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AdvancedTechniques
NetworkingWithShellScripts
2014-03-10   |   Copyright © 2003, 2014 Apple Inc. All Rights Reserved.
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Shellscripts,whencomparedwithcompiledlanguages,generallydonotperformwell.However,mostshell
scriptsalsodonotperformaswellastheycouldwithabitofperformancetuning.Thischaptershowssome
commonpitfallsofshellscriptinganddemonstrateshowtofixthesemistakes.
Avoiding Unnecessary External Commands
Everylineofcodeinashellscripttakestimetoexecute.Thissectionshowstwoexamplesinwhichavoiding
unnecessaryexternalcommandsresultsinasignificantperformanceimprovement.
FindingtheOrdinalRankofaCharacter(MoreQuickly)
TheMonteCarlomethodsamplecode,foundinAnExtremeExample:TheMonteCarlo(Bourne)Methodfor
Pi (page 327),showsanumberofwaystocalculatetheordinalvalueofabyte.Theversionwrittenusingapure
shellapproachispainfullyslow,inlargepartbecauseoftheloopsrequired.
Thebestwaytooptimizeperformanceistofindanexternalutilitywritteninacompiledlanguagethatcan
performthesametaskmoreeasily.Thus,thesolutiontothatperformanceproblemwastousethe
perl
or
awk
interpretertodotheheavylifting.Althoughtheyarenotcompiledlanguages,bothPerlandAWKhave
compiledroutines(
ord
and
index
,respectively)tofindtheindexofacharacterwithinastring.
However,whenusingoutsideutilitiesisnotpossible,youcanstillreducethecomplexitybyexecutingoutside
toolslessfrequently.Forexample,onceyouhaveaninitializedarraycontainingallofthecharactersfrom1–255
(skippingnull),youcanreducethenumberofiterationsbyremovingmorethanonecharacteratatimeuntil
thecharacterdisappears,thengoingbackbyonebatchofcharactersandworkingyourwayforwardagain,
onecharacteratatime.
Thefollowingcoderunsmorethantwiceasfast(onaverage)asthepurelylinearsearch:
ord2()
{
local CH="$1"
local STRING=""
local OCCOPY=$ORDSTRING
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Performance Tuning
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local COUNT=0;
# Delete ten characters at a time. When this loop
# completes, the decade containing the character
# will be stored in LAST.
CONT=1
BASE=0
LAST="$OCCOPY"
while [ $CONT = 1 ] ; do
LAST=`echo "$OCCOPY" | sed 's/^\(..........\)/\1/'`
OCCOPY=`echo "$OCCOPY" | sed 's/^..........//'`
CONT=`echo "$OCCOPY" | grep -c "$CH"`
BASE=`expr $BASE + 10`
done
BASE=`expr $BASE - 10`
# Search for the character in LAST.
CONT=1;
while [ $CONT = 1 ]; do
# Copy the string so we know if we've stopped finding
# nonmatching characters.
OCTEMP="$LAST"
# echo "CH WAS $CH"
# echo "ORDSTRING: $ORDSTRING"
# If it's a close bracket, quote it; we don't want to
# break the regexp.
if [ "x$CH" = "x]" ] ; then
CH='\]'
fi
# Delete a character if possible.
LAST=$(echo "$LAST" | sed "s/^[^$CH]//");
PerformanceTuning
AvoidingUnnecessaryExternalCommands
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# On error, we're done.
if [ $? != 0 ] ; then CONT=0 ; fi
# If the string didn't change, we're done.
if [ "x$OCTEMP" = "x$LAST" ] ; then CONT=0 ; fi
# Increment the counter so we know where we are.
COUNT=$((COUNT + 1)) # or COUNT=$(expr $COUNT '+' 1)
# echo "COUNT: $COUNT"
done
COUNT=$(($COUNT + 1 + $BASE)) # or COUNT=$(expr $COUNT '+' 1)
# If we ran out of characters, it's a null (character 0).
if [ "x$OCTEMP" = "x" ] ; then COUNT=0; fi
# echo "ORD IS $COUNT";
# Return the ord of the character in question....
echo $COUNT
# exit 0
}
Asyoutune,youshouldbecognizantoftheaveragecasetime.Inthecaseofalinearsearch,assumingall
possiblecharactervaluesareequallylikely,theaveragetimeishalfofthenumberofitemsinthelist,orabout
127comparisons.Searchinginunitsof10,theaverageisabout1/10ofthatplushalfof10,orabout17.69
comparisons,withaworstcaseof34comparisons.Theoptimalvalueis16,withanaverageof15.9375
comparisons,andaworstcaseof30comparisons.
