Tuning

Tuning chronicle-map correctly might be a daunting task: How many entries does a particular cache instance need? what is the average key and value for it?

Rather than leaving you only with the trial and error (or the guesswork) approach, this module provides a utility to help you get started in the right direction.

Since chronicle-map is one of the first open-source alternatives to the H2 implementation, it is very likely that your Gerrit instance has been running with the default H2 cache backend.

The idea is to read from the actual H2 persisted files and output the information that will be required to configure chronicle-map as an alternative.

You can do this before installing cache-chroniclemap as a lib module so that your Gerrit server will not need downtime. As follows:

• Drop cache-chroniclemap.jar file in the plugins/ directory.
• Wait for the pluginLoader to acknowledge and load the new plugin. You will see an entry in the error_log:

INFO  com.google.gerrit.server.plugins.PluginLoader : Loaded plugin cache-chroniclemap

• You can now run an analysis on the current status of your H2 caches

ssh -p 29418 admin@<gerrit-server> cache-chroniclemap analyze-h2-caches

The result will be outputted on standard output in a git config format. This is an example (the values are made up):

****************************
** Chronicle-map template **
****************************

[cache "diff_summary"]
	entries = 101
	avgKeySize = 192
	avgValueSize = 1350
[cache "web_sessions"]
	entries = 1
	avgKeySize = 68
	avgValueSize = 332
[cache "pure_revert"]
	entries = 1
	avgKeySize = 112
	avgValueSize = 8
[cache "mergeability"]
	entries = 101
	avgKeySize = 150
	avgValueSize = 8
[cache "diff"]
	entries = 101
	avgKeySize = 188
	avgValueSize = 5035
[cache "persisted_projects"]
	entries = 2
	avgKeySize = 88
	avgValueSize = 4489
[cache "accounts"]
	entries = 5
	avgKeySize = 52
	avgValueSize = 505

Empty caches (if any) will not generate empty config stanzas, rather a warning will be displayed on standard output.

For example:

WARN: Cache diff_intraline is empty, skipping

Please note that the generated configuration is not necessarily final and it might still need adjustments:

• Since chronicle-map file size is pre-allocated, you might want to allow for more entries.
• You might want account for uncertainty by specifying a maxBloatFactor greater than 1.
• any other reason.

Once you gathered the information you wanted you might consider to remove the plugin:

• Remove the jar from the plugins directory

rm plugins/cache-chroniclemap.jar

• Wait for the pluginLoader to acknowledge and unload the plugin. You will see an entry in the error_log:

INFO  com.google.gerrit.server.plugins.PluginLoader : Unloading plugin cache-chroniclemap