runtime/debug: add garbage collection controls

[rsc]

The history information is already accessible using runtime.ReadMemStats, but that is
more expensive than it needs to be just to get some info about past GCs. (In particular
it does not need to stop the world.) Perhaps we should have a separate ReadGCStats that
can be cheaper. It would be nice to have the tukey summary of the pause list as well as
the duration and start time of the most recent pause. The start time may not be recorded
anywhere right now and need to be added.

[rsc]

Comment 1:

// in runtime/debug
package debug
type GarbageStats struct {
    PauseHistory []int64 // most recent first
    LastGC time.Time
    Tukey5 [5]int64
}
func ReadGarbageStats(*GarbageStats)
Putting this in runtime/debug makes it possible to import time.
Parts of the implementation would be in runtime but define symbols in runtime/debug.

[rsc]

Comment 2:

Issue #4091 has been merged into this issue.

[rsc]

Comment 3:

Also, still in runtime/debug
s/GarbageStats/GCStats/
s/ReadGarbageStats/ReadGCStats/
func EnableGC(enabled bool) bool // returns old setting
func SetGCPercent(percent int) int // returns old setting

[rsc]

Comment 4:

Issue #4171 has been merged into this issue.

[rsc]

Comment 5:

Also, still in runtime/debug
// FreeOSMemory attempts to return unused pages of memory back to the
// operating system. It is best effort and not guaranteed to accomplish anything.
func FreeOSMemory()

[rsc]

Comment 6:

Labels changed: added size-m.

[rsc]

Comment 7:

Labels changed: added suggested.

[rsc]

Comment 8:

This issue was closed by revision 472354f.

Status changed to Fixed.