Ofcourse,youcouldwritethecodeasabinarysearch.Becausesplittingastringisnoteasytodoquickly,a
binarysearchworksbestwithstringsofknownlengthinwhichyoucancacheaseriesofstringscontaining
somenumberofperiods.Ifyouaresearchingastringofarbitrarylength,thistechniquewouldprobablybe
much,muchslowerthanalinearsearch(unlessyouuseBASH-specificsubstringexpansion,asdescribedin
TruncatingStrings (page 179)).
PerformanceTuning
AvoidingUnnecessaryExternalCommands
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Cachingthestringsofperiodsusedinthesplittingprocessincreasesinitializationtimeslightly,butafterthat,
theexecutiontimeofthesearchitselfimprovesbyaboutafactorof2comparedtothe“skip16”version.
Whetherthattradeoffisappropriatedependslargelyonhowmanytimesyouneedtoperformthisoperation.
Iftheanswerisonce,thentheextrainitializationtimewilllikelyeraseanyperformancegainfromusingthe
binarysearch.Iftheanswerismorethanonce,thebinarysearchispreferable.
Listing12-1containsthebinarysearchversion.
Listing12-1 AbinarysearchversionoftheBourneshell
ord
subroutine
# Initialize the split strings.
# This block of code should be
# added to the end of ord_init.
SPLIT=128
while [ $SPLIT -ge 1 ] ; do
COUNT=$SPLIT
STRING=""
while [ $COUNT -gt 0 ] ; do
STRING="$STRING""."
COUNT=$((COUNT - 1))
done
eval "SPLIT_$SPLIT=\"$STRING\"";
SPLIT=$((SPLIT / 2))
done
# End of content to add to ord_init
split_str()
{
STR="$1"
NUM="$2"
SPLIT="$(eval "echo \"\$SPLIT_$NUM\"")"
LEFT="$(echo "$STR" | sed "s/^\\($SPLIT\\).*$/\\1/")"
RIGHT="$(echo "$STR" | sed "s/^$SPLIT//")"
}
PerformanceTuning
AvoidingUnnecessaryExternalCommands
2014-03-10   |   Copyright © 2003, 2014 Apple Inc. All Rights Reserved.
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ord3()
{
local CH="$1"
OCCOPY="$ORDSTRING"
FIRST=1;
LAST=257
ord3_sub "$CH" "$ORDSTRING" $FIRST $LAST
}
ord3_sub()
{
local CH="$1"
OCCOPY="$2"
FIRST=$3
LAST=$4
# echo "FIRST: $FIRST, LAST: $LAST"
if [ $FIRST -ne $(($LAST - 1)) ] ; then
SPLITWIDTH=$((($LAST - $FIRST) / 2))
split_str "$OCCOPY" $SPLITWIDTH
if [ $(echo "$LEFT" | grep -c "$CH") -eq 1 ] ; then
# echo "left"
ord3_sub "$CH" "$LEFT" $FIRST $(( $FIRST + $SPLITWIDTH ))
else
# echo "right"
ord3_sub "$CH" "$RIGHT" $(( $FIRST + $SPLITWIDTH )) $LAST
fi
else
echo $(( $FIRST + 1 ))
fi
PerformanceTuning
AvoidingUnnecessaryExternalCommands
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226
}
Asexpected,thisperformssignificantlybetter,decreasingexecutiontimebyabouttenpercentinthiscase.
