Analysis of FreeBSD Ule Scheduler

FreeBSD 5-current has just introduced a kernel dispatch unit scheduler called the Ule Scheduler. This scheduler is much more efficient in SMP systems than FreeBSD previous versions (currently, the scheduler is called the 4BSD Scheduler).

The new Ule Scheduler is designed more like a scheduler for operating systems such as Solaris and Linux. Solaris's SMP performance is very good this is one of its selling points, and its scheduler to use the excellent algorithm is a very important reason. BSD-derived systems, although the overall design of the system is reasonable, and the performance of other parts of the operating system to make up for its shortcomings in the SMP scheduler, even though FreeBSD in most cases performance is more than

Other systems, but it uses the 4BSD Scheduler's low SMP performance is still a criticism.

Happily, the newly designed Ule Scheduler introduced an advanced algorithm for SMP scheduling in Solaris, Linux, and other systems, and it has a fairly good performance in a single-processor system, comparable to the performance of the 4BSD scheduler.

Why is the new Ule scheduler able to achieve higher performance? The main reason is that the new Ule Scheduler maintains the running queues for each CPU separately, and the CPU can "steal" each other's tasks in the ready queue to achieve a better balance. Basically, the design of the Ule scheduler conforms to the following principles: use as much execution resources as possible, use local locks as much as possible, avoid the use of spin locks that waste execution resources as much as possible, and use efficient scheduling algorithms as much as possible (the complexity of the current Ule Scheduler is O(1)).

When I write this article, the Ule Scheduler is still in the process of further improvement. On the one hand, it has been able to provide better scheduling performance than the 4BSD Scheduler--The kernel-state scheduling cost is reduced by about 25%, and user-state computing is reduced to a certain extent.

The CVS tag of the Ule Scheduler (SRC/SYS/KERN/SCHED_ULE.C) described in this article is

$FreeBSD: src/sys/kern/sched_ule.c,v 1.8 2003/02/03 05:30:07 jeff Exp $

Future versions of the Ule scheduler may have some access to this version, but not too large.

If necessary, you can download the required source code here [of course, I personally suggest that you still checkout a copy of Freebsd-current source code ]. Because of the reference

The code in the scheduler, according to the source code license Agreement, the following first restatement of its copyright notice. This code uses a typical BSD-style authorization. In order not to cause the difficulty of reading, this horse does not make grammar light.

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The Chinese text is as follows, for reference, where the English version is the subject of inconsistency with English.
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In this scheduler, there are still a number of sections labeled "XXX", which are currently not well optimized or need to be modified in a larger scale in the future; In addition, there are some inconsistencies with the source code, which I will try to minimize (that is, do not affect the code intent) Correction or avoidance. In addition, I do not intend to elaborate on the details of some of the scheduler-this may include macro definitions that are not closely related to the scheduler itself, and so on. If you are interested in these parts of the scheduler, see your code yourself.