LinuxConf.eu wrapup

The very first LinuxConf Europe event was held in Cambridge, UK, in the first week of September. This conference is the result of a cooperation between the UK Unix User Group and the German Unix User Group; it is, in a sense, a combination of the UKUUG and Linux-Kongress events held in previous years. Talks by Dirk Hohndel and Michael Kerrisk were published last week. Here is a summary of some other LCE events.

Power management remains the focus of a great deal of attention. Arjan van de Ven started off a set of power-related talks with an overview of where the problems are. His biggest point is that software is a critical part of the power consumption picture; contemporary hardware provides a number of power-saving features, but software has a tendency to defeat them. Many of the ways in which this happens have been covered here before, so there is no need to repeat them. The core lesson here is that transitions between power states are expensive, so it is important that hardware components, once put into a power-saving state, be allowed to stay there for some time.

In the case of the CPU, idle periods of 20ms to 50ms are needed for effective power savings. Past kernels have rather defeated that goal, though, by receiving a clock interrupt every 1-10ms. The dynamic tick patches have finally fixed that problem, making it possible for longer sleeps to happen. But then user space comes along and ruins things. Since the advent of PowerTop, though, improvements have been coming quickly. Many distributions now consume at least 30% less power in typical laptop use.

Things may be getting better, but Matthew Garrett started the following session by noting that Linux still sucks - at least, it sucks power. This is a problem, he says, because getting half the battery lifetime as Windows on the same hardware is really embarrassing. Systems are still waking up far too much; the problems exist in both kernel and user space.

On the kernel side, the usual culprits - device drivers - are a big part of the problem. There are quite a few drivers which poll their hardware - sometimes up to 100 times every second. In some cases this cannot be avoided; the hardware may be broken in a way which requires this kind of polling. But in other cases the polling can be made smarter - such as turning it off when the device is not in use. There is still work to be done in this area.

User-space applications remain a problem. People tracking down wakeups often blame the X server, but the real trouble is usually the applications which are causing X to wake up. There is a tool in the works which will identify the real source of X wakeups; this is a good thing: once problems are identified they are usually fixed pretty quickly. Polling for vertical retrace periods (so that the display can be updated without artifacts) seems to be a particular problem; some API work is being done to make it easier to avoid this polling. Evidently there are also some applications which repeatedly ask the server if a particular extension is available; since the set of extensions does not change while the server is running, there is little point in doing this.

There are some interesting things which can be done to better use the power-saving features of the hardware. For example, some framebuffers can compress the video data into a dedicated memory area, then drive the video from the compressed data. This technique reduces video memory bandwidth, saving power (up to half a watt) in the process. An interesting consequence is that the amount of power saved is dependent on how well the screen's contents compress - a user's choice of background wallpaper will affect their power usage.

Finally, there is a lot to be gained if device drivers can communicate more information to user space, making polling unnecessary. Applications which poll for changes to the audio volume are an example here; if the sound system simply told them that the volume had been adjusted, they could update their displays and go back to sleep.

Jörn Engel gave a talk on the death of hard disks. His core point is that flash-based storage is faster, requires less power, makes less noise, and is more robust than rotating storage. It is also more expensive, for now, but flash is getting cheaper much more quickly. Jörn projects that flash-based drives will become more economical than hard drives between 2012 and 2019, depending on which drives one looks at.

Flash makes life easier in a number of ways; the lack of seek delays, for example, means that much of the trouble the kernel goes to in scheduling of block I/O operations can be eliminated. On the other hand, flash has challenges of its own: it is not quite the random-access array of blocks that one would like. In particular, writing to flash requires dealing with wear-leveling issues, erase operations, and more.

Manufacturers have done their best to paper over these issues through the use of translation layers which make a flash array look like a simple disk drive. These layers make it easier to use flash with existing software, but there are problems: performance is not always what one would like, and there can be hidden caches which delay the persistent storage of data. So Jörn has a request to the flash manufacturers: give us direct access to the flash array, without translation layers, and let us figure out how to best support it.

Chris Mason is not waiting for flash to take over; instead, he is working on the next-generation Linux filesystem for rotating disks. The result, Btrfs, was the subject of Chris's talk at LCE. LWN covered Btrfs last June.

Chris's motivation is the fact that disks are, for all practical purposes, getting slower - the time required to read an entire disk is growing. Most systems still store large numbers of small files, leading to a lot of wasted space. Btrfs tries to address these issues and provide a number of interesting features as well. It is extent-based, resulting in more efficient storage of larger files. Small files are packed into the filesystem tree itself, eliminating the internal fragmentation experienced by a number of other filesystems. It has indexed directories, data and metadata checksums, efficient snapshots, sequence numbers in objects (facilitating quick and easy incremental backups), an online filesystem checker in the works, and more.

The directories are actually indexed twice. One index is there for fast filename lookup; the other one, instead, lets the readdir() system call return files in inode-number order, speeding filesystem traversals. Extended attributes are stored as directory entries. Every file has a backpointer to its containing directory - and, yes, multiply-linked files have backpointers to all of the directories in which they are found.

Perhaps the most fun part of the talk was the plots Chris has generated from various benchmark runs. The limiting factor on filesystem performance is generally disk seeks; it is important to minimize disk head movement. In general, ext3 tends to move the disk head all over the platter during benchmark runs while Btrfs and XFS do better. Chris noted that better writeback clustering in the virtual memory subsystem would help ext3.

More benchmark plots (some animated) can be found in the Btrfs benchmark and Seekwatcher pages. Toward the end, Chris was asked whether performance slows down when the disk gets full. The answer was "no" because the system crashes instead. That's a good reminder that Btrfs remains an early-stage development; the on-disk format has not even been finalized yet. But the production version of Btrfs is certainly something to look forward to.

Back in 2000, the British Computer Society awarded its Lovelace Medal to Linus Torvalds. In 2007, the society finally caught up with him to deliver the medal - though, as speaker Dr. David Hartley noted, they probably were almost as quick as the post office would have been. As is typically the case, Linus seemed somewhat embarrassed by the attention.

LinuxConf Europe intends to be a conference on a truly European scale. To that end, next year's event will likely move to Germany; the details were not yet finalized to the point that the location could be announced at this year's conference, though. LCE, helped by the kernel summit, has gotten this institution off to a good start; your editor is looking forward to next year's edition.