It is with a heavy heart that I’m announcing the resignation of my position at Mozilla. Last month marked my 5th year here, and over that time I’ve met some of the most intelligent and driven people in the world. I’m proud to have known you and worked alongside you these years.
As part of my ongoing work I’ve been measuring the size and depth of mozilla-central to extrapolate future repository size for scaling purposes. Part of this was figuring out some details such as average file size, distribution of types of files, and on-disk working copy size versus repository size.
When I posted a graph comparing the size of the mozilla-central repository by Firefox version my colleague gszorc was quick to point out that the 4k blocksize of the filesystem meant that the on-disk size of a working copy might not accurately reflect the true size of the repository. I considered this and compared the working copy size (with blocksize =1) to the typical 4k blocksize. This is the result.
UPDATE: By popular demand I’ve added numbers for beta, aurora, and m-c tip
As part of my recent duties I’ve been looking at trends in Mozilla’s monolithic source code repository mozilla-central. As we’re investigating growth patterns and scalability I thought it would be useful to get metrics about the size of the repositories over time, and in what ways it changes.
It should be noted that the sizes are for all of mozilla-central, which is Firefox and several other Mozilla products. I chose Firefox versions as they are useful historical points. As of this posting (2015-02-06) version 36 is Beta, 37 is Aurora, and 38 is tip of mozilla-central.
UPDATE: This newly generated graph shows that there was a sharp increase in the amount of code around v15 without a similarly sharp rise of working copy size. As this size was calculated with ‘du’, it will not count hardlinked files twice. Perhaps the size of the source code files is insignificant compared to other binaries in the repository. The recent (v34 to v35) increase in working copy size could be due to added assets for the developer edition (thanks hwine!)
My teammate Gregory Szorc has reminded me that since this size is based off a working copy, it is not necessarily accurate as stored in Mercurial. Since most of our files are under 4k bytes they will use up more space (4k) when in a working copy.
From this we can see a few things. The line count scales linearly with the size of a working copy. Except at the beginning, where it was about half the ratio until about Firefox version 18. I haven’t investigated why this is, although my initial suspicion is that it might be caused by there being more image glyphs or other binary data compared to the amount of source code.
Also interesting is that Firefox 5 is about 3.4 million lines of code while Firefox 35 is almost exactly 6.6 million lines. That’s almost a doubling in the amount of source code comprising mozilla-central. For reference, Firefox 5 was released around 2011/06/21 and Firefox 35 was released on 1/13/2015. That’s about two and a half years of development to double the codebase.
If I had graphed back to Firefox 1.5 I am confident that we would see an increasing rate at which code is being committed. You can almost begin to see it by comparing the difference between v5 and v15 to v20 and v30.
I’d like to continue my research into how the code is evolving, where exactly the large size growth came from between v34 and v35, and some other interesting statistics about how individual files evolve in terms of size, additions/removals per version, and which areas show the greatest change between versions.
If you’re interested in the raw data collected to make this graph, feel free to take a look at this spreadsheet.
The source lines of code count was generated using David A. Wheeler’s SLOCCount.
A week ago I was fortunate enough to attend the latest code sprint of the Mercurial project. This was my second sprint with this project, and took away quite a bit from the meeting. The attendance of the sprint was around 20 people and took the form of a large group, with smaller groups splitting out intermittently to discuss particular topics. I had seen a few of the attendees before at a previous sprint I attended.
Joining me at the sprint were two of my colleagues Gregory Szorc (gps) and Mike Hommey (glandium). They took part in some of the serious discussions about core bugfixes and features that will help Mozilla scale its use of Mercurial. Impressively, glandium had only been working on the project for mere weeks, but was able to make serious contributions to the bundle2 format (an upcoming feature of Mercurial). Specifically, we talked to Mercurial developers about some of the difficulties and bugs we’ve encountered with Mozilla’s “try” repository due to the “tens of thousands of heads” and the events that cause a serving request to spin forever.
By trade I’m a sysadmin/DevOps person, but I also do have a coder hat that I don from time to time. Still though, the sprint was full of serious coders who seemingly worked on Mercurial full-time. There were attendees who had big named employers, some of whom would probably prefer that I didn’t reveal their identities here.
Unfortunately due to my lack of familiarity with a lot of the deep-down internals I was unable to contribute to some of the discussions. It was primarily a learning experience for me for both the process which direction-driving decisions are made for the project (mpm’s BDFL status) and all of the considerations that go into choosing a particular method to implement an idea.
That’s not to say I was entirely useless. My knowledge of systems and package management meant I was able to collaborate with another developer (kiilerix) to improve the Docker package building support, including preliminary work for building (un)official Debian packages for the first time.
