Category Archives: Software

Some more git, mercurial, and darcs

Programming

Ted Ts’o had an interesting post about git recently. He has a lot of good thoughts on the subject. He comments that he wound up using git because it’s so Unixy (with its small commands to do things), that he sees the git community developing innovations faster than Mercurial, and that they are working to improve the documentation and user interface problems.

The being so Unixy is a double-edged sword. On the one hand, it can make it easy to write shell scripts to extend Git. That itself can be a double-edged sword (think filename quoting and the like). But one doesn’t have to use the shell. The other downside is that being Unixy makes it hard to run on platforms that aren’t, such as Windows. So if one is working on Unix-only software (X, the kernel, e2fsprogs, etc.), there’s no need to care about it. But if you’re a person like me, who has Windows users using my software, or a large organization like Mozilla, it’s maybe a showstopper. Of course, workarounds exist (cygwin, git-cvsserver), but none of them are particularly nice.

I think that both Git and Mercurial are working to address their shortcomings. I’ve chosen hg for now because it does what I need now. And because there are very nice tools to convert hg to git, and vice-versa. So if Ted’s right, and a year from now git is easier to use, better documented, more featureful, and runs well on Windows, it won’t be that hard to switch over and preserve history. Ted’s the sort of person that usually is right, so maybe I should starting looking at hg2git right now

So following up on my bzr post, here are the things that Mercurial is great at right now:

  1. Performance. Approximately even with git, occasionally faster. Nobody else can compete with these two right now.
  2. Simplicity. It’s almost as easy to get started as with darcs, and with recent patches, will be even closer in the future.
  3. Lots of ways to interact. You can send hg bundles, which preserve all metadata (parents, hash, authors, etc), or you can send git-format email patches, or you can push and pull between repos. The email tools will shortly be able to automatically detect what patches to send. Your choice. git doesn’t seem to support lossless emailing of bundles like this, and bzr doesn’t make emailing of anything easy by default.
  4. Merging. hg seems to be able to automatically resolve more merge conflicts than anything else, and when it can’t automatically resolve them, has a nicely configurable system to let you use your choice of tool to manually resolve them.
  5. Community. The Mercurial community is open and inviting, and open to new/different ideas. It seems similar to Darcs in that respect, and somewhat dissimilar to git.
  6. Rebase does not trash history like it does (barring undocumented manual intervention) in git.

I’ve written before about Darcs, so I won’t duplicate that here.

bzr, again

Programming, , ,

I’ve talked a lot lately about different VCSs.

I got some interesting comments in reply to my most recent post. One person took issue with my complaint that nobody really understood how to specify a revision to git format-patch, and proceeded to issue an incorrect suggestion. And a couple of people complained about my comments about bzr, which generally came down to the released version of bzr didn’t have anything compelling and also didn’t support tags.

So I went into , asked them what bzr has that git, Mercurial, and darcs don’t. And gave bzr the benefit of the doubt that 0.15 will be out soon and will be stable. What I got back were these general items:

  1. Renaming of directories (not in hg, git)
  2. 2-way sync with Subversion (not in hg, darcs)
  3. Checkouts (not in any others by default)
  4. No server-side push requirement

Let’s look at these in more detail.

1. Renaming of directories

All of them can rename files and (excepting git) completely accurately track back the file’s history. But consider this: if person A commits a change to branch A that adds a file, and person B then renames the directory that the file is in on his branch, will a merge cause person A’s file to appear in the new directory name? In darcs and bzr, yes. In Mercurial and git, no.

So yes, this is a nice thing. But I have never actually had this situation crop up in practice, and even if it did, could be trivially remedied. I would say that for me, I don’t really care.

[Update: Current stable releases of Mercurial can do this too. I’m not quite sure how, but it does work. So git is the only one that can’t do that.]

2. 2-way sync with Subversion

This is a really nice feature and is present in both git and bzr. I haven’t tested it either place, but if it works as advertised — and properly supports tracking multiple related svn branches and merges — would be slick. That was enough to make me consider using git, but in retrospect, I so rarely interact with people using svn that it is not that big a deal to me.

