其他

  • gcWork

每个 P 上都有一个 gcw 用来管理灰色对象（get 和 put），gcw 的结构就是 gcWork。gcWork 中的核心是 wbuf1 和 wbuf2，里面存储就是灰色对象，或者说是 work（下面就全部统一叫做 work）。

  1. type p struct {
  2.     ...
  3.     gcw gcWork
  4. }
  6. type gcWork struct {
  7.     // wbuf1 and wbuf2 are the primary and secondary work buffers.
  8.     wbuf1, wbuf2 wbufptr
  10.     // Bytes marked (blackened) on this gcWork. This is aggregated
  11.     // into work.bytesMarked by dispose.
  12.     bytesMarked uint64
  14.     // Scan work performed on this gcWork. This is aggregated into
  15.     // gcController by dispose and may also be flushed by callers.
  16.     scanWork int64
  17. }

既然每个 P 上有一个 work buffer，那么是不是还有一个全局的 work list 呢？是的。通过在每个 P 上绑定一个 work buffer 的好处和 cache 一样，不需要加锁。

  1. var work struct {
  2.     full  uint64                   // lock-free list of full blocks workbuf
  3.     empty uint64                   // lock-free list of empty blocks workbuf
  4.     pad0  [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait
  5.     ...
  6. }

那么为什么使用两个 work buffer （wbuf1 和 wbuf2）呢？例如我现在要 get 一个 work 出来，先从 wbuf1 中取，wbuf1 为空的话则与 wbuf2 swap 再 get。在其他时间将 work buffer 中的 full 或者 empty buffer 移到 global 的 work 中。 这样的好处在于，在 get 的时候去全局的 work 里面取（多个 goroutine 去取会有竞争）。这里有趣的是 global 的 work list 是 lock-free 的，通过原子操作 cas 等实现。下面列举几个函数看一下 gcWrok。

  • 初始化

    1. func (w *gcWork) init() {
    2.   w.wbuf1 = wbufptrOf(getempty())
    3.   wbuf2 := trygetfull()
    4.   if wbuf2 == nil {
    5.       wbuf2 = getempty()
    6.   }
    7.   w.wbuf2 = wbufptrOf(wbuf2)
    8. }

  • put

  1. // put enqueues a pointer for the garbage collector to trace.
  2. // obj must point to the beginning of a heap object or an oblet.
  3. func (w *gcWork) put(obj uintptr) {
  4.     wbuf := w.wbuf1.ptr()
  5.     if wbuf == nil {
  6.         w.init()
  7.         wbuf = w.wbuf1.ptr()
  8.         // wbuf is empty at this point.
  9.     } else if wbuf.nobj == len(wbuf.obj) {
  10.         w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
  11.         wbuf = w.wbuf1.ptr()
  12.         if wbuf.nobj == len(wbuf.obj) {
  13.             putfull(wbuf)
  14.             wbuf = getempty()
  15.             w.wbuf1 = wbufptrOf(wbuf)
  16.             flushed = true
  17.         }
  18.     }
  20.     wbuf.obj[wbuf.nobj] = obj
  21.     wbuf.nobj++
  22. }
  • get 
  1. // get dequeues a pointer for the garbage collector to trace, blocking
  2. // if necessary to ensure all pointers from all queues and caches have
  3. // been retrieved.  get returns 0 if there are no pointers remaining.
  4. //go:nowritebarrier
  5. func (w *gcWork) get() uintptr {
  6.     wbuf := w.wbuf1.ptr()
  7.     if wbuf == nil {
  8.         w.init()
  9.         wbuf = w.wbuf1.ptr()
  10.         // wbuf is empty at this point.
  11.     }
  12.     if wbuf.nobj == 0 {
  13.         w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
  14.         wbuf = w.wbuf1.ptr()
  15.         if wbuf.nobj == 0 {
  16.             owbuf := wbuf
  17.             wbuf = getfull()
  18.             if wbuf == nil {
  19.                 return 0
  20.             }
  21.             putempty(owbuf)
  22.             w.wbuf1 = wbufptrOf(wbuf)
  23.         }
  24.     }
  26.     // TODO: This might be a good place to add prefetch code
  28.     wbuf.nobj--
  29.     return wbuf.obj[wbuf.nobj]
  30. }