"I'm happy to announce that I've implemented a Block I/O bandwidth controller," began Ryo Tsuruta, explaining that it was intended to be used in a cgroup or virtual machine environment, implemented as a device-mapper driver. He detailed a token-based implementation in which dm-band passes out to the various groups, "a group passes on I/O requests that its job issues to the underlying layer so long as it has tokens left, while requests are blocked if there aren't any tokens left in the group. One token is consumed each time the group passes on a request. Dm-band will refill groups with tokens once all of groups that have requests on a given physical device use up their tokens." Ryo explained:

"Dm-band is an I/O bandwidth controller implemented as a device-mapper driver. Several jobs using the same physical device have to share the bandwidth of the device. Dm-band gives bandwidth to each job according to its weight, which each job can set its own value to. At this time, a job is a group of processes with the same pid or pgrp or uid. There is also a plan to make it support cgroup. A job can also be a virtual machine such as KVM or Xen."

"The aim of these four patches is to introduce Virtual Machine time accounting," began Laurent Vivier. He described the first two patches as:

"1) As recent CPUs introduce a third running state, after 'user' and 'system', we need a new field, 'guest', in cpustat to store the time used by the CPU to run virtual CPU. Modify /proc/stat to display this new field.

"2) Like for cpustat, introduce the 'gtime' (guest time of the task) and 'cgtime' (guest time of the task children) fields for the tasks. Modify signal_struct and task_struct. Modify /proc/<pid>/stat to display these new fields."

Both Ingo Molnar and Rik van Riel responded favorably to the patch. Ingo replied, "the concept certainly looks sane to me," adding, "I'd suggest inclusion into 2.6.24." Regarding concerns that the new information at the end of the line could break utilities such as top or ps, Rik assured that it would not, "we have added numbers to the cpu lines in /proc/stat since early 2.6. All the programs parsing /proc/stat should just scan for a number of numbers from the start of the line, without trying to scan for the terminating newline."

The Xen virtual machine monitor was recently merged into the upcoming 2.6.23 Linux kernel in a series of patches from Jeremy Fitzhardinge. The project was originally started as a research project at the University of Cambridge, and has been repeatedly discussed as a merge candidate for the mainline Linux kernel.

Xen is described in the project's FAQ as:

"Xen is a virtual machine monitor (VMM) for x86-compatible computers. Xen can securely execute multiple virtual machines, each running its own OS, on a single physical system with close-to-native performance."

A recently merged KVM patchset included support for guest SMP, various performance improvements, and suspend/resume fixes. KVM stands for Kernel-based Virtual Machine, "a full virtualization solution for Linux on x86 hardware containing virtualization extensions". In regards to the recently merged guest SMP support which will be part of the upcoming 2.6.23 kernel, Avi Kivity noted:

"Guest smp is fully operational. Kernel build on 2-way smp is 40% faster than on a up guest. Expect significant performance improvements from in-kernel apic and from further tuning."

Avi Kivity [interview] announced significant performance improvements and support for running 32-bit Windows Vista as a guest within the latest release of KVM. Originally merged into the 2.6.20 mainline Linux kernel [story], KVM stands for Kernel-based Virtual Machine, "a full virtualization solution for Linux on x86 hardware containing virtualization extensions". Regarding the new release, Avi announced:

"The happy theme of today's kvm is the significant performance improvements, brought to you by a growing team of developers. I've clocked kbuild at within 25% of native. This release also introduces support for 32-bit Windows Vista."

Avi Kivity is the lead developer and maintainer of the Kernel-based Virtual Machine project, better known as kvm. The project was started in mid-2006, and has been part of the Linux kernel since the 2.6.20 release in February of 2007. kvm is a full virtualization system for x86-based Linux hosts, allowing users to run isolated x86 guest operating systems in virtual machines.

Avi Kivity suggested that combining KVM, the Kernel-based Virtual Machine [story], with the dyntick patch [story] could improve overall KVM performance. He noted that it would likely improve performance of both the host by "avoiding expensive vmexits due to useless timer interrupts," as well as on the guest by "reducing the load on the host when the guest is idling (currently an idle guest consumes a few percent cpu)". Ingo Molnar [interview] pointed out that KVM with his -rt kernel already works with dynticks enabled on both the host and the guest, "using the dynticks code from the -rt kernel makes the overhead of an idle guest go down by a factor of 10-15". Ingo added that he hopes the dyntick patch will be ready to be merged into the upcoming mainline 2.6.21 kernel.

Rik van Riel [interview] noted that there were other ways to reduce the load of the guest when it's idling, "you do not need dynticks for this actually. Simple no-tick-on-idle like Xen has works well enough." Ingo explained, "s390 (and more recently Xen too) uses a next_timer_interrupt() based method to stop the guest tick - which works in terms of reducing guest load, but it doesnt stop the host-side interrupt. The highest quality approach is to have dynticks on both the host and the guest, and this also gives high-resolution timers and a modernized time/timer-events subsystem for both the host and the guest."