Arc Lamp
This is the internal runbook for deploying and monitoring Arc Lamp. Arc Lamp collects stack traces from all MediaWiki production traffic, using php-excimer, and publishes daily aggregated flame graphs and trace logs to https://performance.wikimedia.org/php-profiling/.
Service
Arc Lamp data originates from the Arc Lamp client (configured in operations/mediawiki-config.git:/Profiler.php) that runs alongside MediaWiki on the application servers. The samples are collected with php-excimer, and sent to a Redis instance on the arclamp host.
The Arc Lamp service then reads the stream of samples from Redis, and produces trace logs on disk. The flame graphs are generated with brendangregg/FlameGraph.
Stats from the Arc Lamp client are sent to Graphite. Stats from the Arc Lamp service go to Prometheus.
For how to migrate to a new host, refer to past arclamp1001 upgrade (T319434).
Meta
- Source code: https://gerrit.wikimedia.org/g/performance/arc-lamp
- Server provisioning: modules/arclamp and profile::webperf::arclamp.
Hosts
The puppet role for these services is role::webperf:profiling_tools. We currently run one arclamp#### host in each core data center (Eqiad and Codfw). You can monitor these via the Grafana host overview dashboard. There's also an equivalent deployment-webperf## host in the Beta Cluster.
See also
- Excimer, the engine that powers the Arc Lamp.
- WikimediaDebug#Request profiling, which provides on-demand flame graphs for individual debug requests.
- Profiling PHP in production at scale, Timo Tijhof, 2021.
- How we made editing Wikipedia twice as fast, Ori Livneh, 2014.
External links
- Grafana dashboard: Arc Lamp
- Index page: https://performance.wikimedia.org/php-profiling/
- Public file listing: https://performance.wikimedia.org/arclamp/svgs/daily/
- Upstream brendangregg/FlameGraph.