<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML ><HEAD ><TITLE >How does my computer do several things at once?</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="The Unix and Internet Fundamentals HOWTO" HREF="index.html"><LINK REL="PREVIOUS" TITLE="How do input devices and interrupts work?" HREF="devices.html"><LINK REL="NEXT" TITLE="How does my computer keep processes from stepping on each other?" HREF="memory-management.html"></HEAD ><BODY CLASS="sect1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >The Unix and Internet Fundamentals HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="devices.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="memory-management.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="multitasking" ></A >7. How does my computer do several things at once?</H1 ><P >It doesn't, actually. Computers can only do one task (or <I CLASS="firstterm" >process</I >) at a time. But a computer can change tasks very rapidly, and fool slow human beings into thinking it's doing several things at once. This is called <I CLASS="firstterm" >timesharing</I >.</P ><P >One of the kernel's jobs is to manage timesharing. It has a part called the <I CLASS="firstterm" >scheduler</I > which keeps information inside itself about all the other (non-kernel) processes in your zoo. Every 1/60th of a second, a timer goes off in the kernel, generating a clock interrupt. The scheduler stops whatever process is currently running, suspends it in place, and hands control to another process.</P ><P >1/60th of a second may not sound like a lot of time. But on today's microprocessors it's enough to run tens of thousands of machine instructions, which can do a great deal of work. So even if you have many processes, each one can accomplish quite a bit in each of its timeslices.</P ><P >In practice, a program may not get its entire timeslice. If an interrupt comes in from an I/O device, the kernel effectively stops the current task, runs the interrupt handler, and then returns to the current task. A storm of high-priority interrupts can squeeze out normal processing; this misbehavior is called <I CLASS="firstterm" >thrashing</I > and is fortunately very hard to induce under modern Unixes.</P ><P >In fact, the speed of programs is only very seldom limited by the amount of machine time they can get (there are a few exceptions to this rule, such as sound or 3-D graphics generation). Much more often, delays are caused when the program has to wait on data from a disk drive or network connection.</P ><P >An operating system that can routinely support many simultaneous processes is called <SPAN CLASS="QUOTE" >"multitasking"</SPAN >. The Unix family of operating systems was designed from the ground up for multitasking and is very good at it — much more effective than Windows or the old Mac OS, which had multitasking bolted into them as an afterthought and do it rather poorly. Efficient, reliable multitasking is a large part of what makes Linux superior for networking, communications, and Web service.</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="devices.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="memory-management.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >How do input devices and interrupts work?</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >How does my computer keep processes from stepping on each other?</TD ></TR ></TABLE ></DIV ></BODY ></HTML >