《VB2008从入门到精通(PDF格式英文版)》第163章

you　write　your　code。　For　example；　suppose　the　lighting　controller　introduced　in　Chapter　8　had
to　control　3；000　rooms？　To　be　able　to　process　such　a　large　number　of　rooms；　you　would　use　the
same　code；　except　run　it　in　parallel。　And　therein　lies　the　problem　of　code　that　needs　to　multitask。
It’s　more　difficult　to　make　code　run　efficiently　in　parallel。　A　mon　problem　in　multitasking
applications　is　the　dreaded　deadlock。　A　deadlock　occurs　when　two　tasks　need　data　from　each
other　to　continue；　which　means　neither　can　continue。　
Running　code　in　parallel　requires　coding　discipline；　as　you　will　learn　in　this　chapter。　We
will　use　only　a　single　project；　named　JugglingTasks；　which　is　a　console　application　that　imple
ments　the　techniques　demonstrated　in　this　chapter。　
Understanding　Multitasking　
One　of　the　biggest　jumps　in　puting　history　was　when　the　puter　went　from　a　machine
that　carried　out　a　single　task　to　a　multitasking　machine。　The　old　modore　64　and　VIC…20
were　single…tasking　machines。　You　started　the　puter；　and　then　the　puter　waited　for
you　to　do　something。　If　you　decided　to　make　the　puter　run　a　loop　saying　“hello”　millions
of　times；　that　is　all　the　puter　would　do。　The　puter　could　not　do　one　thing　and　then　do
something　else　while　waiting　for　an　answer　from　the　executing　task。　How　could　anyone　get
anything　done？　（On　the　other　hand；　you　could　argue　that　people　might　be　more　productive　if
they　could　work　in　only　a　single　program；　and　not　also　check　their　e…mail；　look　at　the　latest　blog
entry　from　their　favorite　author；　and　so　on。）
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Things　changed　quite　dramatically　when　puters　could　multitask。　Server　puters
running　operating　systems　like　Unix　were　multitasking　from　day　one。　Operating　systems　like
Windows　were　not　100％　preemptive　multitasking　until　Windows　NT　and　Windows　95。　Notice
that　I　used　the　term　preemptive；　which　makes　a　big　difference。
Plain…vanilla　multitasking　is　multitasking　where　individual　tasks　cooperate　and　allocate
resources　as　a　collective。　It　is　essentially　a　dead…end　idea；　and　the　best　example　of　how　it　worked　is
Windows　3。0　and　Windows　3。1　（but　these　were　full　of　hacks）。　Now　let’s　see　what　preemptive
multitasking　means。　
Preemptive　Multitasking　
Preemptive　multitasking　is　when　the　operating　system　controls　which　application　does　what
and　when。　Realize　that　you　can　run　an　application　because　applications　are　treated　as　po
nents。　You　don’t　believe　me？　Create　a　console　application　and　rename　the　method　Main（）　to
Mains（）。　See　what　happens。　The　reality　is　that　the　Main（）　method；　as　it　is　declared；　is　an　API
used　by　the　operating　system　to　run　your　ponent；　which　masquerades　as　a　program。　
So　we　have　this　program　called　the　operating　system　that　runs　ponents　called　programs。
The　next　question　is　how　can　multiple　programs　run　at　the　same　time？　The　answer　is　that　the
operating　system　program　is　no　ordinary　program。　It　is　the　foundation　upon　which　applica
tions　can　be　launched　and　managed。　The　operating　system　hooks　into　special　features　of　the
microprocessor　that　allow　the　operating　system　to　time　slice　the　individual　programs。　Because
your programs　are　ponents；　they　will　never　need　to　access　the　microprocessor　directly。
Time　Slicing　
Time　slicing　is　when　an　operating　system　can　dictate　for　how　much　time　a　program　is　allowed
to　execute。　Between　the　times　of　execution；　the　program　is　in　a　state　of　deep　freeze　and　does
nothing。　You；　as　a　user；　are　not　aware　of　the　time　slices；　because　a　time　slice　operates　on　the
order　of　microseconds。　Because　time　slicing　is　so　fast；　you　think　your　program　is　running
continuously。　To　see　the　time　slicing　in　action；　open　the　Windows　Task　Manager　and　select　the
Processes　tab。　
Look　closely　at　the　Task　Manager　window　shown　in　Figure　13…1；　and　you’ll　see　that
explorer。exe　has　6％　of　the　CPU。　You　can　say　that　the　explorer。exe　application　is　using　the
appropriate　amount　of　time　slices　that　equals　6％　of　the　CPU　time。　How　the　CPU　slices　the　6％
is　a　detail　of　the　operating　system。
Say　you　have　a　program　that　runs　two　tasks：　task　1　and　task　2。　The　microprocessor　is　a　single
core；　and　thus　when　running　two　separate　tasks；　there　will　be　two　time　slices；　as　illustrated　in
Figure　13…2。　In　the　figure；　the　entire　processing　cycle　is　represented　as　a　pie；　and　each　time　slice
is　a　slice　of　the　pie。　
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Figure　13…1。　Windows　time　slicing　in　action　
Figure　13…2。　Single…core　microprocessor　running　two　tasks　
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Notice　how　task　1　and　task　2　run　in　a　serial　manner；　rather　than　concurrently。　This　is
because　the　microprocessor　is　a　single…task　device　made　to　look　like　a　multitask　device。　You
would　run　multiple　tasks　on　a　single…core　microprocessor　when　you　want　to　have　application
background　tasks　that　should　not　affect　the　foreground　task　of　the　application。　For　example；
when　running　a　word　processor；　you　don’t　want　the　spell