functionality to expose page utilization at the julia level (#52390)

---------

Co-authored-by: Valentin Churavy <vchuravy@users.noreply.github.com>

Author: d-netto

base/timing.jl

@@ -98,6 +98,13 @@ function gc_live_bytes()
     Int(ccall(:jl_gc_live_bytes, Int64, ())) + num.allocd + num.deferred_alloc
 end
 
+# must be kept in sync with the value from `src/julia_threads.h``
+const JL_GC_N_MAX_POOLS = 51
+function gc_page_utilization_data()
+    page_utilization_raw = cglobal(:gc_page_utilization_stats, Float64)
+    return Base.unsafe_wrap(Array, page_utilization_raw, JL_GC_N_MAX_POOLS, own=false)
+end
+
 """
     Base.jit_total_bytes()
 

src/gc.c

@@ -1421,19 +1421,17 @@ int jl_gc_classify_pools(size_t sz, int *osize)
 // sweep phase
 
 gc_fragmentation_stat_t gc_page_fragmentation_stats[JL_GC_N_POOLS];
+JL_DLLEXPORT double gc_page_utilization_stats[JL_GC_N_MAX_POOLS];
 
 STATIC_INLINE void gc_update_page_fragmentation_data(jl_gc_pagemeta_t *pg) JL_NOTSAFEPOINT
 {
-#ifdef GC_MEASURE_PAGE_FRAGMENTATION
     gc_fragmentation_stat_t *stats = &gc_page_fragmentation_stats[pg->pool_n];
     jl_atomic_fetch_add(&stats->n_freed_objs, pg->nfree);
     jl_atomic_fetch_add(&stats->n_pages_allocd, 1);
-#endif
 }
 
 STATIC_INLINE void gc_dump_page_utilization_data(void) JL_NOTSAFEPOINT
 {
-#ifdef GC_MEASURE_PAGE_FRAGMENTATION
     for (int i = 0; i < JL_GC_N_POOLS; i++) {
         gc_fragmentation_stat_t *stats = &gc_page_fragmentation_stats[i];
         double utilization = 1.0;
@@ -1442,12 +1440,10 @@ STATIC_INLINE void gc_dump_page_utilization_data(void) JL_NOTSAFEPOINT
         if (n_pages_allocd != 0) {
             utilization -= ((double)n_freed_objs * (double)jl_gc_sizeclasses[i]) / (double)n_pages_allocd / (double)GC_PAGE_SZ;
         }
-        jl_safe_printf("Size class %d: %.2f%% utilization\n", jl_gc_sizeclasses[i], utilization * 100.0);
+        gc_page_utilization_stats[i] = utilization;
         jl_atomic_store_relaxed(&stats->n_freed_objs, 0);
         jl_atomic_store_relaxed(&stats->n_pages_allocd, 0);
     }
-    jl_safe_printf("-----------------------------------------\n");
-#endif
 }
 
 int64_t buffered_pages = 0;

src/gc.h

@@ -238,9 +238,6 @@ STATIC_INLINE jl_gc_pagemeta_t *pop_lf_back(jl_gc_page_stack_t *pool) JL_NOTSAFE
     }
 }
 
-// data structures for tracking fragmentation in the pool allocator
-// #define GC_MEASURE_PAGE_FRAGMENTATION
-
 typedef struct {
     _Atomic(size_t) n_freed_objs;
     _Atomic(size_t) n_pages_allocd;