeline is configured with 2 filters, I -> F1 - F2 -> O, the execution plan is:

  • execute F1
  • it's F1 that invokes the execution of F2, so it's like F2(F1()) (NB this is formally incorrect, but it's useful to explain the nesting of the calls)

Every filter count as a stack frame in the execution thread's stack. So this comment is more general in Logstash execution context, it's not specific to this plugin; the tip is part of the general Logstash documentation, where it's already discussed https://www.elastic.co/guide/en/logstash/current/jvm-settings.html#stacks-size


==== Example: Computing event rate

For a simple example, let's track how many events per second are running
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