Preemption and Xen

Jacob Mathai http://jacobmathai.blogspot.com/

Kernel Preemption and Xen

Preemption is the ability to interupt a kernel mode process. Kernel Preemption strives towards a CPU  that is performing "higest priority processing" at any given time slice and is both art and technique. Most Linux User Mode processes are preemptive in the 'TASK_RUNNING' state. Older Linux 2.4 kernels generally are non-preemptive. Linux 2.6 kernels offer full preemptive capabilities with some exceptions. Preemption is important to "real time"  applications running in Virtual Machines (DomU) as well in the scheduling of  multiple VMs (Dom0). Under specific criteria, certain user programs running in kernel mode cannot be interrupted. This is further complicated by multi processing (SMP) and the challenges are similar with preemption and process scheduling. 

Preemptive kernels are important to operating systems and the Xen Kernel in particular because blocked tasks, flagged with higher priority, can preempt even lowerer priority kernel tasks. In theory, this can impact general performance, scheduler latency, user experience, and real time computing.

Xen Linux Kernel Preempt Options

Preemptive Features in Linux 2.6 Kernels can be enabled in the kernel config.
XEN/xen-unstable/linux-2.6.12-xen0/.config

CONFIG_PREEMPT=y

CONFIG_PREEMPT - This option reduces the latency of the kernel when reacting to 
real-time or interactive events by allowing a low priority process to
be preempted even if it is in kernel mode executing a system call.

Say Y here if you are building a kernel for a desktop, embedded
or real-time system. Say N if you are unsure.

CONFIG_PREEMPT_BKL=y

CONFIG_PREEMPT_BKL - This option reduces the latency of the kernel by making the 
big kernel lock preemptible.

Say Y here if you are building a kernel for a desktop system.
Say N if you are unsure.

Links

Scheduling

Preemptive Kernel Patch 2.4 Kernel ftp://ftp.kernel.org/pub/linux/kernel/people/rml/preempt-kernel/v2.4/