Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 956
SEC. 11.4
To improve the scalability of the scheduling algorithm for multiprocessors with
a high number of processors, the scheduler tries hard not to have to take the lock
that protects access to the global array of priority lists.
Instead, it sees if it can di-
rectly dispatch a thread that is ready to run to the processor where it should run.
For each thread the scheduler maintains the notion of its
ideal processor
attempts to schedule it on that processor whenever possible. This improves the
performance of the system, as the data used by a thread are more likely to already
be available in the cache belonging to its ideal processor.
The scheduler is aware
of multiprocessors in which each CPU has its own memory and which can execute
programs out of any memory—but at a cost if the memory is not local.
These sys-
tems are called
NonUniform Memory Access
) machines. The scheduler
tries to optimize thread placement on such machines.
The memory manager tries
to allocate physical pages in the NUMA node belonging to the ideal processor for
threads when they page fault.
The array of queue headers is shown in Fig. 11-26. The figure shows that there
are actually four categories of priorities: real-time, user, zero, and idle, which is ef-
1. These deserve some comment.
Priorities 16–31 are called system,
and are intended to build systems that satisfy real-time constraints, such as dead-
lines needed for multimedia presentations.
Threads with real-time priorities run
before any of the threads with dynamic priorities, but not before DPCs and ISRs.
If a real-time application wants to run on the system, it may require device drivers
that are careful not to run DPCs or ISRs for any extended time as they might cause
the real-time threads to miss their deadlines.
Ordinary users may not run real-time threads.
If a user thread ran at a higher
priority than, say, the keyboard or mouse thread and got into a loop, the keyboard
or mouse thread would never run, effectively hanging the system.
The right to set
the priority class to real-time requires a special privilege to be enabled in the proc-
ess’ token. Normal users do not have this privilege.
Application threads normally run at priorities 1–15.
By setting the process and
thread priorities, an application can determine which threads get preference.
system threads run at priority 0 and convert free pages into pages of all
zeroes. There is a separate
thread for each real processor.
Each thread has a base priority based on the priority class of the process and
the relative priority of the thread.
But the priority used for determining which of
the 32 lists a ready thread is queued on is determined by its current priority, which
is normally the same as the base priority—but not always. Under certain condi-
tions, the current priority of a nonreal-time thread is boosted by the kernel above
the base priority (but never above priority 15).
Since the array of Fig. 11-26 is
based on the current priority, changing this priority affects scheduling.
No adjust-
ments are ever made to real-time threads.
Let us now see when a thread’s priority is raised.
First, when an I/O operation
completes and releases a waiting thread, the priority is boosted to give it a chance
to run again quickly and start more I/O.
The idea here is to keep the I/O devices

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