Mercurial 3.6 (scheduled for release on or shortly after November 1) contains a number of improvements to cloning. In this post, I will describe a new feature to help server operators reduce load (while enabling clients to clone faster) and some performance work to make clone operations faster on the client.

Cloning repositories can incur a lot of load on servers. For mozilla-central (the main Firefox repository), clones require the server to spend 4+ minutes CPU time and send ~1,230 MB over the network. Multiply by thousands of clients from build and test automation and developers, and you could quickly finding yourself running out of CPU cores or network bandwidth. Scaling Mercurial servers (like many services) can therefore be challenging. (It's worth noting that Git is in the same boat for reasons technically similar to Mercurial's.)

Mozilla previously implemented a Mercurial extension to seed clones from pre-generated bundle files so the Mercurial servers themselves don't have to work very hard for an individual clone. (That linked post goes into the technical reasons why cloning is expensive). We now offload cloning of frequently cloned repositories on hg.mozilla.org to Amazon S3 and a CDN and are diverting 1+ TB/day and countless hours of CPU work away from the hg.mozilla.org servers themselves.

The positive impact from seeding clones from pre-generated, externally-hosted bundles has been immense. Load on hg.mozilla.org dropped off a cliff. Clone times on clients became a lot faster (mainly because they aren't waiting for a server to dynamically generate/stream bits). But there was a problem with this approach: it required the cooperation of clients to install an extension in order for clone load to be offloaded. It didn't just work.

I'm pleased to announce that the ability to seed clones from server-advertised pre-generated bundles is now a core feature in Mercurial 3.6! Server operators can install the clonebundles extension (it is distributed with Mercurial) to advertise the location of pre-generated, externally-hosted bundle files. Compatible clients will automatically clone from the server-advertised URLs instead of creating potentially excessive load on the Mercurial server. The implementation is almost identical to what Mozilla has deployed with great success. If you operate a Mercurial server that needs to serve larger repositories (100+ MB) and/or is under high load, you should be jumping with joy at the existence of this feature, as it should make scaling problems attached to cloning mostly go away.

Documentation for server operators is currently in the extension and can be accessed at the aforementioned URL or with hg help -e clonebundles. It does require a bit of setup work. But if you are at the scale where you could benefit from the feature, the results will almost certainly be worth it.

One caveat is that the feature is currently behind an experimental flag on the client. This means that it doesn't just work yet. This is because we want to reserve the right to change some behaviors without worrying about backwards compatibility. However, I'm pretty confident the server parts won't change significantly. Or if they do, I'm pretty committed to providing an easy transition path since I'll need one for hg.mozilla.org. So, I'm giving server operators a tentative green light to deploy this extension. I can't guarantee there won't be a few bumps transitioning to a future release. But it shouldn't be a break-the-world type of problem. It is my intent to remove the experimental flag and have the feature enabled by default in Mercurial 3.7. At that point, server operators just need clients to run a modern Mercurial release and they can count on drastically reduced load from cloning.

To help with adoption and testing of the clone bundles feature, servers advertising bundles will inform compatible clients of the existence of the feature when they clone:

$ hg clone https://hg.mozilla.org/mozilla-central
requesting all changes
remote: this server supports the experimental "clone bundles" feature that should enable faster and more reliable cloning
remote: help test it by setting the "experimental.clonebundles" config flag to "true"
adding changesets
adding manifests
adding file changes
...

And if you have the feature enabled, you'll see something like:

$ hg clone https://hg.mozilla.org/mozilla-central
applying clone bundle from https://hg.cdn.mozilla.net/mozilla-central/daa7d98525e859d32a3b3e97101e129a897192a1.gzip.hg
adding changesets
adding manifests
adding file changes
added 265986 changesets with 1501210 changes to 223996 files
finished applying clone bundle
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

This new clone bundles feature is deployed on hg.mozilla.org. Users of Mercurial 3.6 can start using it today by cloning from one of the repositories with bundles enabled. (If you have previously installed the bundleclone extension, please be sure your version-control-tools repository is up to date, as the extension was recently changed to better interact with the official feature.)

And that's the clone bundles feature. I hope you are as excited about it as I am!

Mercurial 3.6 also contains numerous performance improvements that make cloning faster, regardless of whether you are using clone bundles! For starters:

• Caching just-added entries made changelog writing 25% faster.
• Reusing file handles when adding revlog entries drastically reduced the number of file opens, closes, and writes.
• Avoiding excessive file flushing when adding revlog entries drastically reduced system call count.

These performance enhancements will make all operations that write new repository data faster. But it will be felt most on clone and pull operations on the client and push operations on the server.