Still, for those that have to work with svn users, this feature in bzr and git could be a big one.

Better yet would be to get all those svn holdouts over to DVCS.

3. Checkouts

A bzr checkout is basically a way to make local commits be pushed to the remote repo immediately, as with svn. This is of no utility to me, though I can see some may have a use for it. But it can be done with hg hooks and probably approximated with scripting in others.

4. No server-side process necessary for pushing repos

bzr has built-in support to push to a server that has sftp only, and doesn’t require a copy of itself on the server. While I believe that none of the other three have that, it is possible to rsync (and probably ftp) darcs and Mercurial repos to a server in a safe fashion by moving repo files in a defined order. Probably also possible with git. All four can pull repos using nothing but regular HTTP.

What bzr still doesn’t have

Integrated patch emailing. The big thing is that it has no built-in emailing of patches support. darcs is extremely strong in this area, followed by hg, and git is probably third. “darcs send” is all it takes to have darcs look at the remote repo, figure out what you have that they don’t, and e-mail a bundle of changesets to them. I posted an extension and later a patchset that does all this for Mercurial except for automatically figuring out what default email address to do (that’ll come in a few days, I think). One feature Mercurial has had for awhile that Darcs hasn’t is sending multiple textual diffs as a thread, with one message per changeset. bzr doesn’t have any support for emailing patches yet, which is disappointing. Because of the strong support for this in darcs and Mercurial, people running those systems feel less of a need to publish their repos.

[Update: There is a plugin for bzr that seems to address some of this. I haven’t tested it, and it’s not in bzr core (so doesn’t quite meet my very friendly for a newbie requirement), but this does exist, though apparently not as advanced as Mercurial]

Performance. Supposedly 0.15 is supposed to be better on this, but even if bzr achieves the claimed doubling of performance, most benchmarks I have seen would rate it as still being significantly behind git and Mercurial, though it may overtake darcs in some tests.

Extensive documentation. I would say that bzr’s docs are better in some ways than git’s (its tutorials especially), but lack depth. If you want to know some detail about how the repository works on-disk, it’s not really documented. Darcs still has David’s excellent manual, and Mercurial has the hg book which is still great as well.

Merging not as advanced. darcs is pretty obviously way on top here, but of the others, Mercurial does a pretty good job with its automatic handling of renames and automatic resolving of different branches that commit the same change (even if that same change is a rename, or an add of the same content). bzr can’t resolve as much automatically.

Summary

Well, I’ll say that bzr still doesn’t look compelling enough for my use cases to use, and the lack of an easy-for-a-newbie-to-use automated email submission feature is a pretty big disappointment. Though I did appreciate the time those on spent with me, and if I needed to sync with svn users frequently, I’d probably choose bzr over git.

For now, I’m happy with sticking with darcs for my code and hg for my Debian work.

But all four communities are aggressively working on their weaknesses, and this landscape may look very different in a year.

More on Git, Mercurial, and Bzr

Programming, , ,

I’ve been writing a lot about this lately, I know, but it’s an interesting landscape.

I had previously discarded git, but in light of git-cvsserver (which provides a plausible way for Windows people to participate), I gave it a try.

The first thing I noticed is that git documentation, in general, is really poor. Some tutorials that claim to cover git actually cover cogito. Still others use commands that are much more complex than those in the current git — and these just the ones linked to from the git homepage.

git’s manpages aren’t much better. There are quite a few git commands (such as log) that take arguments that other git commands accept. Sometimes this fact is documented with a pointer to these other commands, but often not; a person is left guessing what the full range of accepted arguments are.

