Change Details

#### Problem We currently use `microtime(true)` quite a lot. This internally uses `gettimeofday()` in C, which isn't monotonic and subject to clock drift etc. #### Impact The metrics we collect for WANObjectCache, ResourceLoader, Backend Save Timing etc. are all affected by this and thus cannot be trusted. Their mean average might give a rough sense of how long something takes under "boring" circumstances, but it's not a useful way to set alerts because we can't trust the outliers. For backend metrics it's important that we monitor the p99 and max, not the mean average ([why](https://phabricator.wikimedia.org/phame/post/view/83/measuring_wikipedia_page_load_times/)). However, due to clock drift, the metrics that fire these alerts are difficult to trust (e.g. T242024 and T233059). Examples: * [https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group](https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group?orgId=1&fullscreen&panelId=13) * [https://grafana.wikimedia.org/d/000000066/resourceloader](https://grafana.wikimedia.org/d/000000066/resourceloader?orgId=1&fullscreen&panelId=45) * <https://grafana.wikimedia.org/d/000000429/backend-save-timing-breakdown> ##### Requirements PHP 7.3+ introduces `hrtime()` for this purpose, which internally uses `clock_gettime(CLOCK_MONOTONIC)` in C. This is comparable to `performance.now()` in web browsers, and `process.hrtime()` in Node.js. * <https://www.php.net/manual/en/function.hrtime.php> We'll need to provide a way to get access to the monotonic timer in within the MediaWiki run-time. ##### Proposal 1 It looks like it will probably take at least a year before we run MediaWiki on PHP 7.3 by default at WMF. Both for that reason and for third parties, I suggest we invest in supporting a way on PHP 7.2 to get monotonic timers. Assuming this can't be done in pure PHP, it will require a PHP extension (either from Pecl or to developer our own). We would then conditionally define `hrtime()` if absent to use that instead (for older PHP versions), with ultimate falllback to `microtime(true)` if the user is neither PHP 7.3 nor the extension in question.

#### Problem We currently use `microtime(true)` quite a lot. This internally uses `gettimeofday()` in C, which isn't monotonic and subject to clock drift etc. #### Impact The metrics we collect for WANObjectCache, ResourceLoader, Backend Save Timing etc. are all affected by this and thus cannot be trusted. Their mean average might give a rough sense of how long something takes under "boring" circumstances, but it's not a useful way to set alerts because we can't trust the outliers. For backend metrics it's important that we monitor the p99 and max, not the mean average ([why](https://phabricator.wikimedia.org/phame/post/view/83/measuring_wikipedia_page_load_times/)). However, due to clock drift, the metrics that fire these alerts are difficult to trust (e.g. T242024 and T233059). Examples: * [https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group](https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group?orgId=1&fullscreen&panelId=13) * [https://grafana.wikimedia.org/d/000000066/resourceloader](https://grafana.wikimedia.org/d/000000066/resourceloader?orgId=1&fullscreen&panelId=45) * <https://grafana.wikimedia.org/d/000000429/backend-save-timing-breakdown> ##### Requirements PHP 7.3+ introduces `hrtime()` for this purpose, which internally uses `clock_gettime(CLOCK_MONOTONIC)` in C. This is comparable to `performance.now()` in web browsers, and `process.hrtime()` in Node.js. * <https://www.php.net/manual/en/function.hrtime.php> We'll need to provide a way to get access to the monotonic timer in within the MediaWiki run-time. ##### Proposal It looks like it will probably take at least a year before we run MediaWiki on PHP 7.3 by default at WMF. Both for that reason and for third parties, I suggest we invest in supporting a way on PHP 7.2 to get monotonic timers. Assuming this can't be done in pure PHP, it will require a PHP extension (either from Pecl or to developer our own). We would then conditionally define `hrtime()` if absent to use that instead (for older PHP versions), with ultimate falllback to `microtime(true)` if the user is neither PHP 7.3 nor the extension in question.

#### Problem We currently use `microtime(true)` quite a lot. This internally uses `gettimeofday()` in C, which isn't monotonic and subject to clock drift etc. #### Impact The metrics we collect for WANObjectCache, ResourceLoader, Backend Save Timing etc. are all affected by this and thus cannot be trusted. Their mean average might give a rough sense of how long something takes under "boring" circumstances, but it's not a useful way to set alerts because we can't trust the outliers. For backend metrics it's important that we monitor the p99 and max, not the mean average ([why](https://phabricator.wikimedia.org/phame/post/view/83/measuring_wikipedia_page_load_times/)). However, due to clock drift, the metrics that fire these alerts are difficult to trust (e.g. T242024 and T233059). Examples: * [https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group](https://grafana.wikimedia.org/d/lqE4lcGWz/wanobjectcache-key-group?orgId=1&fullscreen&panelId=13) * [https://grafana.wikimedia.org/d/000000066/resourceloader](https://grafana.wikimedia.org/d/000000066/resourceloader?orgId=1&fullscreen&panelId=45) * <https://grafana.wikimedia.org/d/000000429/backend-save-timing-breakdown> ##### Requirements PHP 7.3+ introduces `hrtime()` for this purpose, which internally uses `clock_gettime(CLOCK_MONOTONIC)` in C. This is comparable to `performance.now()` in web browsers, and `process.hrtime()` in Node.js. * <https://www.php.net/manual/en/function.hrtime.php> We'll need to provide a way to get access to the monotonic timer in within the MediaWiki run-time. ##### Proposal 1 It looks like it will probably take at least a year before we run MediaWiki on PHP 7.3 by default at WMF. Both for that reason and for third parties, I suggest we invest in supporting a way on PHP 7.2 to get monotonic timers. Assuming this can't be done in pure PHP, it will require a PHP extension (either from Pecl or to developer our own). We would then conditionally define `hrtime()` if absent to use that instead (for older PHP versions), with ultimate falllback to `microtime(true)` if the user is neither PHP 7.3 nor the extension in question.