194 lines
4.2 KiB
HTML
194 lines
4.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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>How does my computer do several things at once?</TITLE
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><A
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NAME="multitasking"
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>7. How does my computer do several things at once?</H1
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><P
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>It doesn't, actually. Computers can only do one task (or
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<I
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CLASS="firstterm"
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>process</I
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>) at a time. But a computer can change tasks
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very rapidly, and fool slow human beings into thinking it's doing several
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things at once. This is called
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<I
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CLASS="firstterm"
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>timesharing</I
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>.</P
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><P
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>One of the kernel's jobs is to manage timesharing. It has a part
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called the
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<I
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CLASS="firstterm"
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>scheduler</I
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>
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which keeps information inside itself about all the other (non-kernel)
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processes in your zoo. Every 1/60th of a second, a timer goes off in the
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kernel, generating a clock interrupt. The scheduler stops whatever process
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is currently running, suspends it in place, and hands control to another
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process.</P
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><P
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>1/60th of a second may not sound like a lot of time. But on today's
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microprocessors it's enough to run tens of thousands of machine
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instructions, which can do a great deal of work. So even if you have many
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processes, each one can accomplish quite a bit in each of its
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timeslices.</P
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><P
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>In practice, a program may not get its entire timeslice. If an
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interrupt comes in from an I/O device, the kernel effectively stops the
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current task, runs the interrupt handler, and then returns to the current
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task. A storm of high-priority interrupts can squeeze out normal
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processing; this misbehavior is called <I
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CLASS="firstterm"
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>thrashing</I
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> and
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is fortunately very hard to induce under modern Unixes.</P
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><P
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>In fact, the speed of programs is only very seldom limited by the
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amount of machine time they can get (there are a few exceptions to this
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rule, such as sound or 3-D graphics generation). Much more often, delays
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are caused when the program has to wait on data from a disk drive or
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network connection.</P
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><P
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>An operating system that can routinely support many simultaneous
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processes is called <SPAN
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CLASS="QUOTE"
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>"multitasking"</SPAN
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>. The Unix family of operating
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systems was designed from the ground up for multitasking and is very good
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at it — much more effective than Windows or the old Mac OS, which both
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had multitasking bolted into them as an afterthought and do it rather poorly.
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Efficient, reliable multitasking is a large part of what makes Linux
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superior for networking, communications, and Web service.</P
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>How do input devices and interrupts work?</TD
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>How does my computer keep processes from stepping on each other?</TD
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