My complaint that git is overly complex still exists. They’ve made progress, but still have a serious issue here. Part is because of the docuemtnation, and part is because of the interface. I wanted to export to diffs all patches on the current branch in a repo. I asked on , and someone suggested using the revision specifier ..HEAD. Nope, didn’t work. A few other git experts chimed in, and none could come up with the correct recipe. I finally used -500, which worked but is hackish.

git’s lack of even offering support for a human to indicate renames also bothers me, though trustworthy people have assured me that it doesn’t generally cause a problem in practice.

git does have nicer intra-repo branching than Mercurial does, for the moment. But the Mercurial folks are working on that anyway, and branching to new directories still works fine for me.

But in general, git’s philosophy is to make things easy for the upstream maintainer, and doesn’t spend much effort making things easy for contributors (except to make it mildly easier to contribute to a large project like Linux). Most of my software doesn’t have a large developer community, and I want to make it as easy as possible for new developers to join in and participate. git still utterly fails on that.

I tried bzr again. It seems that every time I try it, after just a few minutes, I am repulsed. This time, I stopped when I realized that bzr doesn’t support tags and has no support for emailing changesets whatsoever. As someone that has really liked darcs send (and even used tags way back with CVS!), this is alarming. The tutorial on the bzr website referenced a command “bzr help topics”, which does not work.

So I’ll stick with my mercurial and darcs combination for now.

I announced the first version of a hg send extension yesterday as well. I think Mercurial is very close to having a working equivalent to darcs send.

Want to try living in vim

Programming

I’ve been an Emacs user for many years, though of course I know some vi and vim commands out of necessity.

I want to try taking the plunge by spending a month using vim only, no Emacs.

Sadly the vim documentation isn’t very helpful for me in a number of areas. I’m hoping someone can point me to some resources or recipes that will help with:

  • Turning off that stupid “hide most of the Debian changelog” thing. I have no idea why it does that or how to make it stop.
  • Turn on or off autoindent, syntax highlighting, etc. in various languages (really, I want to set global defaults for all of them)
  • Be able to edit another file without closing or saving the first (:e doesn’t seem to do what I want)
  • Integrate it with Mercurial and Darcs

Re-Examining Darcs & Mercurial

Programming,

I recently wrote an article or two about distributed version control systems.

I’ve been using Darcs since 2005. I switched to Darcs, in fact, 10 days after the simultaneous founding announcements of git and Mercurial.

Overall, I have been happy. I continue to believe that it is the most distributed of the distributed VCSs, which is a Good Thing.

However, I have lately started having trouble with Darcs hanging while working on my Debian packages. My post to the Darcs user list drew out a few other people whith this problem, which is a design flaw of Darcs.

So I revisited the VCS landscape. I re-examined git, Mercurial, and bzr. I eventually decided to give Mercurial a try. I avoided git because I write some code that is portable to Windows, and git isn’t (or isn’t very well). Also, git is complex to pick up for me, and I certainly don’t want to force something complex onto my contributors. bzr seemed to still have some strange behaviors that it’s had for awhile, and I couldn’t find even one advantage of it over Mercurial. So off I went with Mercurial.

I quickly learned a bit of a philosophical difference from Darcs to Mercurial.

Darcs avoids conflicts at all costs. Mercurial makes handling conflict easy and, in many cases, automatic.

It is exactly this Darcs behavior that permits both is excellent “darcs send” feature (still unmatched in any other VCS), but also causes its hang problems.

I found Mercurial quite pleasant to work with, and *fast*. It seems to be edging out git in speed tests sometimes these days.

It is easy to get started with Mercurial. The mq system — similar to quilt or other patch-management programs — is really quite an amazing hybrid between patch management and version control. I frankly don’t see any need for other patch-management tools anymore.

Mercurial has a “patchbomb” feature where you can select a range of changesets to send off, and it will generate nice emails with one changeset per email, and send them to your selected destination, optionally with an introductory message. The normal way of interacting with other Mercurial users is via the hg export/import commands, which send around simple unified diffs plus some additional header information, optionally in the git extended diff format.

I am happy with Mercurial and am in the process of converting my Debian repositories from Darcs to Mercurial. I’m going to keep my personal code in Darcs for the moment because “darcs send” is still easier than “hg email”, but that may change before long, depending on how my experience goes.

