expose metric to report reasons why full GCs were triggered (JuliaLan…

…g#55826)

Additional GC observability tool.

This will help us to diagnose why some of our servers are triggering so
many full GCs in certain circumstances.

base/timing.jl

@@ -107,6 +107,39 @@ function gc_page_utilization_data()
     return Base.unsafe_wrap(Array, page_utilization_raw, JL_GC_N_MAX_POOLS, own=false)
 end
 
+# must be kept in sync with `src/gc.h``
+const FULL_SWEEP_REASONS = [:FULL_SWEEP_REASON_SWEEP_ALWAYS_FULL, :FULL_SWEEP_REASON_FORCED_FULL_SWEEP,
+                            :FULL_SWEEP_REASON_ALLOCATION_INTERVAL_ABOVE_MAXMEM, :FULL_SWEEP_REASON_LIVE_BYTES_ABOVE_MAX_TOTAL_MEMORY,
+                            :FULL_SWEEP_REASON_LARGE_INTERGEN_FRONTIER]
+
+"""
+    Base.full_sweep_reasons()
+
+Return a dictionary of the number of times each full sweep reason has occurred.
+
+The reasons are:
+- `:FULL_SWEEP_REASON_SWEEP_ALWAYS_FULL`: Full sweep was caused due to `always_full` being set in the GC debug environment
+- `:FULL_SWEEP_REASON_FORCED_FULL_SWEEP`: Full sweep was forced by `GC.gc(true)`
+- `:FULL_SWEEP_REASON_ALLOCATION_INTERVAL_ABOVE_MAXMEM`: Full sweep was forced by the allocation interval being above the total
+    memory in the machine (as returned by LibUV) divided by the number of mutator threads
+- `:FULL_SWEEP_REASON_LIVE_BYTES_ABOVE_MAX_TOTAL_MEMORY`: Full sweep was caused due to live bytes being above the
+    soft heap limit size (which is either automatically computed at initialization based on the total memory provided by LibUV,
+    or set by the user via `--heap-size-hint`)
+- `:FULL_SWEEP_REASON_LARGE_INTERGEN_FRONTIER`: Full sweep was forced by the intergenerational frontier being too large
+    (i.e. too many pointers in the remembered set)
+
+Note that the set of reasons is not guaranteed to be stable across minor versions of Julia.
+"""
+function full_sweep_reasons()
+    reason = cglobal(:jl_full_sweep_reasons, UInt64)
+    reasons_as_array = Base.unsafe_wrap(Vector{UInt64}, reason, length(FULL_SWEEP_REASONS), own=false)
+    d = Dict{Symbol, Int64}()
+    for (i, r) in enumerate(FULL_SWEEP_REASONS)
+        d[r] = reasons_as_array[i]
+    end
+    return d
+end
+
 """
     Base.jit_total_bytes()
 

src/gc.c

@@ -37,6 +37,8 @@ uv_sem_t gc_sweep_assists_needed;
 uv_mutex_t gc_queue_observer_lock;
 // Tag for sentinel nodes in bigval list
 uintptr_t gc_bigval_sentinel_tag;
+// Table recording number of full GCs due to each reason
+JL_DLLEXPORT uint64_t jl_full_sweep_reasons[FULL_SWEEP_NUM_REASONS];
 
 // Linked list of callback functions
 
@@ -3551,6 +3553,7 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
         if (large_frontier) {
             sweep_full = 1;
             gc_num.interval = last_long_collect_interval;
+            gc_count_full_sweep_reason(FULL_SWEEP_REASON_LARGE_INTERGEN_FRONTIER);
         }
         if (not_freed_enough || large_frontier) {
             gc_num.interval = gc_num.interval * 2;
@@ -3566,6 +3569,7 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
         if (gc_num.interval > maxmem) {
             sweep_full = 1;
             gc_num.interval = maxmem;
+            gc_count_full_sweep_reason(FULL_SWEEP_REASON_ALLOCATION_INTERVAL_ABOVE_MAXMEM);
         }
     }
 
@@ -3574,13 +3578,16 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
     if (live_bytes > max_total_memory) {
         under_memory_pressure = 1;
         sweep_full = 1;
+        gc_count_full_sweep_reason(FULL_SWEEP_REASON_LIVE_BYTES_ABOVE_MAX_TOTAL_MEMORY);
     }
     if (gc_sweep_always_full) {
         sweep_full = 1;
+        gc_count_full_sweep_reason(FULL_SWEEP_REASON_SWEEP_ALWAYS_FULL);
     }
     if (collection == JL_GC_FULL && !prev_sweep_full) {
         sweep_full = 1;
         recollect = 1;
+        gc_count_full_sweep_reason(FULL_SWEEP_REASON_FORCED_FULL_SWEEP);
     }
     if (sweep_full) {
         // these are the difference between the number of gc-perm bytes scanned

src/gc.h

@@ -560,6 +560,22 @@ FORCE_INLINE void gc_big_object_link(bigval_t *sentinel_node, bigval_t *node) JL
     sentinel_node->next = node;
 }
 
+// Must be kept in sync with `base/timing.jl`
+#define FULL_SWEEP_REASON_SWEEP_ALWAYS_FULL (0)
+#define FULL_SWEEP_REASON_FORCED_FULL_SWEEP (1)
+#define FULL_SWEEP_REASON_ALLOCATION_INTERVAL_ABOVE_MAXMEM (2)
+#define FULL_SWEEP_REASON_LIVE_BYTES_ABOVE_MAX_TOTAL_MEMORY (3)
+#define FULL_SWEEP_REASON_LARGE_INTERGEN_FRONTIER (4)
+#define FULL_SWEEP_NUM_REASONS (5)
+
+extern JL_DLLEXPORT uint64_t jl_full_sweep_reasons[FULL_SWEEP_NUM_REASONS];
+STATIC_INLINE void gc_count_full_sweep_reason(int reason) JL_NOTSAFEPOINT
+{
+    assert(reason >= 0 && reason < FULL_SWEEP_NUM_REASONS);
+    jl_full_sweep_reasons[reason]++;
+}
+
+extern uv_mutex_t gc_perm_lock;
 extern uv_mutex_t gc_threads_lock;
 extern uv_cond_t gc_threads_cond;
 extern uv_sem_t gc_sweep_assists_needed;

src/threading.c

@@ -1035,7 +1035,7 @@ JL_DLLEXPORT int jl_heartbeat_resume(void)
     if (uv_sem_trywait(&heartbeat_off_sem) != 0) {
         return -1;
     }
-    
+
     // reset state as we've been paused
     n_hbs_missed = 0;
     n_hbs_recvd = 0;

test/gc.jl

@@ -28,6 +28,13 @@ function run_pg_size_test()
     @test page_size == (1 << 12) || page_size == (1 << 14)
 end
 
+function full_sweep_reasons_test()
+    GC.gc()
+    reasons = Base.full_sweep_reasons()
+    @test reasons[:FULL_SWEEP_REASON_FORCED_FULL_SWEEP] >= 1
+    @test keys(reasons) == Set(Base.FULL_SWEEP_REASONS)
+end
+
 # !!! note:
 #     Since we run our tests on 32bit OS as well we confine ourselves
 #     to parameters that allocate about 512MB of objects. Max RSS is lower
@@ -43,3 +50,7 @@ end
     run_nonzero_page_utilization_test()
     run_pg_size_test()
 end
+
+@testset "Full GC reasons" begin
+    full_sweep_reasons_test()
+end