HHVM

HHVM, sometimes known as HipHop Virtual Machine, is a virtual machine for PHP, with an associated just-in-time compiler (JIT). Deploying HHVM on a MediaWiki wiki should lead to performance improvements across the board for most users.

This page is about Wikimedia-sponsored work on HipHop support in MediaWiki, and its deployment to Wikimedia production wikis.

Historically, the HipHop compiler was a project by Facebook which involved compiling PHP code into C++ for purposes of speeding up the language. Facebook has since abandoned this project, and now their development efforts are focused on HHVM itself instead.

Roadmap
Here is the general plan for deploying HHVM to the production cluster: Each step along the way will likely cause discovery of new bugs that need to be fixed before the next step can be completed, so dates are difficult to venture at this time.
 * Deployment to Beta Cluster (currently ongoing work, in parallel with other work) ✅
 * Week of July 21: deployment to a few job runners in production ✅
 * Deploy to test.wikipedia.org application server ✅
 * Deploy Varnish module allowing partial deployment to a fraction of application servers✅
 * Limited deployment to small number of application servers✅
 * Ramp up deployment to more application servers until most servers use HHVM
 * Deploy to remainder of services

Current work
https://bugzilla.wikimedia.org/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=PATCH_TO_REVIEW&bug_status=REOPENED&bug_status=VERIFIED&f1=keywords&list_id=288958&o1=substring&order=changeddate%20DESC%2Cbug_status%2Cpriority%2Cassigned_to%2Cbug_id&query_based_on=&query_format=advanced&title=Bug%20List&v1=hhvm&ctype=atom

Bugzilla: Open bugs, All bugs.

Rationale
It is a well-studied phenomenon that even small delays in response time (e.g. half of a second) can result in sharp declines in web user retention. As a result, popular websites such as Google and Facebook invest heavily in site performance initiatives, and partially as a result, remain popular. Formerly popular sites (such as Friendster) suffered due to lack of attention to these issues. Wikipedia and its sister projects must remain usable and responsive in order for the movement to sustain its mission.

Facebook, as a big user of PHP, has recognized this problem, and invested heavily in a solution: HHVM, a virtual machine that compiles PHP bytecode to native instructions at runtime, the same strategy used by Java and C# to achieve their speed advantages. We're quite confident that this will result in big performance improvements on our sites as well.

What will HipHop do for our end users?
MediaWiki is written in PHP, a language that is interpreted at run-time. The overhead of running this PHP code every time someone views a page necessitates the usage of caching servers, running software such as Varnish, which cache the HTML generated by running this PHP, so that the PHP does not have to run every time a page is viewed. These caches only serve users that are not logged in. Actions which are not affected by the cache, and therefore are affected by the run time of PHP code, include: Therefore, any action we can take to reduce the time it takes for MediaWiki's PHP code will therefore also decrease the loading times of our site for all of our logged in users and anyone who edits anonymously.
 * Any page you view while logged in.
 * Saving pages that you've edited, whether you are logged in or not.

HipHop was written to be a faster, more efficient PHP interpreter than our current interpreter (Zend). It is our hope that by implementing HipHop as a replacement for Zend, our users will notice a tangible increase in the performance of our sites.

How does our development work on HipHop affect MediaWiki developers?
In our initial sprint of work, due to be finished at the end of March 2014, we hope to make it so that anyone can elect to use HipHop on Beta Labs instead of Zend. This will be on a totally opt-in basis which can be disabled at any time. This will allow the MediaWiki Core team to gauge the performance of HipHop against that of Zend directly using our current test infrastructure, instead of just estimating theoretical performance increases. It will also create a development environment that will help us see how much work is needed to make HipHop compatible with MediaWiki, and as such let us create an estimate for how long it will take us to get HipHop live on production as a full replacement for Zend.

For other MediaWiki developers, the consequence of HipHop being deployed in this manner is that if they are using the Beta Cluster as a test environment, they will find it trivial to test how their patches perform using HipHop instead of Zend if they wish to. However, to minimise the disruption of our work, the opt-in nature of the infrastructure will allow developers will be able to continue to develop totally agnostic of the future HipHop migration if they wish to do so.