Re: Context switch times

From: Hubertus Franke (
Date: Tue Oct 09 2001 - 15:37:24 EST

* Mika Liljeberg <> [20011007 05;57]:"
> Alan Cox wrote:
> > This isnt idle speculation - I've done some minimal playing with this but
> > my initial re-implementation didnt handle SMP at all and I am still not 100%
> > sure how to resolve SMP or how SMP will improve out of the current cunning
> > plan.
> Here's some idle speculation on SMP to top it off. :) I tend to think
> that the load balancing between CPUs should be a completely separate
> algorithim and should not necessarily be run at every schedule(). The
> idea is to compeletely decouple the problem of scheduling a single CPU
> between tasks and the problem of load balancing between the CPUs, making
> each problem simpler to solve.

This is what we implemented as an extension to our MQ scheduler.
I will present on the results for this during ALS technical session:
"CPU Pooling and Load Balancing in Linux MultiQueue Scheduling".
If interested I can already put an earlier version on

It basically does (1) and (2). (3) is done unintelligently right now.

> Consider the following basic rules:
> A) When a new task comes along, pick the "least loaded" CPU and lock the
> new task onto that.
> B) Whenever the load imbalance between least loaded CPU and most loaded
> CPU becomes too great, move one or more tasks from most loaded CPU to
> the least loaded CPU.
> The rules themselves should be self-explanatory: A provides initial load
> balancing, while B tries to keep the balance (with a sensible hysteresis
> to avoid thrashing). However, there are a few minor details to solve:
> 1) How to determine the load of a CPU? If we can quantify this clearly,
> we can easily set a hysteresis level to trigger load balancing between
> two CPUs.
> 2) When and how often to check for load imbalance?
> 3) How to select the task(s) that should be moved between two CPUs to
> correct an imbalance?
> For problems 1 and 2 I propose the following solution: Insert the the
> load balancing routine itself as a (fake) task on each CPU and run it
> when the CPU gets around to it. The load balancer should behave almost
> like a CPU-bound task, scheduled on the lowest priority level with other
> runnable tasks. The last bit is important: the load balancer should not
> be allowed to starve but should be invoked approximately once every
> "full rotation" of the scheduler.
> With the above it is easy to estimate the load of a CPU. We can simply
> use the elapsed time between two invokations of the load balancer task.
> When the load balancer task of a particular CPU gets run, it chalks up
> the elapsed time on a score board somewhere, and checks whether there is
> a significant imbalance between itself and some other CPU. If there is,
> it commences to move some tasks between itself and the other CPU (note
> rule B, though, it should be enough to mess with just two CPU queues at
> a time to minimize balancing and locking overhead).
> Problem 3 is tricky. Basically, there should be a cost/benefit function
> F(tasks to move) that should be minimized. Ideally F(task_i), the
> cost/benefit of moving a single task, would be calculated as a byproduct
> of the CPU scheduler algorithm.
> F(task_i) might be function of elapsed time since task_i was last
> scheduled and the average time slice used by task_i, to account for the
> probable cache hit. This would leave it up to the load balancer to move
> as many lowest cost tasks to a new CPU as is needed to correct the
> imbalance (average time slices used by each task would be needed in
> order to make this decision).
> Naturally, some additional rules might be necessary to make a task
> eligible for moving, e.g., never move the only/last CPU bound task to
> another CPU. In addition, it might actually make sense to move at most
> one task at each invocation of the load balancer, to further reduce the
> probability of thrashing. The load would still converge fairly quickly
> towards a balanced state. It would also scale fairly well with the
> number of CPUs.
> How does that sound?
> MikaL
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