I’d encourage others to give Mercurial a try. The community is also very nice and helpful.

I have contributed patches to Tailor to make it make exact copies of Darcs repos into Mercurial, which are now in its Darcs repo. There is also a thread on the Mercurial list with some of my initial questions/concerns coming from a Darcs perspective.

A better environment for shell scripting

Programming

Shell scripts are good for a lot of things. It’s quick and easy to design shell scripts that take input from one program, pass it to another program, munge it for filenames, etc.

But there are a few drawbacks to shell scripts.

The drawback, in my opinion, is that it is extremely difficult to get quoting and escaping right. I often see things like $@ in shell scripts (breaks if a parameter has a space in it). I also see people failing to check for errors properly (set -e helps that). It’s also difficult to do a more modern style of exception handling (do a sequence of actions in a temporary directory, and always remove that directory, even if there’s an error, but stop processing and propogate the error). Command-line parsing is esoteric and odd, even with getopt. That’s not to say that it’s impossible to make a secure shell script that handles filenames with spaces in them properly. Just that it’s difficult, and makes using common operators like backticks difficult.

Awhile back, I toyed with the idea of making Haskell a shell scripting language. This week, I spent some time to make this a reality. I released HSH, a shell scripting environment for Haskell.

HSH makes it easy to run shell commands, set up pipelines, etc. straight from Haskell. You can either use simple strings to invoke commands (they’ll be passed to sh -c), or you can specify arguments as a list (like exec…() takes), which eliminates the strange filename problems.

But the really cool thing is that HSH doesn’t just let you pipe from one external program to another. It also lets you pipe to/from pure Haskell functions. Yes, you can pipe the output of ls -l straight into a Haskell version of grep. I’ve found it to be very nice, especially for more complex processing tasks.

I put these simple examples on the HSH homepage:

run $ "echo /etc/pass*" :: IO String
 -> "/etc/passwd /etc/passwd-"

runIO $ "ls -l" -|- "wc -l"
 -> 12

runIO $ "ls -l" -|- wcL
 -> 12

In this example, wcL is a pure-Haskell line-counting function.

The results were surprising. According to SLOCCount, porting hg-buildpackage from a shell script to a HSH script achieved a 20% reduction in source lines of code. And at the same time, gained better error handling, better safety of filenames, better type safety (compile-time type checking), etc. Yet it does exactly the same thing in almost exactly the same way.

Even greater savings will occur too. I decided to reimplement a small part of sed just for fun, and that code is still in my tree. If I removed that and replaced it with a call to sed as in the shell version, that would probably buy another 5% savings.

I didn’t really expect to achieve a reduction in lines of code. I thought that I’d be lucky to come close to breaking even. After all, who’d expect something other than the shell to be better at shell scripting?

I don’t know if these results are generalizable, but I’m really excited about it.

Rebase Considered Harmful

Programming

Today I was musing about different version control systems and merge algorithms. I’ve been thinking specifically about how I maintain Debian packages in Darcs. I tend to import upstream tarballs into one branch, and maintain the Debian packages in another, simply merging when a new upstream is released.

Now, there seem to be two prevailing philosophies on how to handle merges in this case. I’m thinking here about merges back to upstream. Say I want to contribute my Debian patches to them.

  1. Commit “clean” patches upstream. Don’t have a bunch of history — the fixing typos commits, the fixing bugs commits, or the merging to track new upstream releases. Just something like a series of diffs against the current head.
  2. Bring across the full history, warts and all, and keep it around permanently.

git encourages option , with its rebase option. Darcs encourages option (though some use its amend-record option to work more like ).

As I got to thinking about it, it occured to me that git-rebase would be very nice if you are going to use philosophy . In short, rebase will remove your local patches from a repo, update it to the latest upstream, then re-apply your local changesets — aborting to have you fix any conflicts. This is as opposed to a more traditional merge, where you add the upstream changesets to your local branch and then commit new changesets to resolve conflicts. (So a rebase would be totally useless in situation )

I got to thinking about this, and started wondering what would happen to people that I’m working with that in turn work off my branches. And sure enough, the git-rebase manpage says, “When you rebase a branch, you are changing its history in a way that will cause problems for anyone who already has a copy of the branch in their repository and tries to pull updates from you.”