I also learned about some infrequently used features or tips about Mercurial. For example, folks who come from a background of using git often complain about Mercurial’s lack of interactive rebase functionality. The “histedit” extension provides this feature. Much like many other features of Mercurial, this is technically “in core”, but not enabled by default. Adding a line in the “[extensions]” section your “hgrc” file such as “histedit =” enables it. It allows all the expected picking, folding, dropping, editing, or modifying commit messages.
Changeset evolution is another feature that’s been coming for a long time. It enables developers to safely modify history and be able to propagate those changes to any down/upstream clones. It’s still disabled by default, but is available as an extension. Gregory Szorc, a colleague of mine, has written about it before. If you’re curious you can read more about it here.
One of the features I’m most looking forward to is sparse checkouts. Imagine a la Perforce being able to only check out a subtree or subtrees of a repository using ‘–include subdir1/’ and –exclude subdir2/’ arguments during cloning/updating. This is what sparse checkouts will allow. Additionally, functionality is being planned to enable saved ‘profiles’ of subdirs for different uses. For instance, specifying the ‘–enable-profile mobile’ argument will allow a saved list of included and excluded items. This seems like a really powerful way of building lightweight build profiles for each different type of build we do. Unfortunately to be properly implemented it is waiting on some other code to be developed such as sharded manifests.
One last thing I’d like to tell you about is an upcoming free software project for Mercurial hosting named Kallithea. It was borne from the liberated code from the RhodeCode project. It is still in its infancy (version 0.1 as of the writing of this post), but has some attractive features for viewing repositories, such visualizations of changelog graphs, diffs, code reviews, a built-in editor, LDAP support, and even a JSON-RPC API for issue tracker integration.
In all I feel it was a valuable experience for me to attend that benefited both the Mercurial project and myself. I was able to lend some of my knowledge about building packages and familiarity with operations of large-scale hgweb serving, and was able to learn a lot about the internals of Mercurial and understand that even the deep core code of the project isn’t very scary.
I’m very thankful for my ability to attend and look forward to attending the next Sprint in the following year.
At Mozilla we use Mercurial for Firefox development. We have several repositories/trees that are used depending on where the code should be. If a developer wishes to test the code they have been developing, they can submit it to a Mercurial repository called ‘try‘, since running our entire test suite is not feasible on developer machines.
We have quite a bit of infrastructure around this including Tinderbox Pushlog (TBPL) and more. This post deals with the infrastructure and problem we face while trying to scale the ‘try’ repository.
A few statistics:
- The try repository currently has 17943 heads. These heads are never removed.
- The try repository is about 3.6 GB in size.
- Due to Mercurial’s on-wire HTTP protocol, this number of heads causes HTTP cloning to fail
- There are roughly 81000 HTTP requests to try per day
- To fix problems (mentioned below), the try repository is deleted and re-cloned from mozilla-central every few months
There are a number of problems associated with such a repository. One particularly nasty one has been present through several years of Mercurial development, and has been tricky in that it is seemingly unreproducible. The scenario is something like:
- User ‘hg push’es some changes to a new head onto try
- The push process takes a long time (sometimes between 10 minutes and hours)
- A developer could issue an interrupt signal (ctrl+C) which causes the client to gracefully hang up and exit (his typically has no effect on the server
- Subsequent pushes will hang with something similar to ‘remote: waiting for lock on repository /repo/hg/mozilla/try/ held by ‘hgssh1.dmz.scl3.mozilla.com:23974’
- When this happens a hg process is running on the server has the following characteristics:
- A ‘hg serve’ process runs single-threaded using 100% CPU
- strace-ing and ltrace-ing reveal that the process is not making any system calls or external library calls
- perf reveals that the process is spending all of its time inside some ambiguous python function
- pdb yields that the process is spending all of its time in a function that (along some point in the stack trace) is going through ancestor calculations
- The process will eventually exit cleanly
- As operators there is nothing we can do that to alleviate the situation once the repository gets in this state. We simply inform developers and monitor the situation.
There have been several ideas on ways to alleviate the problem:
- Periodically reset ‘try’. This is considered bad because 1) it loses history, and 2) it is disruptive to developers, who might have to re-submit try jobs again
- Reset try on the SSH servers, but keep old try repositories on the HTTP servers. This has the potential to create unforeseen problems of growing these repositories even further on the HTTP servers. If reset (staggered from SSH server resets) this will remove unforeseen problem potential, but still lose history.
- Creating bundle files out of pushes to ‘try’, then hosting these in an accessible location (S3, http webroot, etc). I will detail this method in a future blog post.
As of now though, try will periodically need to be reset as a countermeasure to the hangs mentioned in this post. Getting a reproducible test case might allow us to track down a bug or inefficiency in Mercurial to fix this problem after all. If you’d like to help us with this, please ping fubar or me (bkero) on irc.mozilla.org.
Earlier this year I was able to move mozilla.org’s Mercurial infrastructure from using NFS-mounted storage to transitioning to local disk. This was done for several reasons. Continue reading “Measuring the performance improvement of Mercurial (NFS vs local disk)”