runtime: update big mgc.go comment

The comment describing the overall GC algorithm at the top of mgc.go
has gotten woefully out-of-date (and was possibly never
correct/complete). Update it to reflect the current workings of the
GC and the set of phases that we now divide it into.

Change-Id: I02143c0ebefe9d4cd7753349dab8045f0973bf95
Reviewed-on: https://go-review.googlesource.com/34711Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(rsc): The code having to do with the heap bitmap needs very serious cleanup.
-// It has gotten completely out of control.
-
 // Garbage collector (GC).
 //
 // The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
@@ -24,67 +21,73 @@
 // Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
 // Concurrency and Computation: Practice and Experience 15(3-5), 2003.
 //
-// TODO(austin): The rest of this comment is woefully out of date and
-// needs to be rewritten. There is no distinct scan phase any more and
-// we allocate black during GC.
+// 1. GC performs sweep termination.
+//
+//    a. Stop the world. This causes all Ps to reach a GC safe-point.
+//
+//    b. Sweep any unswept spans. There will only be unswept spans if
+//    this GC cycle was forced before the expected time.
+//
+// 2. GC performs the "mark 1" sub-phase. In this sub-phase, Ps are
+// allowed to locally cache parts of the work queue.
+//
+//    a. Prepare for the mark phase by setting gcphase to _GCmark
+//    (from _GCoff), enabling the write barrier, enabling mutator
+//    assists, and enqueueing root mark jobs. No objects may be
+//    scanned until all Ps have enabled the write barrier, which is
+//    accomplished using STW.
+//
+//    b. Start the world. From this point, GC work is done by mark
+//    workers started by the scheduler and by assists performed as
+//    part of allocation. The write barrier shades both the
+//    overwritten pointer and the new pointer value for any pointer
+//    writes (see mbarrier.go for details). Newly allocated objects
+//    are immediately marked black.
+//
+//    c. GC performs root marking jobs. This includes scanning all
+//    stacks, shading all globals, and shading any heap pointers in
+//    off-heap runtime data structures. Scanning a stack stops a
+//    goroutine, shades any pointers found on its stack, and then
+//    resumes the goroutine.
+//
+//    d. GC drains the work queue of grey objects, scanning each grey
+//    object to black and shading all pointers found in the object
+//    (which in turn may add those pointers to the work queue).
+//
+// 3. Once the global work queue is empty (but local work queue caches
+// may still contain work), GC performs the "mark 2" sub-phase.
+//
+//    a. GC stops all workers, disables local work queue caches,
+//    flushes each P's local work queue cache to the global work queue
+//    cache, and reenables workers.
+//
+//    b. GC again drains the work queue, as in 2d above.
+//
+// 4. Once the work queue is empty, GC performs mark termination.
+//
+//    a. Stop the world.
+//
+//    b. Set gcphase to _GCmarktermination, and disable workers and
+//    assists.
+//
+//    c. Drain any remaining work from the work queue (typically there
+//    will be none).
+//
+//    d. Perform other housekeeping like flushing mcaches.
+//
+// 5. GC performs the sweep phase.
+//
+//    a. Prepare for the sweep phase by setting gcphase to _GCoff,
+//    setting up sweep state and disabling the write barrier.
+//
+//    b. Start the world. From this point on, newly allocated objects
+//    are white, and allocating sweeps spans before use if necessary.
+//
+//    c. GC does concurrent sweeping in the background and in response
+//    to allocation. See description below.
 //
-//  0. Set phase = GCscan from GCoff.
-//  1. Wait for all P's to acknowledge phase change.
-//         At this point all goroutines have passed through a GC safepoint and
-//         know we are in the GCscan phase.
-//  2. GC scans all goroutine stacks, mark and enqueues all encountered pointers
-//       (marking avoids most duplicate enqueuing but races may produce benign duplication).
-//       Preempted goroutines are scanned before P schedules next goroutine.
-//  3. Set phase = GCmark.
-//  4. Wait for all P's to acknowledge phase change.
-//  5. Now write barrier marks and enqueues black, grey, or white to white pointers.
-//       Malloc still allocates white (non-marked) objects.
-//  6. Meanwhile GC transitively walks the heap marking reachable objects.
-//  7. When GC finishes marking heap, it preempts P's one-by-one and
-//       retakes partial wbufs (filled by write barrier or during a stack scan of the goroutine
-//       currently scheduled on the P).
-//  8. Once the GC has exhausted all available marking work it sets phase = marktermination.
-//  9. Wait for all P's to acknowledge phase change.
-// 10. Malloc now allocates black objects, so number of unmarked reachable objects
-//        monotonically decreases.
-// 11. GC preempts P's one-by-one taking partial wbufs and marks all unmarked yet
-//        reachable objects.
-// 12. When GC completes a full cycle over P's and discovers no new grey
-//         objects, (which means all reachable objects are marked) set phase = GCoff.
-// 13. Wait for all P's to acknowledge phase change.
-// 14. Now malloc allocates white (but sweeps spans before use).
-//         Write barrier becomes nop.
-// 15. GC does background sweeping, see description below.
-// 16. When sufficient allocation has taken place replay the sequence starting at 0 above,
-//         see discussion of GC rate below.
-
-// Changing phases.
-// Phases are changed by setting the gcphase to the next phase and possibly calling ackgcphase.
-// All phase action must be benign in the presence of a change.
-// Starting with GCoff
-// GCoff to GCscan
-//     GSscan scans stacks and globals greying them and never marks an object black.
-//     Once all the P's are aware of the new phase they will scan gs on preemption.
-//     This means that the scanning of preempted gs can't start until all the Ps
-//     have acknowledged.
-//     When a stack is scanned, this phase also installs stack barriers to
-//     track how much of the stack has been active.
-//     This transition enables write barriers because stack barriers
-//     assume that writes to higher frames will be tracked by write
-//     barriers. Technically this only needs write barriers for writes
-//     to stack slots, but we enable write barriers in general.
-// GCscan to GCmark
-//     In GCmark, work buffers are drained until there are no more
-//     pointers to scan.
-//     No scanning of objects (making them black) can happen until all
-//     Ps have enabled the write barrier, but that already happened in
-//     the transition to GCscan.
-// GCmark to GCmarktermination
-//     The only change here is that we start allocating black so the Ps must acknowledge
-//     the change before we begin the termination algorithm
-// GCmarktermination to GSsweep
-//     Object currently on the freelist must be marked black for this to work.
-//     Are things on the free lists black or white? How does the sweep phase work?
+// 6. When sufficient allocation has taken place, replay the sequence
+// starting with 1 above. See discussion of GC rate below.
 
 // Concurrent sweep.
 //