Theimprovedperformance,however,isalmostpreciselyoffsetbytheextrainitializationcoststoenableyou
tosplitthelist.Thatiswhyyoushouldneverassumethatatheoreticallyoptimalalgorithmwillperformbetter
thanatheoreticallylessoptimalalgorithm.Inshellscripting,theperformanceimpactofconstantcostdifferences
canandoftendoeasilyoutweighimprovementsinalgorithmiccomplexity.
Ofcourse,usingaPerlorAWKscripttofindtheordinalrankismuchfasterthananyofthesemethods.The
purposeofthisexampleistodemonstratemethodsforimprovingefficiencyofsimilaroperations,nottoshow
thebestwaytofindtheordinalrankofacharacter.
ReducingUseoftheevalBuiltin
The
eval
builtinisaverypowerfultool.However,itaddsconsiderableoverheadwhenyouuseit.
Ifyouareexecutingthe
eval
builtinrepeatedlyinaloopanddonotneedtousetheresultsforintermediate
calculations,itissignificantlyfastertostoreeachexpressionasaseriesofsemicolon-separatedcommands,
thenexecutethemallinasinglepassattheend.
Forexample,thefollowingcodeshiftstheentriesinapseudo-arraybyonerow:
test1()
{
X=1; XA=0
while [ $X -lt 5 ] ; do
Y=1;
while [ $Y -lt 5 ] ; do
eval "FOO_$X""_$Y=FOO_$XA""_$Y"
Y=`expr $Y + 1`
done
X=`expr $X + 1`
XA=`expr $XA + 1`
done
}
PerformanceTuning
AvoidingUnnecessaryExternalCommands
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227
Youcanspeedupthissubroutinebyabout20%byconcatenatingtheassignmentstatementsintoasingle
stringandrunning
eval
onlyonce,asshowinthefollowingexample:
test3()
{
X=1; XA=0
LIST=""
while [ $X -lt 5 ] ; do
Y=1;
while [ $Y -lt 5 ] ; do
LIST="$LIST$SEMI""FOO_$X""_$Y=\$FOO_$XA""_$Y"
SEMI=";"
Y=`expr $Y + 1`
done
X=`expr $X + 1`
XA=`expr $XA + 1`
done
# echo $LIST
eval $LIST
}
Anevenmoredramaticperformanceimprovementcomeswhenyoucanprecachethesecommandsintoa
variable.Ifyouneedtorepeatedlyexecuteafairlywell-definedseriesofstatementsinthisway(butdon’twant
towastehundredsoflinesofspaceinyourcode),youcancreatethelistofcommandsonce,thenuseit
repeatedly.
Bycachingthelistofcommands,thesecondandsubsequentexecutionsimprovebyaboutafactorof200,
whichputsitsperformanceatornearthespeedofasubroutinecallwithalloftheassignmentstatements
writtenout.
Anotherusefultechniqueistoprecacheadummyversionofthecommands,withplaceholdertextinsteadof
certainvalues.Forexample,intheabovecodeyoucouldcacheaseriesofstatementsintheform
ROW_X_COL_1=ROW_Y_COL_1;
,repeatingforeachcolumnvalue.Then,whenyouneededtocopyonerow
toanother,youcoulddothis:
eval `echo $ROWCOPY | sed "s/X/$DEST_ROW/g" | sed "s/Y/$SRC_ROW/g"`
PerformanceTuning
AvoidingUnnecessaryExternalCommands
2014-03-10   |   Copyright © 2003, 2014 Apple Inc. All Rights Reserved.
228
Ifyoudon’thaveseparatevariablesforsourceanddestinationrows,youmightwritesomethinglikethe
following:
eval `echo $ROWCOPY | sed "s/X/$ROW/g" | sed "s/Y/$(expr $ROW + 1)/g"`
Bywritingthecodeinthisway,youhavereplacedseverallinesofiteratorcodeanddozensof
eval
instructions
withasingle
eval
instructionandtwoexecutionsof
sed
.Theresultingperformanceimprovementisdramatic.
Other PerformanceTips
Hereareafewmoreperformancetuningtips.
BackgroundorDeferOutput
Outputtofilestakestime,outputtotheconsoledoublyso.Ifyouarewritingcodewhereperformanceisa
consideration,youshouldeitherexecuteoutputcommandsinthebackgroundbyaddinganampersand(
&
)
totheendofthecommandorgroupmultipleoutputstatementstogether.
