Information Technology Reference
In-Depth Information
Stop and Go means that the thread starting the enumeration will join on
all the threads running the query expression. That method is used when
you request that immediate execution of a query by using ToList() or
To A r r a y ( ) , o r a n y t i m e P L I N Q n e e d s t h e f u l l r e s u l t s e t b e f o r e c o n t i n u i n g
such as ordering and sorting. Both of the following queries use Stop and Go:
var
stopAndGoArray = (
from
n
in
data.AsParallel()
where
n <
150
select
Factorial(n)).ToArray();
var
stopAndGoList = (
from
n
in
data.AsParallel()
where
n <
150
select
Factorial(n)).ToList();
Using Stop and Go processing you'll often get slightly better performance
at a cost of a higher memory footprint. However, notice that I've still con-
structed the entire query before executing any of the query expressions.
Yo u ' l l s t i l l w a n t t o c o m p o s e t h e e n t i r e q u e r y, r a t h e r t h a n p r o c e s s i n g e a c h
portion using Stop and Go and then composing the final results using
another query. That will often cause the threading overhead to overwhelm
performance gains. Processing the entire query expression as one com-
posed operation is almost always preferable.
The final algorithm used by the parallel task library is Inverted Enumeration.
Inverted Enumeration doesn't produce a result. Instead, it performs some
action on the result of every query expression. In my earlier samples, I
printed the results of the Factorial computation to the console:
var
numsParallel =
from
n
in
data.AsParallel()
where
n <
150
select
Factorial(n);
foreach
(
var
item
in
numsParallel)
Console
.WriteLine(item);
LINQ to Objects (nonparallel) queries are evaluated lazily. That means
each value is produced only when it is requested. You can opt into the par-
allel execution model (which is a bit different) while processing the result
of the query. That's how you ask for the Inverted Enumeration model:
var
nums2 =
from
n
in
data.AsParallel()
where
n <
150
select
Factorial(n);
nums2.ForAll(item =>
Console
.WriteLine(item));
