Category Archives: bugzilla

bugzilla and performance, 2014

bugzilla is a large legacy perl application initially developed at the birth of the world wide web. it has been refactored significantly in the 15 years since its release; however, some design decisions made at the start of the project carry significant technical debt in today’s modern web.

while there have been a large number of micro-optimisations over the course of bugzilla’s life (generally as a result of performance profiling), there are limits to the benefits these sorts of optimisations can provide.

this year has seen a lot of focus on improving bugzilla’s performance within mozilla, centred around the time it takes to display a bug to authenticated users.

response-times-may-2013response-times-may-2014

tl;dr bugzilla is faster

memcached

the design of a modern large web application is generally centred around caches, between the business logic layer and the data sources, as well as between the users and the user interface. while bugzilla has been refactored to use objects, database abstraction, templates, etc, it had zero caching capabilities. this coupled with completely stateless processing of each request meant that every byte of html returned to the user was regenerated from scratch, starting with a new connection to the database.

towards the end of 2013 i worked on implementing a memcached framework into bugzilla [bug 237498].

retrofitting a caching mechanism into a large extendible framework proved to be a significant challenge. bugzilla provides the ability for developers to extend bugzilla’s functionality via extensions, including but not limited to adding new fields, user interface, or process. the extensions system conflicts with caching as it’s possible for an extension to conditionally alter an object in ways that would render it impossible to cache (eg. add a new field to an object only if the current user is a member of a particular group).

some compromises had to be made. instead of caching fully constructed objects, the cache sits between the object’s constructor and the database. we avoid a trip to the database, but still have to construct objects from that data (which allows extensions to modify the object during construction).

code which updated the database directly instead of using bugzilla’s objects had to be found and rewritten to use the objects or updated to manually clear the cache entries. extra care had to be taken as returning stale data could silently result in data loss. to placate concerns that these errors would be impossible to detect, the caller of an object’s constructor must pass in a parameter to “opt-in” to caching.

in 2014 i built upon the memcached framework to support most of bugzilla’s objects [bug 956233], with the “bugs” object being the only notable exception. memcached also caches bugzilla’s configuration parameters (classifications, products, components, groups, flags, …) [bug 987032]. although caching the “bugs” object would be beneficial given its central role in all things bugzilla, it is highly likely that enabling this by default would break bugzilla extensions and customisations as a high proportion of them update the database directly instead of using bugzilla’s object layer. this would manifest as data which is silently stale, making undetectable dataloss a high probability.

memcached support will be released with bugzilla 5.0, but has been live on bugzilla.mozilla.org (bmo) since february 2014.

instrumentation

while profilling tools such as nytprof have often been pointed at bugzilla, bmo’s high number of concurrent requests and usage patterns time and time again resulted in performance optimisations performing worse than expected once deployed to production.

we took the decision to deploy instrumentation code into bugzilla itself, reporting on each http request, database query, and template execution. as bugzilla is written in perl, support was absent for off-the-shelf instrumentation tools such as new relic, so we had to roll our own data collection and reporting system [bug 956230].

the collector wraps specific Bugzilla::DB, Bugzilla::Memcached and Bugzilla::Template calls via subclassing, then reports data to an elasticsearch cluster. currently all reporting solutions are ad-hoc and involve running of scripts which identify the most costly database queries and templates.

this data identified areas of bugzilla which require optimisation or caching. examples include the optimisation of a single query which shaved 200ms (~15%) off page load times for most users [bug 993894], caching of all queries which read an entire table [part of bug 987032], and caching of user groups [bugs 993939] and settings [bug 993926].

we continue to revisit the instrumentation reports in order to guide further improvements.

stylesheet concatenation and minification

improving the performance of a web applications isn’t limited to focusing on server-side execution speed.

due to bugzilla’s extensions we ended up in a situation where bugzilla was serving multiple small css files – on bmo we loaded 17 stylesheets as part of show_bug in comparison with 5 for an installation of bugzilla without any extensions installed.

similar to the issue encountered with memcached, extensions have complete control with regards to optionally loading stylesheets, which means any css concatenation and minification solution needed to be implemented at run-time.

