LKML Summary Podcast

Audio: COMING SOON

For the weekend of May 2nd 2010, I’m Jon Masters with a summary of the past week’s LKML traffic.

In today’s issue: Linux 2.6.34-rc6, vger.kernel.org, Checkpoint and Restart, Frontswap, FUSE, and the Scheduler.

Linux 2.6.34-rc6. Linus Torvalds announced the latest 2.6.34 RC kernel on Thursday April 29th at 8:18pm PDT (Best Coast Time). The latest release is bloated by an updated PowerPC defconfig but does containing other fixes.

vger.kernel.org. There was a vger.kernel.org outage this week, from the 28th through the weekend, due to a power failure in the datacenter that hosts the equipment. This disrupted traffic to LKML, although some folks on IRC noted that their productivity had improved due to the lack of distraction.

Checkpoint and restart. Oren Laadan posted the latest version (21) of the “Kernel based checkpoint/restart” patch series, all 100 of the patches. He included various hints about which bits should be reviewed by whom, but the sheer size of the series boggled a few people. Although there wasn’t much discussion on the list, it does seem unlikely that a 100-part patch series of this kind would be pulled whole any time soon. http://www.linux-cr.org.

Frontswap. Discussion continued on some patches we missed in last week’s episode, on a rewritten piece of the previous “Transcendent Memory” patch series, named “Frontswap”. This piece of the large patch series – which is apparently shipping now in both OpenSuSE and Oracle Enterprise Linux – adds a new generic means to register what is the “opposite” of a swap-like backing store. Frontswap is essentially non-addressable RAM that is provided by a hypervisor (or perhaps a compressed in-kernel RAM device) and which may grow and shrink over time according to the availability of system resources. For example, a hypervisor may grant guests large amounts of otherwise unused RAM in the form of such “frontswap”able devices that may need to be reclaimed later on if other guests require the resources. Using frontswap, one can potentially avoid additional disk overhead usually associated with “swap”. One of the biggest criticisms, from Avi Kivity – was that these patches assume access to the frontswap device is synchronous and not being performed using DMA or some other asynchronous process. Dan Magenheimer confirmed that this is an intential design limitation in order to make the implementation much simpler for its use case(s) dealing with real physical RAM. Dan noted that the conversation had gone off on a tangent, discussing such other (interesting, but not directly relevant) issues as swap-to-flash.

Fuse. Miklos Szeredi posted an RFC patchset implementing splice(2) support for FUSE (Filesystems in USErspace). This means that is is possible to move an existing page directly into the page cache of the FUSE filesystem without ever having to perform a copy. Given that there is obvious overhead in having filesystems implemented in userspace, adding splice support is a nice touch. Apparently the early tests show improved bandwdith and reduced system time but it will be interesting to see what further testing reveals over time.

Scheduler. Ted Baker, Joerg Roedel, Doug Niehaus, and Peter Zijlstra discussed scheduler policy and classes available in the kernel in a followup to a much earlier thread entitled “RFC for a new Scheduling policy/class in the Linux-kernel”, specifically about any plans to support SCHED_SPORADIC. Both Ted Baker and Doug Niehaus had plans for the ability to assign a task a priority that is specifically non-runnable without having to send it a signal – such as SIGSTOP – that requires the task to run in order to process the STOP. Peter Zijlstra stated that the current plan involved supporting the sporadic task model through the use of SCHED_DEADLINE rather than POSIX’s SCHED_SPORADIC (the name of which, according to Peter, was jokingly “stole[n] [...] from us”). Ted Baker replied to Peter, noting that deadline scheduling and sporadic server scheduling are “two quite different things” – the latter belonging to the existing fixed priority scheduling domain (that is a separate problem domain from that of the deadline scheduling folks). Ted thought issues with the POSIX SCHED_SPORADIC API that may have problems could be corrected through “interpretation” of the standard such that a solution were available in short order rather than longer term, especially if Linux were to do something with implementation that he could feed to the Austin Group (the POSIX folks).

In today’s miscellaneous items:

* Mike Travis (SGI) posted a patch providing a kernel parameter to increase pid_max from 32k for early-in-boot use, before it can be otherwise set to a higher value. Otherwise, on a system with 1664 CPUs, Mike finds that there are 25163 processes started before the login prompt!

