什么是协程栈
每个协程都需要有自己的栈空间,来存放变量,函数,寄存器等信息。所以系统需要给协程分配足够的栈空间。
栈分配方式
固定大小的栈
每个协程都有相同的,固定大小的栈。
优点:实现简单;
缺点:每个协程需要的栈空间不尽相同,如果一概而论,那么有些是浪费,有些是不够用。
创建时指定
由开发者在创建时指定协程栈大小。java, c++在创建线程时可以指定其栈大小。
优点:实现简单
缺点:对开发者要求比较高,需要根据栈变量,请求量预估。但是有些场景不太好预估,比如递归调用,这种情况通常只能往大的估计。
Segmented stacks
分配和释法额外的内存空间。初始分配的比较小的空间,如4k。不够了再增加,用完即释放。以下是一个例子:
当G调用H的时候,没有足够的栈空间来让H运行,这时候Go运行环境就会从堆里分配一个新的栈内存块去让H运行。在H返回到G之前,新分配的内存块被释放回堆。这种管理栈的方法一般都工作得很好。但对有些代码,特别是递归调用,它会造成程序不停地分配和释放新的内存空间。举个例子,在一个程序里,函数G会在一个循环里调用很多次H函数。每次调用都会分配一块新的内存空间。这就是热分裂问题(hot split problem)。
优点:动态扩展,初始成本小,可以将协程当作廉价资源使用。
缺点:存在热分裂问题(hot split problem)。
Stack copying
动态扩展,分配更大的内存,做指针迁移。
优点:动态扩展,初始成本小,可以将协程当作廉价资源使用,且不存在hot split problem问题
缺点:由于通常以2倍扩展,当请求量密集,内存敏感的情况下,内存会消耗比较多,容易oom,当然,通常的业务量是ok的,不会有任何问题。同时100w连接才要考虑优化。
golang 栈分配方式
1.3之前采用的是Segmented stacks的方式。之后采用的Stack copying,也叫continuous stack(连续栈)
栈扩容
触发时机
运行时,发现栈不够用了
关键步骤
- 将状态从 _Grunning 更新至 _Gcopystack
- 计算出需要申请的数据大小
- copystack,进行栈复制,后面会详细分析
- 将协程状态恢复至_Grunning
- 走一遍协程调度
关键源码
func newstack() { thisg := getg() ...... gp := thisg.m.curg ...... // Allocate a bigger segment and move the stack. oldsize := gp.stack.hi - gp.stack.lo newsize := oldsize * 2 // 比原来大一倍 ...... // The goroutine must be executing in order to call newstack, // so it must be Grunning (or Gscanrunning). casgstatus(gp, _Grunning, _Gcopystack) //修改协程状态 // The concurrent GC will not scan the stack while we are doing // the copy since the gp is in a Gcopystack status. copystack(gp, newsize, true) //在下面会讲到 ...... casgstatus(gp, _Gcopystack, _Grunning) gogo(&gp.sched) } 复制代码
栈缩容
触发时机
gc进行时,非运行中协程,栈使用不超过1/4的,会缩容为原来1/2
关键步骤
- 检查协程状态,如果已经结束,则释放空间
- 确定新空间size,目前为原来1/2
- 检查栈使用是否超过1/4,若没有,则放弃
- copystack,进行栈复制,后面会详细分析
关键源码
func shrinkstack(gp *g) { gstatus := readgstatus(gp) if gstatus&^_Gscan == _Gdead { if gp.stack.lo != 0 { // Free whole stack - it will get reallocated // if G is used again. stackfree(gp.stack) gp.stack.lo = 0 gp.stack.hi = 0 } return } ...... oldsize := gp.stack.hi - gp.stack.lo newsize := oldsize / 2 // 比原来小1倍 if newsize < _FixedStack { return } // Compute how much of the stack is currently in use and only // shrink the stack if gp is using less than a quarter of its // current stack. The currently used stack includes everything // down to the SP plus the stack guard space that ensures // there's room for nosplit functions. avail := gp.stack.hi - gp.stack.lo //当已使用的栈占不到总栈的1/4 进行缩容 if used := gp.stack.hi - gp.sched.sp + _StackLimit; used >= avail/4 { return } copystack(gp, newsize, false) //在下面会讲到 } 复制代码
copystack栈拷贝过程
原来内容上的拷贝
关键步骤
- 申请新的栈空间:new := stackalloc(uint32(newsize));
- 调整指针指向,将sudog,ctx等,指向新位置,计算方式为原地址+delta(delta为new.hi-old.hi);
- gentraceback,调整栈帧到新位置;
- memmove老栈数据到新栈;
- 删除老栈。
func copystack(gp *g, newsize uintptr, sync bool) { ...... old := gp.stack ...... used := old.hi - gp.sched.sp // allocate new stack new := stackalloc(uint32(newsize)) ...... // Compute adjustment. var adjinfo adjustinfo adjinfo.old = old adjinfo.delta = new.hi - old.hi //用于旧栈指针的调整 //后面有机会和 select / chan 一起分析 // Adjust sudogs, synchronizing with channel ops if necessary. ncopy := used if sync { adjustsudogs(gp, &adjinfo) } else { ...... adjinfo.sghi = findsghi(gp, old) // Synchronize with channel ops and copy the part of // the stack they may interact with. ncopy -= syncadjustsudogs(gp, used, &adjinfo) } //把旧栈数据复制到新栈 // Copy the stack (or the rest of it) to the new location memmove(unsafe.Pointer(new.hi-ncopy), unsafe.Pointer(old.hi-ncopy), ncopy) // Adjust remaining structures that have pointers into stacks. // We have to do most of these before we traceback the new // stack because gentraceback uses them. adjustctxt(gp, &adjinfo) adjustdefers(gp, &adjinfo) adjustpanics(gp, &adjinfo) ...... // Swap out old stack for new one gp.stack = new gp.stackguard0 = new.lo + _StackGuard // NOTE: might clobber a preempt request gp.sched.sp = new.hi - used gp.stktopsp += adjinfo.delta // Adjust pointers in the new stack. gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, adjustframe, noescape(unsafe.Pointer(&adjinfo)), 0) ...... //释放旧栈 stackfree(old) } 复制代码