I maintain, therefore, that git-rebase is evil and should be avoided. It only works for a situation where someone maintains a private branch of a project, never shared in any way except to submit patches to an upstream. Forget it if you have a team maintaining that branch, or want to post that branch online for others to help with (as I do with my Debian darcs package). Even if you keep it private now, do you really want to adopt a work process that forces you to keep it private forever, or else completely change how you work?

And this brings me back to the original question of patch philosophy. Personally, I dislike philosophy . I’d much rather have the full history of a change, warts and all. Look at the Linux kernel example: changesets that introduced bugs that made it into the official tree have their fixes documented, but changesets that introduced bugs that were fixed before being merged into the official tree could be lost to the public due to rebasing by submitters. Is that really what we want? I don’t think so.

With Darcs, tagging is very cheap and it is quite trivial to write an “apply a changeset bundle” script that makes a before tag, applies a series of patches, and makes an after tag. One could then run a darcs diff between the two tags to see the net effect on the repository, or could still look at the individual patches. (Or, you can avoid tagging and manually specify the “from” and “to” patches.) I find that a much better model: you can have it both ways. I’d think that most modern VCSs ought to support some variant on that, too.

And I think that git-rebase should be removed on the grounds that it encourages poor version tracking practices.

Haskell Time Travel

Programming

There is something very cool about a language in which the easiest, most direct way to explain how it solves a problem is to say, “When we pass the output of [this function] into the input for the oracle we are actually sending the data backwards in time. So when [the code] queries the oracle we get a result from the future.”

Sweet.

The story goes on to say, however, “Time travel is a very dangerous business. One false move and you can create a temporal paradox that will destroy the universe (which in this case means that the computation will diverge). When programming with values from the future, it is important never, never, to do anything with the values that might change the future. This is the temporal prime directive.”

Saving Power with CPU Frequency Scaling

Linux, Uncategorized

Yesterday I wrote about the climate crisis. Today, let’s start doing something about it.

Electricity, especially in the United States and China, turns out to be a pretty dirty energy source. Most of our electricity is generated using coal, which despite promises of “clean coal” to come, burns dirty. Not only does it contribute to global warming, but it also has been shown to have an adverse impact on health.

So let’s start simple: reduce the amount of electricity our computers consume. Even for an individual person, this can add up to quite a bit of energy (and money) savings in a year. When you think about multiplying this over companies, server rooms, etc., it adds up fast. This works on desktops, servers, laptops, whatever.

The easiest way to save power is with CPU frequency scaling. This is a technology that lets you adjust how fast a running CPU runs, while it’s running. When CPUs run at slower speeds, they consume less power. Most CPUs are set to their maximum speed all the time, even when the system isn’t using them. Linux has support for keeping the CPU at maximum speed unless it is idle. By turning on this feature, we can save power at virtually no cost to performance. The Linux feature to handle CPU frequency scaling is called cpufreq.

Set up modules

Let’s start by checking to see whether cpufreq support is already enabled in your kernel. These commands will need to be run as root.

# cd /sys/devices/system/cpu/cpu0
# ls -l

If you see an entry called cpufreq, you are good and can skip to the governor selection below.

If not, you’ll need to load cpufreq support into your kernel. Let’s get a list of available drivers:

# ls /lib/modules/`uname -r`/kernel/arch/*/kernel/cpu/cpufreq

Now it’s guess time. It doesn’t really hurt if you guess wrong; you’ll just get a harmless error message. One hint, though: try acpi-cpufreq last; it’s the option of last resort.

