Updated 32 bit heuristics (#44805)

Co-authored-by: Christine H. Flood <[email protected]>
Co-authored-by: Jameson Nash <[email protected]>
Co-authored-by: Dilum Aluthge <[email protected]>

src/gc.c

@@ -585,12 +585,20 @@ static void gc_sweep_foreign_objs(void)
 // GC knobs and self-measurement variables
 static int64_t last_gc_total_bytes = 0;
 
+// max_total_memory is a suggestion.  We try very hard to stay
+// under this limit, but we will go above it rather than halting.
 #ifdef _P64
 #define default_collect_interval (5600*1024*sizeof(void*))
 static size_t max_collect_interval = 1250000000UL;
+// Eventually we can expose this to the user/ci.
+static uint64_t max_total_memory = (uint64_t) 2 * 1024 * 1024 * 1024 * 1024 * 1024;
 #else
 #define default_collect_interval (3200*1024*sizeof(void*))
 static size_t max_collect_interval =  500000000UL;
+// Work really hard to stay within 2GB
+// Alternative is to risk running out of address space
+// on 32 bit architectures.
+static uint32_t max_total_memory = (uint32_t) 2 * 1024 * 1024 * 1024;
 #endif
 
 // global variables for GC stats
@@ -3158,6 +3166,13 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
             gc_num.interval = max_collect_interval;
         }
     }
+
+    // If the live data outgrows the suggested max_total_memory
+    // we keep going with minimum intervals and full gcs until
+    // we either free some space or get an OOM error.
+    if (live_bytes > max_total_memory) {
+        sweep_full = 1;
+    }
     if (gc_sweep_always_full) {
         sweep_full = 1;
     }
@@ -3237,6 +3252,17 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
       if (gc_num.interval < default_collect_interval) gc_num.interval = default_collect_interval;
     }
 
+    // We need this for 32 bit but will be useful to set limits on 64 bit
+    if (gc_num.interval + live_bytes > max_total_memory) {
+        if (live_bytes < max_total_memory) {
+            gc_num.interval = max_total_memory - live_bytes;
+        } else {
+            // We can't stay under our goal so let's go back to
+            // the minimum interval and hope things get better
+            gc_num.interval = default_collect_interval;
+       }
+    }
+
     gc_time_summary(sweep_full, t_start, gc_end_t, gc_num.freed, live_bytes, gc_num.interval, pause);
 
     prev_sweep_full = sweep_full;