doc: remove mention of default GOMAXPROCS(1) in Effective Go
Fixes #11781
Change-Id: Idc46a6a4fb5bf1c4d394eadf2d860d7ef75c8ccf
Reviewed-on: https://go-review.googlesource.com/12390
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/doc/effective_go.html b/doc/effective_go.html
index 5465fa3..5a522f6 100644
--- a/doc/effective_go.html
+++ b/doc/effective_go.html
@@ -3172,40 +3172,44 @@
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 < 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 < numCPU; i++ {
+ go v.DoSome(i*len(v)/numCPU, (i+1)*len(v)/numCPU, u, c)
}
// Drain the channel.
- for i := 0; i < NCPU; i++ {
+ for i := 0; i < numCPU; i++ {
<-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