On my system, I see:

acpi-cpufreq.ko     longrun.ko      powernow-k8.ko         speedstep-smi.ko
cpufreq-nforce2.ko  p4-clockmod.ko  speedstep-centrino.ko
gx-suspmod.ko       powernow-k6.ko  speedstep-ich.ko
longhaul.ko         powernow-k7.ko  speedstep-lib.ko

For each guess, you’ll run modprobe with the driver name. I have an Athlon64, which is a K8 machine, so I run:

#modprobe powernow-k8

Note that you leave off the “.ko” bit. If you don’t get any error message, it worked.

Once you find a working module, edit /etc/modules and add the module name there (again without the “.ko”) so it will be loaded for you on boot.

Governor Selection

Next, we need to load the driver that tells the kernel what governor to use. The governor is the thing that monitors the system and adjusts the speed accordingly.

I’m going to suggest the ondemand governor. This governor keeps the system’s speed at maximum unless it is pretty sure that the system is idle. So this will be the one that will let you save power with the least performance impact.

Let’s load the module now:

# modprobe cpufreq_ondemand

You should also edit /etc/modules and add a line that says simply cpufreq_ondemand to the end of the file so that the ondemand governor loads at next boot.

Turning It On

Now, back under /sys/devices/system/cpu/cpu0, you should see a cpufreq directory. cd into it.

To turn on the ondemand governor, run this:

# echo echo ondemand > scaling_governor

That’s it, your governor is enabled. You can see what it’s doing like this:

# cat cpuinfo_min_freq
800000
# cat cpuinfo_max_freq
2200000
# cat cpuinfo_cur_freq
800000

That shows that my CPU can go as low as 800MHz, as high as 2.2GHz, and that at the present moment, it’s running at 800MHz presently.

Now, check your scaling governor settings:

# cat scaling_min_freq
800000
# cat scaling_max_freq
800000

This is showing that the system is constraining the governor to only ever operate on an 800MHz to 800MHz range. That’s not what I want; I want it to scale over the entire range of the CPU. Since my cpuinfo_max_freq was 2200000, I want to write that out to scaling_max_freq as well:

echo 2200000 > scaling_max_freq

Making This The Default

The last step is to make this happen on each boot. Open up your /etc/sysfs.conf file. If you don’t have one, you will want to run a command such as apt-get install sysfsutils (or the appropriate one for your distribution).

Add a line like this:

devices/system/cpu/cpu0/cpufreq/scaling_governor = ondemand
devices/system/cpu/cpu0/cpufreq/scaling_max_freq = 2200000

Remember to replace the 2200000 with your own cpu_max_freq value.

IMPORTANT NOTE: If you have a dual-core CPU, or more than one CPU, you’ll need to add a line for each CPU. For instance:

devices/system/cpu/cpu1/cpufreq/scaling_governor = ondemand
devices/system/cpu/cpu1/cpufreq/scaling_max_freq = 2200000

You can see what all CPU devices you have with ls /sys/devices/system/cpu.

Now, save this file, and you’ll have CPU frequency scaling saving you money, and helping the environment, every time you boot. And with the ondemand governor, chances are you’ll never notice any performance loss.

This article showed you how to save power using CPU frequency scaling on Linux. I have no idea if it’s possible to do the same on Windows, Mac, or the various BSDs, but it would be great if someone would leave comments with links to resources for doing that if so.

Updated: added scaling_max_freq info

The Haskell Blog Tutorial

Programming

The first installment of Mark C. Chu-Carroll’s Haskell tutorial series went up last week.

It begins this way:

Before diving in and starting to explain Haskell, I thought it would be good to take a moment and answer the most important question before we start:

Why should you want to learn Haskell?

It’s always surprised me how many people don’t ask questions like that.

Farther down:

So what makes Haskell so wonderful? Or, to ask the question in a slightly better way: what is so great about the pure functional programming model as exemplified by Haskell?

The answer is simple: glue.

Languages like Haskell have absolutely amazing support for modular development.

An interesting and though-provoking article, even for someone that’s been using Haskell for more than 2 years now. (Yikes, I had no idea it was that long)

You can also see all his posts on Haskell, which include a couple more installments.