[bug 977969] does exactly that – the template passes an array of stylesheets to load to bugzilla’s global header, where a hash of the array is used to find a unified stylesheet. simple minification is performed which dropped the stylesheet size from 54kb to 43kb on show_bug on bmo.

stylesheet concatenation and minification support will be released with bugzilla 5.0, and has been live on bugzilla.mozilla.org since may 2014.

database

in order to address performance issues caused by bugzilla’s use of the myisam table type, in march our DBAs upgraded our database cluster to mysql version 5.6. this was the result of analysis by the DBAs into replication and stability issues around our myisam table.

as mysql 5.6 adds support for fulltext indexing to its innodb table type, bugzilla was able to switch away from myisam. this immediately fixed the database replication issues, and provided a noticeable performance boost to searches involving comments.

looking forward

the next large project is to update bugzilla so the bug object can use memcached on an unmodified installation without any non-default extensions enabled. for reasons previously covered it’s unlikely we’ll ship a version of bugzilla with this enabled by default, however this will allow sites to audit their own code (if any) and enable caching of the bug object if required.

we will build on the css concatenation and minification work to provide the same benefits to javascript files.

we periodically enable instrumentation and use it to identify the next set of targets for optimisation. this will continue for the foreseeable future.

a tale of bugzilla performance

a high-level goal across multiple teams this year is to improve bugzilla.mozilla.org’s performance, specifically focusing on the time it takes to load a bug (show_bug.cgi).

towards this end, in q1 2014 i focused primarily on two things: implementing a framework for bugzilla to use memcached, and deep instrumentation of bmo in our production environment to help identify areas which require optimisation and could leverage memcached.

i’ll talk more about memcached in a later blog post.  today i’ll talk about a single little query.

the data gathered quickly identified a single query used to determine a user’s group membership was by far the slowest query, averaging more than 200 ms to complete, and was executed on every page:

SELECT DISTINCT groups.id 
  FROM groups, 
       user_group_map, 
       group_group_map 
 WHERE user_group_map.user_id = 3881 
       AND (
           (user_group_map.isbless = 1 AND groups.id=user_group_map.group_id)
           OR (groups.id = group_group_map.grantor_id
               AND group_group_map.grant_type = 1
               AND group_group_map.member_id IN (20,19,10,9,94,23,49,2,119,..))
       )

in bug 993894 i rewrote this query to:

SELECT DISTINCT group_id
  FROM user_group_map
 WHERE user_id = 3881
       AND isbless = 1
UNION
SELECT DISTINCT grantor_id
  FROM group_group_map
 WHERE grant_type = 1
       AND member_id IN (20,19,10,9,94,23,49,2,119,..)

which executes almost instantly.

the yellow bar on the following graph shows when this fix was deployed to bugzilla.mozilla.org:

bug993894

bugzilla 4.2 – what’s new – searching

a major change between bugzilla 4.0 and bugzilla 4.2 comes in the form of changes to searching. the searching implementation in 4.2 was rewritten from scratch, removing some seriously convoluted code and adding a large amount of test cases.

notable differences include:

search results limiting

search results are initially limited to 500 bugs, with an additional click required to show up to 10,000 bugs.  it is no longer possible to generate a query which results in massive bug lists.

searching via APIs remains unlimited.

changes to relative dates

when using relative dates and times, -1w is now a synonym for -7d and means exactly 7 days.

previously, -1w meant the beginning of the week, which was confusing some users. the same confusion existed for -1d which was different from -24h, and for -1m which was different from -30d. now if you really want the beginning of the day, week or month, you must use -1ds, -1ws, and -1ms respectively, where “s” means “start of”.

this change will affect existing saved searches using relative dates.

improved custom search builder

the custom search builder on the advanced search tab has been improved; it’s now easier and quicker to construct queries.

predictable results

complex queries built by the custom search interface are easier to build and have more sensible results, as they are built using a more intuitive logic.

bugzilla.mozilla.org source

there’s been a few questions recently regarding the location of the source which runs bugzilla.mozilla.org.

you can find it at bzr.mozilla.org/bmo – we’re currently running the 4.0 branch.

timeless has pointed mxr-test to the bmo source: mxr-test.mozilla.org/bugzillamozilla-bzr

bugzilla inline history extension

Unlike most bug tracking systems, Bugzilla doesn’t display changes made to fields when it displays the comments.  This has been a often requested feature, and is currently targeted for Bugzilla version 5.0.  For some time the Bugzilla Tweaks Firefox add-on has provided this functionality to bugzilla.mozilla.org, and it’s working well for those who have discovered this add-on.

Using the Tweaks implementation as a guide, I have implemented an Inline History Bugzilla extension:

This extension has just been deployed to BMO and is currently disabled by default.  To enable you have to be logged in, then go your User Preferences, and set When viewing a bug, show all bug activity to on.

Once we’re happy with it it will be enabled by default for all logged-in users.  [Edit: Now enabled by default]

The latest version of Bugzilla Tweaks (v1.11) will detect when this Bugzilla extension is active, and will disable its inline history implementation.  The option to disable the extension will always be available should someone perfer Bugzilla Tweaks’ formatting over the extension’s.

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