One of the most impressive performance optimizations was to a Python class that converts a generator of raw data chunks to something that resembles a file object so it can be read() from. Refactoring read() to avoid collections.deque operations and an extra string slice and allocation made unbundle operations 15-20% faster. Since this function can handle hundreds of megabytes or even gigabytes of data across hundreds of thousands of calls, small improvements like this can make a huge difference! This patch was a stark reminder that function calls, collection mutations, string slicing, and object allocation all can have a significant cost in a higher-level, garbage collected language like Python.

The end result of all this performance optimization on applying a mozilla-central gzip bundle on Linux on an i7-6700K:

• 35-40s wall time faster (~245s to ~205s) (~84% of original)
• write(2) calls reduced from 1,372,411 to 679,045 (~49% of original)
• close(2) calls reduced from 405,147 to 235,039 (~58% of original)
• total system calls reduced from 5,120,893 to 2,938,479 (~57% of original)

And the same operation on Windows 10 on the same machine:

• ~300s wall time faster (933s to 633s) (~68% of original)

You may have noticed the discrepancy between Linux and Windows wall times, where Windows is 2-4x slower than Linux. What gives? The reason is closing file handles that have been appended to is slow on Windows. For more, read my recent blog post.

Mercurial writes ~226,000 files during a clone of mozilla-central (excluding the working copy). Assuming 2ms per file close operation, that comes out to ~450s just for file close operations! (All operations are on the same thread.) The current wall time difference between clone times on Windows and Linux is ~428s. So it's fair to say that waiting on file closes accounts for most of this.

Along the same vein, the aforementioned performance work reduced total number of file close operations during a mozilla-central clone by ~165,000. Again assuming 2ms per file close, that comes to ~330s, which is in the same ballpark as the ~300s wall time decrease we see on Windows in Mercurial 3.6. Writing - and therefore closing - hundreds of thousands of files handles is slower on Windows and accounts for most of the performance difference on that platform.

Empowered by this knowledge, I wrote some patches to move file closing to a background thread on Windows. The results were promising (minutes saved when writing 100,000+ files). Unfortunately, I didn't have time to finish these patches for Mercurial 3.6. Hopefully they'll make it into 3.7. I also have some mad scientist ideas for alternate storage mechanisms that don't rely on hundreds of thousands of files. This should enable clones to run at 100+ MB/s on all platforms - basically as fast as your network and system I/O can keep up (yes, Python and Windows are capable of this throughput). Stay tuned.

And that's a summary of the cloning improvements in Mercurial 3.6!

Mercurial 3.6 is currently in release candidate. Please help test it by downloading the RC at https://www.mercurial-scm.org/. Mercurial 3.6 final is due for release on or shortly after November 1. There is a large gathering of Mercurial contributors in London this weekend. So if a bug is reported, I can pretty much guarantee a lot of eyeballs will see it and there's a good chance it will be acted upon.

A few weeks ago, some coworkers were complaining about the relative performance of Mercurial cloning on Windows. I investigated on my brand new i7-6700K Windows 10 desktop machine and sure enough they were correct: cloning times on Windows were several minutes slower than Linux on the same hardware. What gives?

I performed a few clones with Python under a profiler. It pointed to a potential slowdown in file I/O. I wanted more details so I fired up Sysinternals Process Monitor (strace for Windows) and captured data for a clone.

As I was looking at the raw system calls related to I/O, something immediately popped out: CloseFile() operations were frequently taking 1-5 milliseconds whereas other operations like opening, reading, and writing files only took 1-5 microseconds. That's a 1000x difference!

I wrote a custom Python script to analyze an export of Process Monitor's data. Sure enough, it said we were spending hundreds of seconds in CloseFile() operations (it was being called a few hundred thousand times). I posted the findings to some mailing lists. Follow-ups in Mozilla's dev-platform list pointed me to an old MSDN blog post where it documents behavior similar to what I was seeing.

Long story short, closing file handles that have been appended to is slow on Windows. This is apparently due to an implementation detail of NTFS. Writing to a file in place is fine and only takes microseconds for the open, write, and close. But if you append a file, closing the associated file handle is going to take a few milliseconds. Even if you are using Overlapped I/O (async I/O on Windows), the CloseHandle() call to close the file handle blocks the calling thread! Seriously.

This behavior is in stark contrast to Linux and OS X, where system I/O functions take microseconds (assuming your I/O subsystem can keep up).

There are two ways to work around this issue:

1. Reduce the amount of file closing operations on appended files.
2. Use multiple threads for I/O on Windows.

Armed with this knowledge, I dug into the guts of Mercurial and proceeded to write a number of patches that drastically reduced the amount of file I/O system calls during clone and pull operations. While I intend to write a blog post with the full details, cloning the Firefox repository with Mercurial 3.6 on Windows is now several minutes faster. Pretty much all of this is due to reducing the number of file close operations by aggressively reusing file handles.

