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High Fetcher Usage Although the fetcher helper count is already high

If you face the following problems:

Fetcher helper usage is permanently above 96%, and fetcher count is already high (i.e., >50 or 100 or more) and
the service "Check_MK" runs into constant "CRIT with fetcher timeouts
- You can also use this command as site user to narrow down and find slow-running active checks.
  Code Block
  language bash
  theme RDark
  lq "GET services\nColumns: execution_time host_name display_name" | awk -F';' '{ printf("%.2f %s %s\n", $1, $2, $3)}' | sort -rn | head

This can have several reasons:

Firewalls are dropping traffic from Checkmk to the monitored systems. If the packets are dropped rather than blocked, Checkmk must wait for a timeout instead of instantly terminating the fetching process.
You might have too many DOWN hosts, which are still being checked. Checkmk still tries to query those hosts, and the fetchers need to wait for a timeout every time. This can bind a lot of fetcher helpers, which are blocked for that time. Remove hosts which are in a DOWN state from your monitoring. Either permanently or by setting their Criticality to "Do not monitor this host".
For classical operating systems (Linux/Windows/etc.), this indicates that you might have plugins/local checks with quite a long runtime. Increasing the number of fetchers further here is not constructive. Instead, you must identify the long-running plugins/local checks and set them to asynchronous execution and/or define (generous) cache settings or even timeouts, especially for them.
For SNMP devices, you might have poorly performing SNMP devices. To troubleshoot those, have a look at this blog post.