Go-Slice的扩容机制
查看Go的不同版本:https://go.googlesource.com/go/+refs
源码位置
src/runtime/slice.go :func growslice(xxx)
1.18之前
newcap := old.cap doublecap := newcap + newcap if cap > doublecap { newcap = cap } else { if old.cap < 1024 { newcap = doublecap } else { // Check 0 < newcap to detect overflow // and prevent an infinite loop. for 0 < newcap && newcap < cap { newcap += newcap / 4 } // Set newcap to the requested cap when // the newcap calculation overflowed. if newcap <= 0 { newcap = cap } } }
1.18之后
go在这次提交中 runtime: make slice growth formula a bit smoother修改了扩容规则
用一个更平滑的公式来计算扩容因子,在256个元素之前还是2倍扩容因子,在256个元素后逐步减少,不同容量下的扩容因子如下:
从小切片的增长2x过渡到大切片的增长1.25x
这个公式给出了两者之间的平滑过渡
newcap := oldCap doublecap := newcap + newcap if newLen > doublecap { newcap = newLen } else { const threshold = 256 if oldCap < threshold { newcap = doublecap } else { // Check 0 < newcap to detect overflow // and prevent an infinite loop. for 0 < newcap && newcap < newLen { // Transition from growing 2x for small slices // to growing 1.25x for large slices. This formula // gives a smooth-ish transition between the two. newcap += (newcap + 3*threshold) / 4 } // Set newcap to the requested cap when // the newcap calculation overflowed. if newcap <= 0 { newcap = newLen } } }
总结
-
1.18以前:
- 容量小于1024时为2倍扩容,大于等于1024时为1.25倍扩容
-
1.18以后:
- 小于256时为2倍扩容,大于等于256时的扩容因子逐渐从2减低为1.25