I also experimented with moving file close operations to a separate thread on Windows. While this change didn't make it into Mercurial 3.6, the results were very promising. Even on Python (which doesn't have real asynchronous threads due to the GIL), moving file closing to a background thread freed up the main thread to do the CPU heavy work of processing data. This made clones several minutes faster. (Python does release the GIL when performing an I/O system call.) Furthermore, simply creating a dedicated thread for closing file handles made Mercurial faster than 7-zip at writing tens of thousands of files from an uncompressed tar archive. (I'm not going to post the time for tar on Windows because it is embarassing.) That's a Python process on Windows faster than a native executable that is lauded for its speed (7-zip). Just by offloading file closing to a single separate thread. Crazy.

I can optimize file closing in Mercurial all I want. However, Mercurial's storage model relies on several files. For the Firefox repository, we have to write ~225,000 files during clone. Assuming 1ms per file close (which is generous), that's 225s (or 3:45) wall time performing file closes. That's not going to scale. I've already started experimenting with alternative storage modes that initially use 1-6 files. This should enable Mercurial clones to run at over 100 MB/s (yes, Python and Windows can do I/O that quickly if you are smart about things).

My primary takeaway is that creating/appending to thousands of files is slow on Windows and should be addressed at the architecture level by not requiring thousands of files and at the implementation level by minimizing the number of file close operations after write. If you absolutely must create/append to thousands of files, use multiple threads for at least closing file handles.

My secondary takeaway is that Sysinternals Process Monitor is amazing. I used it against Firefox and immediately found performance concerns. It can be extremely eye opening to see how your higher-level code is translated into function calls into your operating system and where the performance hot spots are or aren't at the OS level.

Starting today, a Mozilla LDAP account with Mercurial SSH access is no longer required to hg push into MozReview to initiate code review with Mozilla projects.

The instructions for configuring your client to use MozReview have been updated to reflect how you can now push to MozReview over HTTP using a Bugzilla API Key for authentication.

This change effectively enables first-time contributors to use MozReview for code review. Before, you had to obtain an LDAP account and configure your SSH client, both of which could be time consuming processes and therefore discourage people from contributing. (Or you could just use Bugzilla/Splinter and not get the benefits of MozReview, which many did.)

I encourage others to update contribution docs to start nudging people towards MozReview over Bugzilla/patch-based workflows (such as bzexport).

Bug 1195856 tracked this feature.

For the past few months, Mozilla has been serving Mercurial clones from Amazon S3. We upload snapshots (called bundles) of large and/or high-traffic repositories to S3. We have a custom Mercurial extension on the client and server that knows how to exchange the URLs for these snapshots and to transparently use them to bootstrap a clone. The end result is drastically reduced Mercurial server load and faster clone times. The benefits are seriously ridiculous when you operate version control at scale.

Amazon CloudFront is a CDN. You can easily configure it up to be backed by an S3 bucket. So we did.

https://hg.cdn.mozilla.net/ is Mozilla's CDN for hosting Mercurial data. Currently it's just bundles to be used for cloning.

As of today, if you install the bundleclone Mercurial extension and hg clone a repository on hg.mozilla.org such as mozilla-central (hg clone https://hg.mozilla.org/mozilla-central), the CDN URLs will be preferred by default. (Previously we preferred S3 URLs that hit servers in Oregon, USA.)

This should result in clone time reductions for Mozillians not close to Oregon, USA, as the CloudFront CDN has servers all across the globe and your Mercurial clone should be bootstrapped from the closest and hopefully therefore fastest server to you.

Unfortunately, you do need the the aforementioned bundleclone extension installed for this to work. But, this should only be temporary: I've proposed integrating this feature into the core of Mercurial so if a client talks to a server advertising pre-generated bundles the clone offload just works. I already have tentative buy-in from one Mercurial maintainer. So hopefully I can land this feature in Mercurial 3.6, which will be released November 1. After that, I imagine some high-traffic Mercurial servers (such as Bitbucket) will be very keen to deploy this so CPU load on their servers is drastically reduced.

I added a feature to Mercurial 3.4 that exposes JSON from Mercurial's various web APIs. Unfortunately, due to the presence of legacy code on hg.mozilla.org providing similar functionality, we weren't able to deploy this feature to hg.mozilla.org when we deployed Mercurial 3.4 several weeks ago.

I'm pleased to announce that as of today, JSON is now exposed from hg.mozilla.org!

To access JSON output, simply add json- to the command name in URLs. e.g. instead of https://hg.mozilla.org/mozilla-central/rev/de7aa6b08234 use https://hg.mozilla.org/mozilla-central/json-rev/de7aa6b08234. The full list of web commands, URL patterns, and their parameters are documented in the hgweb help topic.

Not all web commands support JSON output yet. Not all web commands expose all data available to them. If there is data you need but isn't exposed, please file a bug and I'll see what I can do.

Thanks go to Steven MacLeod for reviewing the rather large series it took to make this happen.