栈帧调整
golang栈帧
package main func myFunction(a, b int) (int, int) { return a + b, a - b } func main() { myFunction(66, 77) } 复制代码
栈帧调整
gentraceback里回调了adjustframe函数,我们所需要了解的即golang的栈空间中,有存放函数参数,返回值,函数返回地址等信息,这些地址都需要调节,该函数就是针对原来的栈指针进行的调节。代码如下:
// Note: the argument/return area is adjusted by the callee. func adjustframe(frame *stkframe, arg unsafe.Pointer) bool { adjinfo := (*adjustinfo)(arg) targetpc := frame.continpc if targetpc == 0 { // Frame is dead. return true } f := frame.fn ......... pcdata := pcdatavalue(f, _PCDATA_StackMapIndex, targetpc, &adjinfo.cache) if pcdata == -1 { pcdata = 0 // in prologue } // Adjust local variables if stack frame has been allocated. size := frame.varp - frame.sp var minsize uintptr switch sys.ArchFamily { case sys.ARM64: minsize = sys.SpAlign default: minsize = sys.MinFrameSize } if size > minsize { var bv bitvector stackmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps)) if stackmap == nil || stackmap.n <= 0 { print("runtime: frame ", funcname(f), " untyped locals ", hex(frame.varp-size), "+", hex(size), "\n") throw("missing stackmap") } // Locals bitmap information, scan just the pointers in locals. if pcdata < 0 || pcdata >= stackmap.n { print("runtime: pcdata is ", pcdata, " and ", stackmap.n, " locals stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n") throw("bad symbol table") } bv = stackmapdata(stackmap, pcdata) size = uintptr(bv.n) * sys.PtrSize if stackDebug >= 3 { print(" locals ", pcdata, "/", stackmap.n, " ", size/sys.PtrSize, " words ", bv.bytedata, "\n") } adjustpointers(unsafe.Pointer(frame.varp-size), &bv, adjinfo, f) } // Adjust saved base pointer if there is one. if sys.ArchFamily == sys.AMD64 && frame.argp-frame.varp == 2*sys.RegSize { if !framepointer_enabled { print("runtime: found space for saved base pointer, but no framepointer experiment\n") print("argp=", hex(frame.argp), " varp=", hex(frame.varp), "\n") throw("bad frame layout") } if stackDebug >= 3 { print(" saved bp\n") } if debugCheckBP { // Frame pointers should always point to the next higher frame on // the Go stack (or be nil, for the top frame on the stack). bp := *(*uintptr)(unsafe.Pointer(frame.varp)) if bp != 0 && (bp < adjinfo.old.lo || bp >= adjinfo.old.hi) { println("runtime: found invalid frame pointer") print("bp=", hex(bp), " min=", hex(adjinfo.old.lo), " max=", hex(adjinfo.old.hi), "\n") throw("bad frame pointer") } } adjustpointer(adjinfo, unsafe.Pointer(frame.varp)) } // Adjust arguments. if frame.arglen > 0 { var bv bitvector if frame.argmap != nil { bv = *frame.argmap } else { stackmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps)) if stackmap == nil || stackmap.n <= 0 { print("runtime: frame ", funcname(f), " untyped args ", frame.argp, "+", frame.arglen, "\n") throw("missing stackmap") } if pcdata < 0 || pcdata >= stackmap.n { print("runtime: pcdata is ", pcdata, " and ", stackmap.n, " args stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n") throw("bad symbol table") } bv = stackmapdata(stackmap, pcdata) } if stackDebug >= 3 { print("args\n") } adjustpointers(unsafe.Pointer(frame.argp), &bv, adjinfo, funcInfo{}) } return true }
作者:牛牛码特
链接:https://juejin.cn/post/6844903969253244936
来源:掘金
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