Question

直接从堆提取大小合适的内存块。

// malloc.go

// Allocate an object of size bytes.
// Small objects are allocated from the per-P cache's free lists.
// Large objects (> 32 kB) are allocated straight from the heap.

func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {

    c := getMCache(mp)
  
    if size <= maxSmallSize {
        // tiny & small ...
    } else {
        span = c.allocLarge(size, noscan)
    
        span.freeindex = 1
        span.allocCount = 1
        size = span.elemsize
      
        x = unsafe.Pointer(span.base())
    }

    return x
}

// mcache.go

// allocLarge allocates a span for a large object.
func (c *mcache) allocLarge(size uintptr, noscan bool) *mspan {
    
    // 计算页数和等级。
	npages := size >> _PageShift
	spc := makeSpanClass(0, noscan)
    
    // 从堆直接分配。
	s := mheap_.alloc(npages, spc)

	// Put the large span in the mcentral swept list so that it's
	// visible to the background sweeper.
	mheap_.central[spc].mcentral.fullSwept(mheap_.sweepgen).push(s)
    
	return s
}