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doc: remove mention of default GOMAXPROCS(1) in Effective Go

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Fixes #11781

Change-Id: Idc46a6a4fb5bf1c4d394eadf2d860d7ef75c8ccf
Reviewed-on: https://go-review.googlesource.com/12390
Reviewed-by: Rob Pike <r@golang.org>
This commit is contained in:
Brad Fitzpatrick 2015-07-18 09:49:23 -07:00
parent 7ebcf5eac7
commit aa5118b129
1 changed files with 27 additions and 23 deletions
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50
doc/effective_go.html
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@ -3172,40 +3172,44 @@ count the completion signals by draining the channel after
launching all the goroutines.
</p>
<pre>
const NCPU = 4 // number of CPU cores
const numCPU = 4 // number of CPU cores
func (v Vector) DoAll(u Vector) {
c := make(chan int, NCPU) // Buffering optional but sensible.
for i := 0; i &lt; NCPU; i++ {
go v.DoSome(i*len(v)/NCPU, (i+1)*len(v)/NCPU, u, c)
c := make(chan int, numCPU) // Buffering optional but sensible.
for i := 0; i &lt; numCPU; i++ {
go v.DoSome(i*len(v)/numCPU, (i+1)*len(v)/numCPU, u, c)
}
// Drain the channel.
for i := 0; i &lt; NCPU; i++ {
for i := 0; i &lt; numCPU; i++ {
&lt;-c // wait for one task to complete
}
// All done.
}
</pre>
<p>
The current implementation of the Go runtime
will not parallelize this code by default.
It dedicates only a single core to user-level processing. An
arbitrary number of goroutines can be blocked in system calls, but
by default only one can be executing user-level code at any time.
It should be smarter and one day it will be smarter, but until it
is if you want CPU parallelism you must tell the run-time
how many goroutines you want executing code simultaneously. There
are two related ways to do this. Either run your job with environment
variable <code>GOMAXPROCS</code> set to the number of cores to use
or import the <code>runtime</code> package and call
<code>runtime.GOMAXPROCS(NCPU)</code>.
A helpful value might be <code>runtime.NumCPU()</code>, which reports the number
of logical CPUs on the local machine.
Again, this requirement is expected to be retired as the scheduling and run-time improve.
Rather than create a constant value for numCPU, we can ask the runtime what
value is appropriate.
The function <code><a href="/pkg/runtime#NumCPU">runtime.NumCPU</a></code>
returns the number of hardware CPU cores in the machine, so we could write
</p>
<pre>
var numCPU = runtime.NumCPU()
</pre>
<p>
There is also a function
<code><a href="/pkg/runtime#GOMAXPROCS">runtime.GOMAXPROCS</a></code>,
which reports (or sets)
the user-specified number of cores that a Go program can have running
simultaneously.
It defaults to the value of <code>runtime.NumCPU</code> but can be
overridden by setting the similarly named shell environment variable
or by calling the function with a positive number. Calling it with
zero just queries the value.
Therefore if we want to honor the user's resource request, we should write
</p>
<pre>
var numCPU = runtime.GOMAXPROCS(0)
</pre>
<p>
Be sure not to confuse the ideas of concurrency—structuring a program
as independently executing components—and parallelism—executing