Welcome to the Linux Weather Forecast

This page is an attempt to track ongoing developments in the Linux development community that have a good chance of appearing in a mainline kernel and/or major distributions sometime in the near future. Your "chief meteorologist" is Jonathan Corbet, Executive Editor at LWN.net. If you have suggestions on improving the forecast (and particularly if you have a project or patchset that you think should be tracked), please add your comments to the Discussion page. There's a blog that reports on the main changes to the forecast. You can view it directly or use a feed reader to subscribe to the blog feed. You can also subscribe directly to the changes feed for this page to see feed all forecast edits.

Forecast Summaries

Current conditions: the 3.8 kernel was released on February 18, 2013.  Some of the significant features added in this development cycle include:

• Support for the 386 architecture has been removed at long last.  Note that 486 processors are still supported and will be indefinitely.

• Callback-free read-copy-update is an important step toward the goal of being able to offload all kernel overhead from a subset of CPUs on a multiprocessor system.

• The ongoing debate over improving NUMA performance has been resolved in favor of merging the "balancenuma" patch set for 3.8.  "Balancenuma" provides the infrastructure for improved NUMA placement and balancing; much of the policy-related work needed for better NUMA performance is expected for 3.9.

• The huge zero page feature greatly reduces memory requirements for some workloads.

• Kernel memory usage accounting has been added.  This feature allows control groups to be used to limit the amount of kernel memory consumed by a group of processes.

• Inline data support in the ext4 filesystem allows for more efficient storage of small files.

• The Btrfs filesystem has a new "replace" operation allowing much quicker replacement of a single drive in a volume.

• User namespaces are now almost completely implemented.  This feature allows the creation of containers where it is safe to give processes root privileges without risking the system as a whole.

• The f2fs filesystem for flash devices has been merged.  This filesystem is still quite new, though, and will not be ready for production work for some time yet.

3.8 was the busiest development cycle in Linux history with 12,394 individual changes merged.  Well over 1200 developers representing over 200 companies contributed to this release; see this article for more information on where the code in 3.8 came from.  

Short-term forecast: the 3.9 kernel can be expected sometime around the beginning of May.  Some of the significant features in this release will be:

• Virtualization with KVM is now supported on ARM Cortex-A15 processors.

• Dynamic frequency selection suppport is being added to the wireless networking subsystem, allowing high-frequency networking to coexist safely with air traffic control radars.

• VMware's "VM Sockets" and "Virtual Machine Communication Interface" subsystems have been merged into the mainline kernel.

• User namespace support is now almost entirely complete now that all major filesystems except XFS have been updates.

• The SO_REUSEPORT networking option makes it easier to set up scalable, multi-process network servers.

• Intel's PowerClamp driver can control power consumption in data center settings through the forced injection of idle cycles.

• The new "dm-cache" device mapper target can use a solid-state device as a fast cache for slower storage devices.  See the associated documentation file for details.

• The Btrfs filesystem has finally gained RAID5 and RAID6 support on an experimental basis.

The 3.9 merge window closed with the announcement of 3.9-rc1 on March 3.  Some 10,265 changes were pulled into the mainline for the 3.9 release, making this another busy development cycle.

Longer-term forecasts

As with the weather, there are no certainties about what may be merged into the Linux kernel going forward; every change is evaluated on both its merits and its long-term maintenance costs.  Here are a few things on the horizon that are worth watching, though.

The Android kernel patches.  These changes have been the subject of extensive discussions and have, thus far, not been accepted into the mainline kernel.  At the 2011 Kernel Summit, though, it was decided that this code should probably go in.  Longstanding tradition says that code which is actively shipped, supported, and used should eventually be merged, even if it has problems. Nothing is certain until it happens, but we will probably see some movement in this area, perhaps for the 3.3 release.

The Btrfs filesystem is heading into active use.  Oracle has confirmed that it intends to use it as the default filesystem in its enterprise distribution.  Remaining issues (as discussed at LinuxCon Europe) include a functional filesystem repair tool and the merging of RAID5/6 support.

Realtime preemption - the provision of deterministic response in the Linux kernel - has been developed out of tree since 2004.  Current plans call for the merging of much of that code within the next few development cycles, leaving only the core realtime work out of the mainline.  See the report from the October 22 realtime minisummit for more information.

x32 is a new binary interface standard for 64-bit architectures; it is intended to provide better performance for applications that do not actually need 64-bit address spaces.  Since most applications have no such need, the benefits could be significant.  There are some remaining issues to be worked out, but the bulk of the work is done and we should be able to expect x32 support to be merged sometime in 2012.  Some more information can be found in this article.

Control groups are the mechanism by which the kernel gathers processes into hierarchical groups; it can then apply policies and resource usage limits to those groups.  This feature has been somewhat controversial within the kernel community, but its use is growing and shows every sign of continuing to do so.  Over the next year or so we can expect to see a lot of work in this area along two lines: (1) adding new controllers to apply policies to groups, and (2) improving the implementation of control groups and the controllers to make kernel developers happier.  Some more information can be found in the 2011 Kernel Summit control groups discussion.

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