Thanks for the clarification. On Mon, Oct 22, 2018 at 13:13 Ian Lance Taylor <i...@golang.org> wrote:
> On Sun, Oct 21, 2018 at 8:34 PM, Fei Ding <fding...@gmail.com> wrote: > > > > I am studying golang source code, literally `go/src/runtime/chan.go`, the > > question is in function `makechan`, when it comes to malloc memory for > > channel with/without pointers, you guys did it as: > > > > > > case elem.kind&kindNoPointers != 0: > > // Elements do not contain pointers. > > // Allocate hchan and buf in one call. > > c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil, true)) > > c.buf = add(unsafe.Pointer(c), hchanSize) > > default: > > // Elements contain pointers. > > c = new(hchan) > > c.buf = mallocgc(uintptr(size)*elem.size, elem, true) > > > > So, my question is why to use two different ways to do this, (continuous > vs > > separated) ? and what's the difference/relationship between malloc and > new? > > The hchan struct is designed so that it doesn't contain any pointers > that the garbage collector needs to know about, as explained in the > comment a few lines above the code you quote. If the channel elements > don't contain any pointers, then the garbage collector doesn't have to > scan them. So in that case we can allocate a single large block of > memory to hold the hchan struct and the channel buffer, and the > garbage collector will never to scan that large buffer. That tends to > give better memory cache behavior and slightly lessens the load on the > garbage collector. When the channel elements do have pointers, we > allocate the channel buffer separately, as that is easier than > building up a GC structure that handles both the hchan struct and the > buffer. The key difference is that in the no-pointer case we pass nil > as the type argument to mallocgc, and in the pointer case we pass the > element type. > > Calling new is equivalent to calling mallocgc(sizeof_type, type, > true). The only reason to call mallocgc is when the runtime wants to > vary one of those arguments. > > Ian > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
