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golang核心原理-协程栈

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

  1. 将状态从 _Grunning 更新至 _Gcopystack
  2. 计算出需要申请的数据大小
  3. copystack,进行栈复制,后面会详细分析
  4. 将协程状态恢复至_Grunning
  5. 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)
}
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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

  1. 检查协程状态,如果已经结束,则释放空间
  2. 确定新空间size,目前为原来1/2
  3. 检查栈使用是否超过1/4,若没有,则放弃
  4. 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
}
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copystack栈拷贝过程

原来内容上的拷贝

关键步骤

  1. 申请新的栈空间:new := stackalloc(uint32(newsize));
  2. 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);
  3. gentraceback,调整栈帧到新位置;
  4. memmove老栈数据到新栈;
  5. 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)
}
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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)
}
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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
}
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