[Git][ghc/ghc][wip/gc/root-evac-stats] rts: Track root evacuation statistics

[Ben Gamari]

Ben Gamari pushed to branch wip/gc/root-evac-stats at Glasgow Haskell Compiler / GHC


Commits:
b3917b31 by Ben Gamari at 2020-12-17T13:18:49-05:00
rts: Track root evacuation statistics

- - - - -


3 changed files:

- rts/sm/GC.c
- rts/sm/GCThread.h
- rts/sm/MarkWeak.c


Changes:

=====================================
rts/sm/GC.c
=====================================
@@ -217,6 +217,35 @@ addMutListScavStats(const MutListScavStats *src,
 }
 #endif /* DEBUG */
 
+/* -----------------------------------------------------------------------------
+   Statistics from root evacuation
+   -------------------------------------------------------------------------- */
+
+/*
+ * Note [Root evacuation statistics]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * When diagnosing concurrent GC pauses it can often be useful
+ * to know how many GC roots we get from various sources.
+ * Consequently, track this information in each GC thread and
+ * emit eventlog events if ROOT_EVAC_STATS is #define'd.
+ *
+ * To keep track of root evacuations mark_root() bumps a
+ * counter in gc_thread, which gets zero'd after we finish
+ * evacuating each class of roots. We also bump the counter in
+ * markWeakPtrList(), since this codepath doesn't use mark_root().
+ */
+
+#if defined(DEBUG) || defined(THREADED_RTS)
+#define ROOT_EVAC_STATS
+#endif
+
+#if defined(ROOT_EVAC_STATS)
+#define TRACE_EVACD_ROOTS(label) \
+    trace(nonmoving_gc, "root:" label ":%d", gct->n_roots_evacd); \
+    gct->n_roots_evacd = 0;
+#else
+#define TRACE_EVACD_ROOTS(label)
+#endif
 
 /* -----------------------------------------------------------------------------
    GarbageCollect: the main entry point to the garbage collector.
@@ -441,10 +470,12 @@ GarbageCollect (uint32_t collect_gen,
           }
       }
   }
+  TRACE_EVACD_ROOTS("MutList");
 
   // follow roots from the CAF list (used by GHCi)
   gct->evac_gen_no = 0;
   markCAFs(mark_root, gct);
+  TRACE_EVACD_ROOTS("CAF");
 
   // follow all the roots that the application knows about.
   gct->evac_gen_no = 0;
@@ -456,15 +487,20 @@ GarbageCollect (uint32_t collect_gen,
   } else {
       markCapability(mark_root, gct, cap, true/*don't mark sparks*/);
   }
+  TRACE_EVACD_ROOTS("Cap");
 
+  // Mark runnable threads
   markScheduler(mark_root, gct);
+  TRACE_EVACD_ROOTS("Sched");
 
   // Mark the weak pointer list, and prepare to detect dead weak pointers.
   markWeakPtrList();
   initWeakForGC();
+  TRACE_EVACD_ROOTS("WeakPtr");
 
   // Mark the stable pointer table.
   markStablePtrTable(mark_root, gct);
+  TRACE_EVACD_ROOTS("StablePtr");
 
   // Remember old stable name addresses.
   rememberOldStableNameAddresses ();
@@ -1296,7 +1332,11 @@ gcWorkerThread (Capability *cap)
     // Every thread evacuates some roots.
     gct->evac_gen_no = 0;
     markCapability(mark_root, gct, cap, true/*prune sparks*/);
+    TRACE_EVACD_ROOTS("Cap");
+
+    // Scavenge mutable lists
     scavenge_capability_mut_lists(cap);
+    TRACE_EVACD_ROOTS("MutList");
 
     scavenge_until_all_done();
 
@@ -1753,6 +1793,7 @@ init_gc_thread (gc_thread *t)
     t->any_work = 0;
     t->no_work = 0;
     t->scav_find_work = 0;
+    t->n_roots_evacd = 0;
 }
 
 /* -----------------------------------------------------------------------------
@@ -1774,6 +1815,9 @@ mark_root(void *user USED_IF_THREADS, StgClosure **root)
     SET_GCT(user);
 
     evacuate(root);
+#if defined(ROOT_EVAC_STATS)
+    gct->n_evacd_roots++;
+#endif
 
     SET_GCT(saved_gct);
 }


=====================================
rts/sm/GCThread.h
=====================================
@@ -184,6 +184,7 @@ typedef struct gc_thread_ {
     W_ any_work;
     W_ no_work;
     W_ scav_find_work;
+    W_ n_roots_evacd;              // See Note [Root evacuation statistics] in GC.c.
 
     Time gc_start_cpu;             // thread CPU time
     Time gc_end_cpu;               // thread CPU time


=====================================
rts/sm/MarkWeak.c
=====================================
@@ -427,6 +427,7 @@ markWeakPtrList ( void )
             }
 #endif
 
+            gct->n_roots_evacd++;  // See Note [Root evacuation statistics]
             evacuate((StgClosure **)last_w);
             w = *last_w;
             last_w = &(w->link);



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b3917b313325d3cb9b4cc6788ce41356c9659868

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b3917b313325d3cb9b4cc6788ce41356c9659868
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