Make cycle detector block checking incremental

In PR ponylang#2709, @sylvanc pointed out a concern regarding the cycle
detector creating a message storm due to how it would ask all
actors whether they are blocked or not on every interval.

This commit changes the logic so that the cycle detector only
asks set number of actors whether they are blocked or not. If
it is not able to ask all actors, it will resume iterating actors
on the next iteration.

This commit also addresses @SeanTAllen's concern regarding the
conditional check order using `ponyint_actor_getnoblock`.

src/libponyrt/gc/cycle.c

@@ -10,6 +10,8 @@
 #include <string.h>
 #include <inttypes.h>
 
+#define CD_MAX_CHECK_BLOCKED 1000
+
 typedef struct block_msg_t
 {
   pony_msg_t msg;
@@ -137,6 +139,7 @@ typedef struct detector_t
 
   size_t next_token;
   size_t detect_interval;
+  size_t last_checked;
 
   viewmap_t views;
   viewmap_t deferred;
@@ -542,7 +545,7 @@ static void collect(pony_ctx_t* ctx, detector_t* d, perceived_t* per)
 
 static void check_blocked(pony_ctx_t* ctx, detector_t* d)
 {
-  size_t i = HASHMAP_BEGIN;
+  size_t i = d->last_checked;
   view_t* view;
 
   while((view = ponyint_viewmap_next(&d->views, &i)) != NULL)
@@ -553,8 +556,20 @@ static void check_blocked(pony_ctx_t* ctx, detector_t* d)
     {
       pony_send(ctx, view->actor, ACTORMSG_ISBLOCKED);
     }
+
+    // if we've hit the max limit for # of actors to check
+    if(i > CD_MAX_CHECK_BLOCKED)
+      break;
   }
 
+  // if we've reached the end of the map, reset and start from
+  // the beginning next time else continue from where we left
+  // off
+  if(view == NULL)
+    d->last_checked = HASHMAP_BEGIN;
+  else
+    d->last_checked = i;
+
   // process all deferred view stuff
   deferred(ctx, d);
 }
@@ -911,6 +926,9 @@ void ponyint_cycle_create(pony_ctx_t* ctx, uint32_t detect_interval)
   // 1 second = 2000000000 cycles (approx.)
   // based on same scale as ponyint_cpu_core_pause() uses
   d->detect_interval = detect_interval * 2000000;
+
+  // initialize last_checked
+  d->last_checked = HASHMAP_BEGIN;
 }
 
 bool ponyint_cycle_check_blocked(pony_ctx_t* ctx, uint64_t tsc, uint64_t tsc2)

src/libponyrt/sched/scheduler.c

@@ -744,7 +744,7 @@ static pony_actor_t* steal(scheduler_t* sched)
     }
 
     // if we're scheduler 0 and cycle detection is enabled
-    if((sched->index == 0) && !ponyint_actor_getnoblock())
+    if(!ponyint_actor_getnoblock() && (sched->index == 0))
     {
       // trigger cycle detector by sending it a message if it is time
       uint64_t current_tsc = ponyint_cpu_tick();
@@ -786,7 +786,7 @@ static void run(scheduler_t* sched)
   while(true)
   {
     // if we're scheduler 0 and cycle detection is enabled
-    if((sched->index == 0) && !ponyint_actor_getnoblock())
+    if(!ponyint_actor_getnoblock() && (sched->index == 0))
     {
       // trigger cycle detector by sending it a message if it is time
       uint64_t current_tsc = ponyint_cpu_tick();