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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 39] 39.8 A Big Environment Can Slow You Down </TITLE><METANAME="DC.title"CONTENT="UNIX Power Tools"><METANAME="DC.creator"CONTENT="Jerry Peek, Tim O'Reilly & Mike Loukides"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1998-08-04T21:50:17Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-260-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch39_01.htm"TITLE="39. Time and Performance"><LINKREL="prev"HREF="ch39_07.htm"TITLE="39.7 Checking System Load: uptime "><LINKREL="next"HREF="ch39_09.htm"TITLE='39.9 Know When to Be "nice" to OTher Users...and WhenNot to'></HEAD><BODYBGCOLOR="#FFFFFF"TEXT="#000000"><DIVCLASS="htmlnav"><H1><IMGSRC="gifs/smbanner.gif"ALT="UNIX Power Tools"USEMAP="#srchmap"BORDER="0"></H1><MAPNAME="srchmap"><AREASHAPE="RECT"COORDS="0,0,466,58"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="467,0,514,18"HREF="jobjects/fsearch.htm"ALT="Search this book"></MAP><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch39_07.htm"TITLE="39.7 Checking System Load: uptime "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 39.7 Checking System Load: uptime "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 39<BR>Time and Performance</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch39_09.htm"TITLE='39.9 Know When to Be "nice" to OTher Users...and WhenNot to'><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT='Next: 39.9 Know When to Be "nice" to OTher Users...and WhenNot to'BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-5710">39.8 A Big Environment Can Slow You Down </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-43956"></A>Part of starting a new child process (starting a program, for instance)is making a copy of the environment from the parent process.Some computers, especially busy ones, aren't very fast at making newprocesses.(In the early 1980s, I worked on a VAX 11/750 running 4.1 BSD; the loadaverage got above 40. Sometimes, after a command finished, it could take 10or 20 seconds just to get the next shell prompt. Sheesh!)</P><PCLASS="para">Filling up your environment with lots of variables (the <EMCLASS="emphasis">csh</EM>command <EMCLASS="emphasis">setenv</EM> or the <EMCLASS="emphasis">sh</EM> command<EMCLASS="emphasis">export</EM>) can be handy.But it can slow you down - especially in shell scripts that run loops,starting lots of subprocesses.</P><PCLASS="para">I did a test on our 386-based computer running Interactive UNIX SystemV/386 Release 3.2 late one night when I was the only user logged on.First, I cleaned out my environment to around 300 characters.Then I did:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch06_01.htm"TITLE="6.1 What Environment Variables Are Good For ">env</A> <ACLASS="co"HREF="ch29_06.htm"TITLE="29.6 Counting Lines, Words, and Characters: wc ">wc</A> <ACLASS="co"HREF="ch39_03.htm"TITLE="39.3 The csh time variable ">time</A> <ACLASS="co"HREF="ch09_25.htm"TITLE="9.25 The C Shell repeat Command ">repeat</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">% <CODECLASS="userinput"><B>env | wc -c</B></CODE> 335% <CODECLASS="userinput"><B>set time</B></CODE>% <CODECLASS="userinput"><B>repeat 50 /bin/true</B></CODE>0.0u 0.1s 0:00 15%0.1u 0.1s 0:00 18%0.0u 0.2s 0:00 20% ...</PRE></TD></TR></TABLE></P><PCLASS="para">That started the short <EMCLASS="emphasis">/bin/true</EM> shell script 50 times.I added up the system times (from the second column) and got 6.9 CPU seconds.Then I used a C shell<EMCLASS="emphasis">while</EM> loopto quickly add a bunch of huge environment variables named <EMCLASS="emphasis">FOO1</EM>,<EMCLASS="emphasis">FOO2</EM>, and so on, like this:</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"><ACLASS="co"HREF="ch06_08.htm"TITLE="6.8 Shell Variables ">set</A> <ACLASS="co"HREF="ch47_04.htm"TITLE="47.4 C Shell Variable Operators and Expressions ">@</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">% <CODECLASS="userinput"><B>set n = 0</B></CODE>% <CODECLASS="userinput"><B>while ($n < 30)</B></CODE>1? <CODECLASS="userinput"><B>setenv FOO$n xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx...</B></CODE>? <CODECLASS="userinput"><B>end</B></CODE> ...% <CODECLASS="userinput"><B>env | wc -c</B></CODE> 4934</PRE></TD></TR></TABLE></P><PCLASS="para">and ran <CODECLASS="literal">repeat 50 /bin/true</CODE> again.With a 5000-character environment, it took 8.9 system CPU seconds - that's about 30% longer.</P><PCLASS="para">A thorough test?Nope.But if you have a big environment on a slow computer, you might run a testlike this to seewhether cleaning it out - replacing environment variables with shellvariables, for instance - can make your subprocesses start faster.</P><PCLASS="para">Article<ACLASS="xref"HREF="ch38_07.htm"TITLE="Why ps Prints Some Commands in Parentheses ">38.7</A>explains the problem a big environment can cause for <EMCLASS="emphasis">ps</EM>.Article<ACLASS="xref"HREF="ch02_09.htm"TITLE="Speeding Up Your C Shell with set prompt Test ">2.9</A>shows how to start C shells more quickly.</P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">JP</SPAN></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch39_07.htm"TITLE="39.7 Checking System Load: uptime "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 39.7 Checking System Load: uptime "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="book"HREF="index.htm"TITLE="UNIX Power Tools"><IMGSRC="gifs/txthome.gif"SRC="gifs/txthome.gif"ALT="UNIX Power Tools"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch39_09.htm"TITLE='39.9 Know When to Be "nice" to OTher Users...and WhenNot to'><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT='Next: 39.9 Know When to Be "nice" to OTher Users...and WhenNot to'BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">39.7 Checking System Load: uptime </TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="index"HREF="index/idx_0.htm"TITLE="Book Index"><IMGSRC="gifs/index.gif"SRC="gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172">39.9 Know When to Be "nice" to OTher Users...and WhenNot to</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><IMGSRC="gifs/smnavbar.gif"SRC="gifs/smnavbar.gif"USEMAP="#map"BORDER="0"ALT="The UNIX CD Bookshelf Navigation"><MAPNAME="map"><AREASHAPE="RECT"COORDS="0,0,73,21"HREF="../index.htm"ALT="The UNIX CD Bookshelf"><AREASHAPE="RECT"COORDS="74,0,163,21"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="164,0,257,21"HREF="../unixnut/index.htm"ALT="UNIX in a Nutshell"><AREASHAPE="RECT"COORDS="258,0,321,21"HREF="../vi/index.htm"ALT="Learning the vi Editor"><AREASHAPE="RECT"COORDS="322,0,378,21"HREF="../sedawk/index.htm"ALT="sed & awk"><AREASHAPE="RECT"COORDS="379,0,438,21"HREF="../ksh/index.htm"ALT="Learning the Korn Shell"><AREASHAPE="RECT"COORDS="439,0,514,21"HREF="../lrnunix/index.htm"ALT="Learning the UNIX Operating System"></MAP></DIV></BODY></HTML>⌨️ 快捷键说明
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