* Jack Steiner (SGI) noted that the existing SLAB allocator implementation of cpuset_mem_spread_node used a single rotor for allocating both file pages and SLAB pages, so that (on a multi-node memory system), writing a particular test file results in advancing the rotor 2 nodes per allocation and skipping e.g. odd number nodes in the SLAB pages allocation. The patc introduces a second rotor just for the SLAB page allocation.

* Philip Langdale (VM) noted that he has been following the Transparent Hugepage work over the past few weeks and is very encouraged. He claims a 22% improvement in ops/sec reported by SPECjbb under virtualization.

* A kernel developer posted a somewhat distressing thread suggesting some emotional disturbance caused by a particular relationship. In the interest of not being the US Weekly of LKML I shall refrain from further comment, and agree with the suggestion of using the “It’s Complicated” button on Facebook next time something like this comes up instead.

* Ying Huang posted initial support for APEI (ACPI Platform Error Interface).

* Joerg Roedel posted the second version of the “Nested Paging support for Nested SVM” patchset.

* Steven J. Magnani posted version 2 of a stack unwinder for Microblaze.

* A second series of viafb patches for OLPC from Jonathan Corbet, who later pushed a version 2.1 of the series, containing three additional patches fixing issues pointed out by Bruno Prémont. The patches are available from git://git.lwn.net/linux-2.6.git in the branch viafb-posted. Jon wondered if the patches were ready to go into viafb-next.

In today’s announcements:

* DRM. Stefan Bader posted to let everyone know that he is now maintaining a 2.6.32-based tree on kernel.org containing backported DRM improvements for 2.6.32 based kernels, since a number of vendors are using that tree. Luis R. Rodriguez replied saing that this was “Great stuff! Thanks for putting this up!”. One wonders if this is more sign of a growing trend.

* Linux 2.6.33-rt19. Thomas Gleixner announced verion 2.6.33.3-rt19 of the Real Time patchset, containing mostly VFS scalability bits. This followed a previous 2.6.33-rt16 release also this week containing largely a merge with upstream 2.6.33, and -rt17 and -rt18 releases that contained a few fixes. Thomas notes in his posting that he had previously pushed out rt14 and rt15 without sending an announcement out to the list, so he included changelogs from -rt13 to 16, and rt17-rt18 (in the separate emails he made announcing -rt16, and -rt17). Patches are available at http://www.kernel.org/pub/linux/kernel/projects/rt/ and the tip git tree on git.kernel.org contains existing rt/head and rt/2.6.33 release branches.

* Upstart 0.6.6. Scott James Remnant announced the 0.6.6 release of the “upstart” SYSV init daemon replacement that supports modern asycnhronous event driven operation rather than traditional runlevels (though it does also support emulating those for backward compatibility). Upstart is used by a number of distributions, and is available at upstart.ubuntu.com/

The latest kernel release was 2.6.34-rc6.

Greg Kroah-Hartman released stable series kernels 2.6.32.12 and 2.6.33.3. The former came with some thanks (and possibly an indirect dig at vendors) to Maximilian Attems for his “hard work digging out patches from the various vendor kernel trees for this release”. Maximilian was also thanked specifically in the latter case for contributing patches also. Separately, Greg requested of Stephen Rothwell that he begin pulling a new staging-next tree into his daily Linux -staging tree (a nice present for Stephen as he returned from vacation).

Frederic Weisbecker replied (in an innocuous thread otherwise containing a patch email thread of conversation entitled “ptrace: Cleanup useless header”) noting that things touching the BKL should CC both him and Arnd Bergmann. They are still working on Big Kernel Lock (BKL) removal, which you can keep track of via http://kernelnewbies.org/BigKernelLock. There was some other BKL removal traffic over the past week, also, including some patches from Arnd entitled “Push down BKL into device drivers” (similar to the FS patches he had posted previously that did the same in that layer – nice).

That’s a summary of today’s Linux Kernel Mailing List traffic, for further information visit www.kernel.org. I’m Jon Masters.