Scheduler Activations: Effective Kernel Support for the User-Level

From: Green, Gregory P (gregory.p.green_at_boeing.com)
Date: Mon Jan 26 2004 - 13:11:40 PST

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    This paper relates a technique for tying together user-level threads
    with the kernel on a multiprocessor system to prevent problems
    presented by using either kernel threads or user-space threads. The
    design is to use a construct called a scheduler activation which calls
    back into user-space thread management with all kernel events that can
    affect user space threads. With this information in hand, the
    user-space thread management can then do the appropriate
    action. Another part of the method is for the user threads to tell the
    kernel that it needs more or less processors.

    The specific events that are up-called into user-space are: processor
    has been preempted, the kernel has blocked this activation, the kernel
    has unblocked this activation. A complication that can arise is if a
    user thread is in a critical section, and is preempted by the
    kernel. The critical section needs to be resumed until exited when the
    activation gets the processor back so that it can release the lock. A
    clever method of copying the instructions of the critical section is
    used so that when the activation is resumed, it calls the user thread
    management routine for rescheduling after the critical section is
    exited. To sum up, activations allow the developer to have the
    advantages of kernel-threads with the speed of user-level threads.

    I knew little about user threads vs kernel threads before reading this
    paper, so most of the information was new to me. In that respect
    alone, it was quite enlightening. I thought the design detailed here
    was quite clever. This is similar to the concept of wake-up predicates
    in the exokernel idea, but in reverse. It also educated me on the
    dangers of user space threads, none of which I knew about before. The
    mechanism seems secure, simple, and general. I would like to know if
    any os has actually used this concept. I was also unaware of the large
    speed differences between kernel threads and user threads.

    Greg Green
    Advanced Design Systems
    Mathematics and Computing Technology
    The Boeing Co.
    bellevue: (425) 865-2790 everett: (425) 342-1361 mobile:
    (206)992-0316


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