Plugin to cache Git repository's access to refdb

When Serialize AccountCache series was introduced it simplified the cache eviction by always reaching out to JGit for data. Unfortunately it comes with price which is especially high when All-Users repository is accessed through the NFS and core.trustFolderStat = false is configured in ${GERRIT_SITE}/etc/jgit.config (quite common setup for HA/Multi-Site ens).

This plugin was developed to introduce the in-memory cache (managed by Gerrit so that evictions could be coordinated to multiple nodes) that reduces the price for reaching to refs in JGit. It is a Gerrit native alternative (that can be applied to Gerrit 3.2) to work that is currently under progress for caching Refs in JGit.

Here is the short comparison of heavy-refs-related operations performance. The test scenario was to get random change details (over the same REST API that is used in Gerrit's details page) in 8 parallel threads over 5mins period of time. The core.trustFolderStat = false was set in ${GERRIT_SITE}/etc/jgit.config. It was called against:

vanilla Gerrit 3.1.16 version (marked as stable-3.1 in the results)
vanilla Gerrit 3.2.12 version (marked as stable-3.2 in the results)
Gerrit 3.2.14 with libCache module loaded (marked as stable-3.2-libCache in the results).

Note that TRS is Reqs/Sec for each Thread.

| version                           | TRS Avg | TRS Std Dev | TRS Max | Total Reqs/sec | Transfer/sec(MB)|
| --------------------------------- | ------- | ----------- | ------- | -------------- | --------------- |
| stable-3.1                        | 57,33   | 8,26        | 80      | 456,95         | 4,34            |
| stable-3.2                        | 13,87   | 4,92        | 20      | 110,18         | 1,07            |
| stable-3.2-libCache               | 105,27  | 14,55       | 150     | 834,88         | 8,41            |
| stable-3.1 vs stable-3.2          | 313,34% | 67,89%      | 300,00% | 314,73%        | 305,61%         |
| stable-3.2-libCache vs stable-3.2 | 658,98% | 195,73%     | 650,00% | 657,74%        | 685,98%         |
| stable-3.2-libCache vs stable-3.1 | 83,62%  | 76,15%      | 87,50%  | 82,71%         | 93,78%          |

One can clearly see that in this setup using this library module outperforms both Gerrit 3.2 and 3.1 by factor of 6 and 2 correspondingly. The test script, detailed description and more results are available here.

How to build

Clone or link this plugin to the plugins directory of Gerrit‘s source tree, and then run bazel build on the plugin’s directory.

Example:

git clone --recursive https://gerrit.googlesource.com/gerrit
cd plugins
git clone "https://gerrit.googlesource.com/modules/cached-refdb"
cd .. && bazel build plugins/cached-refdb

The output plugin jar is created in:

bazel-bin/plugins/cached-refdb/cached-refdb.jar

How to install

Copy the cached-refdb.jar into the ${GERRIT_SITE}/lib/ so that it is being loaded when the Gerrit instance is started. Note that the following configuration options need to be added

git config --file ${GERRIT_SITE}/etc/gerrit.config --add gerrit.installDbModule\
  com.googlesource.gerrit.plugins.cachedrefdb.LibDbModule
git config --file ${GERRIT_SITE}/etc/gerrit.config --add gerrit.installModule\
  com.googlesource.gerrit.plugins.cachedrefdb.LibSysModule

By default cache can hold up to 1024 refs which will not be sufficient for any production site therefore one can configure it through the standard Gerrit cache configuration means e.g.

git config --file ${GERRIT_SITE}/etc/gerrit.config cache.ref_by_name.memoryLimit 10240

Note that library module requires the Gerrit instance restart in order to pick up the configuration changes.