Not all these features will get in, showing the stop-and-go road for improvements to make their way into the Linux kernel.
To a business user of Linux, the development of its kernel may appear so Byzantine, with dozens of people maintaining different pieces and hundreds more volunteers submitting code, that it's hard to see where new features are headed.
There is no Linux road map, per se. To give a glimpse of the process, here are seven areas of development worth watching, based on interviews with developers and kernel maintainers, and time on www.kernelnewbies.org. Not all are moving ahead smoothly, illustrating the stop-and-go path improvements must travel to get into the kernel.
Recognizing virtualization as a "megatrend" of the decade, Linux kernel maintainers have made it a priority to add virtualization features to the kernel at a rapid pace. The hypervisor KVM, contributed by Avi Kivity of startup Qumranet, was included in the kernel of late 2006 and updated in last month's release. But it's an example of the conflict between rapid kernel releases and the slower-advancing enterprise editions.
"KVM is a very good example of things we think are not enterprise-ready," says Holger Dryoff, VP of management at Novell. KVM needs more testing on how it interacts with kernel subsystems, including the scheduler, he adds, before it gets into SUSE Linux Enterprise Server.
XenSource, the commercial open source virtualization company recently bought by Citrix Systems for $500 million, has lobbied to get the Xen hypervisor in the kernel with its own architecture, much as a new chip would. Kernel maintainers contend that's a maintenance-heavy way to add a virtualization feature. XenSource engineers have conceded, but work remains to get Xen aligned with the kernel's operations. It hasn't made it into the kernel, beyond just-added support that lets Linux recognize when it's running in a virtualized environment.
Other virtualization features are moving faster, including KVM and Lguest, a minimalist 5,000-line hypervisor written by IBM engineer Rusty Russell that's included in the most recent kernel. Like KVM, it taps the virtualization hooks in the latest chips from Intel and Advanced Micro Devices. Unlike VMware's ESX Server, however, Lguest creates a virtual machine whose operating system realizes it has been virtualized. This architecture lets the operating system more efficiently pass some calls for CPU cycles straight to the hardware instead of slowing things down by acting as an intermediary.
Linux has been rapidly improving in real-time operations and is now a frequently used embedded system in cell phones and other devices. But the recently issued 2.6.23 kernel shows "a little bit of regression" in real-time operations, says Jim Ready, CTO and founder of MontaVista, a maker of commercial embedded Linux. A new process scheduler tilted more toward "fairness"--the notion that tasks the end user tells the processor to do should get more priority.
"A real-time guy doesn't want fairness," says Ready, since real-time advocates want the operating system to interrupt whatever the CPU is doing and assert a new priority over it. A simple example is that the software in a medical device monitoring a patient's breathing should send an immediate alert if breathing stops, interrupting whatever process the software's doing. MontaVista won't incorporate the new kernel into its product line until performance is restored, Ready says. Gartner analyst George Weiss predicts standard Linux will be competitive as a real-time system in 2008.
One reason Weiss can say that is because kernel developers are working on giving the scheduler another real-time characteristic. One key role for the operating system is to manage interrupts--to decide which tasks should grab the CPU's attention and how to prioritize different actions. If all the interrupt handlers can be combined into their own thread, that thread can be scheduled and prioritized instead of occurring unpredictably and delaying real-time responses.
Work on such an approach has been going on for three years. MontaVista's Sven-Thorsten Dietrich submitted code back in 2004 in hopes of preventing interrupt handlers from tying up the kernel for routine tasks, since they hurt real-time response. But that code was too disruptive to get past Ingo Molnar, the kernel's scheduler domain expert. The code trespassed on a key kernel feature, spinlocks, which tie up the CPU as a process waits for a needed bit of data or an event. Many processes rely on spinlocks. Dietrich's code reduced hundreds of spinlocks to 30; Molnar's revision kept 90 spinlocks, a less disruptive change.
The collection of interrupt handlers into a separate thread now appears ready to go into the kernel. "Ingo replaced what we did, but his work is good," says Ready. MontaVista wouldn't mind more credit for the work it did, but Ready knows this is how open source collaboration works, and he'll settle for real-time changes progressing into the kernel.
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