Forexample,ifyouaredrawingagameboard,thefastestwayistostoreyourdrawcommandsinasingle
variableandoutputthedataatonce.Inthisway,youavoidtakingmultipleexecutionpenalties.Averyfast
waytodothisistodisablebufferingandsetnewlinetoshiftdownalinewithoutreturningtotheleftedge
(run
stty raw
tosetbothoftheseparameters),thenstorethefirstrowintoavariable,followedbyanewline,
followedbybackspacecharacterstoshiftlefttothestartofthenextrow,followedbythenextrow,andsoon.
Defer PotentiallyUnnecessaryWork
Iftheresultsofaseriesofinstructionsmayneverbeused,donotperformthoseinstructions.
Forexample,considercodethatusesthe
eval
builtintoobtainthevaluesfromaseriesofvariablesina
pseudo-array.Supposethatthecodereturnsimmediatelyifanyofthevariableshasavalueof2ormore.
Unlessyouareaccumulatingmultipleassignmentstatementsintoasinglecallto
eval
(asdescribedinReducing
UseoftheevalBuiltin (page 227)),youshouldcall
eval
onthefirststatementbyitself,makethecomparison,
run
eval
forthenextstatement,andsoon.Bydoingso,youarereducingtheaveragenumberofcallsto
eval
.
PerformComparisonsOnlyOnce
Ifyouhaveasubroutinethatperformsanexpensivetesttwoormoretimes,cachetheresultsofthattestand
performthemostlightweightcomparisonpossiblefromthenon.
PerformanceTuning
OtherPerformanceTips
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229
Also,ifyouhavetwopossibleexecutionpathsthroughyourcodethatsharesomecodeincommon,itmay
befastertouseonlyasingle
if
statementandduplicatethesmallamountofcommoncoderatherthan
repeatedlyperformingthesamecomparison.Ingeneral,however,suchchangeswillonlyresultinasingle-digit
percentageimprovementinperformance,soitisusuallynotworththedecreaseinmaintainabilitytoduplicate
codeinthisway.
Theperformanceimpactvariesdependingontheexpenseofthetest.Teststhatperformcomputationsor
outsideexecutionareparticularlyexpensiveandthusshouldbeminimizedasmuchaspossible.Ofcourse,
youcanreducetheadditionalimpactbyperformingthecalculationonceanddoingalightweighttestmultiple
times.
AsimpletestcaseproducedtheresultsshowninTable12-1.
Table12-1
Performance(inseconds)impactofduplicatingcommoncodetoavoidredundanttests
Testperformedoncewithtwocopiesofshared
code
Testperformedtwicewithonecopyofsharedcode
in-between
6.957
7.003
ChooseControlStatementsCarefully
Inmostsituations,theappropriatecontrolstatementisobvious.Totesttoseewhetheravariablecontains
oneoftwoorthreevalues,yougenerallychoosean
if
statementwithasmallnumberof
elif
statements.
Forlargernumberofvalues,yougenerallychoosea
case
statement.Thisnotonlyleadstomorereadable
code,butalsoresultsinfastercode.
Forsmallnumbersofcases(5),asexpected,thedifferencebetweenaseriesof
if
statements,an
if
statement
withaseriesof
elif
statements,anda
case
statementislargelylostinthenoise,performance-wise,even
after1000iterations.AlthoughtheresultsshowninTable12-2areintheexpectedorder,thiswasonlytrue
approximatelyhalfthetime.Forasmallernumberofcases,thedifferencescanlargelybeignored.
Table12-2
Performance(inseconds)comparisonsof1000executionsofvariouscontrolstatementsequences
casestatement
if,thenseriesofelif
statements
seriesofif
statements
evalbuiltinexecuting
multiplesubroutines
6.807
6.831
6..846
6.945
Fivecases
6.903
6.980
7.224
7.094
Tencases
6.704
7.392
8.03
7.023
Fiftycases
PerformanceTuning
OtherPerformanceTips
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230
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