Why are the default values of max_searches_per_cpu and base_max_searches in limits.conf lowered in 5.x?

[mchang_splunk]

After upgrading to 5.0, I find the default value of max_searches_per_cpu and base_max_searches in /etc/system/default/limits.conf have been changed.

In $SPLUNK_HOME/etc/system/default/limits.conf:

4.x:

# the maximum number of concurrent searches per CPU
max_searches_per_cpu = 4

# the base number of concurrent searches
base_max_searches = 4

5.x:

# the maximum number of concurrent searches per CPU
max_searches_per_cpu = 1

# the base number of concurrent searches
base_max_searches = 6

This means that on a server with 4 CPU cores, Splunk 5.x will limit the number of concurrent searches to 10 (6 + 4 * 1) where the limit would have been 20 (4 + 4 * 4) with Splunk 4.x.

Why was this change made?

[mchang_splunk · Accepted solution]

These values were changed because in 5.x and beyond, search jobs started from the UI can now be queued when the search concurrency limit is reached instead of being refused. The back-end (splunkd) had this capability since 4.2, but the UI can only handle queued jobs since 5.0.

The bottom line is that in 5.x, the maximum number of concurrent searches has been lowered but it should be compensated by the full implementation of search job queueing. Overall, the goal is to improve the search experience on systems with high search concurrency: your search might get queued up for a bit (ideally, no more than a few seconds) but it should run faster when launched as there will be fewer searches contending for the same resources (most notably, disk I/O).

Further reading on this topic:

• Accommodate concurrent users and searches
• How saved searches affect Splunk performance