Add fallback if we have make a weird GC decision. (#50682)

If something odd happens during GC (the PC goes to sleep) or a very big
transient the heuristics might make a bad decision. What this PR
implements is if we try to make our target more than double the one we
had before we fallback to a more conservative method. This fixes the new
issue @vtjnash found in #40644
for me.

Make.inc

@@ -1461,6 +1461,12 @@ endef
 # Overridable in Make.user
 WINE ?= wine
 
+ifeq ($(BINARY),32)
+HEAPLIM := --heap-size-hint=1000M
+else
+HEAPLIM :=
+endif
+
 # many of the following targets must be = not := because the expansion of the makefile functions (and $1) shouldn't happen until later
 ifeq ($(BUILD_OS), WINNT) # MSYS
 spawn = $(1)

src/gc-debug.c

@@ -1410,17 +1410,17 @@ void _report_gc_finished(uint64_t pause, uint64_t freed, int full, int recollect
  if (!gc_logging_enabled) {
  return;
  }
- jl_safe_printf("GC: pause %.2fms. collected %fMB. %s %s\n",
+ jl_safe_printf("\nGC: pause %.2fms. collected %fMB. %s %s\n",
  pause/1e6, freed/(double)(1<<20),
  full ? "full" : "incr",
  recollect ? "recollect" : ""
  );
 
- jl_safe_printf("Heap stats: bytes_mapped %.2f MB, heap_size %.2f MB, heap_target %.2f MB, live_bytes %.2f MB\n, Fragmentation %.3f",
+ jl_safe_printf("Heap stats: bytes_mapped %.2f MB,\nheap_size %.2f MB, heap_target %.2f MB, Fragmentation %.3f\n",
  jl_atomic_load_relaxed(&gc_heap_stats.bytes_mapped)/(double)(1<<20),
+ // live_bytes/(double)(1<<20), live byes tracking is not accurate.
  jl_atomic_load_relaxed(&gc_heap_stats.heap_size)/(double)(1<<20),
  jl_atomic_load_relaxed(&gc_heap_stats.heap_target)/(double)(1<<20),
- live_bytes/(double)(1<<20),
  (double)live_bytes/(double)jl_atomic_load_relaxed(&gc_heap_stats.heap_size)
  );
 }

src/gc.c

@@ -1,6 +1,7 @@
 // This file is a part of Julia. License is MIT: https://julialang.org/license
 
 #include "gc.h"
+#include "julia.h"
 #include "julia_gcext.h"
 #include "julia_assert.h"
 #ifdef __GLIBC__
@@ -804,8 +805,8 @@ static uint64_t old_heap_size = 0;
 static uint64_t old_alloc_diff = 0;
 static uint64_t old_freed_diff = 0;
 static uint64_t gc_end_time = 0;
-
-
+static int thrash_counter = 0;
+static int thrashing = 0;
 // global variables for GC stats
 
 // Resetting the object to a young object, this is used when marking the
@@ -1337,7 +1338,10 @@ static void combine_thread_gc_counts(jl_gc_num_t *dest) JL_NOTSAFEPOINT
  dest->bigalloc += jl_atomic_load_relaxed(&ptls->gc_num.bigalloc);
  uint64_t alloc_acc = jl_atomic_load_relaxed(&ptls->gc_num.alloc_acc);
  uint64_t free_acc = jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
+ dest->freed += jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
  jl_atomic_store_relaxed(&gc_heap_stats.heap_size, alloc_acc - free_acc + jl_atomic_load_relaxed(&gc_heap_stats.heap_size));
+ jl_atomic_store_relaxed(&ptls->gc_num.alloc_acc, 0);
+ jl_atomic_store_relaxed(&ptls->gc_num.free_acc, 0);
  }
  }
 }
@@ -3533,9 +3537,6 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
  // 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;
  }
@@ -3569,8 +3570,7 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
  uint64_t sweep_time = gc_end_time - start_sweep_time;
  gc_num.total_sweep_time += sweep_time;
  gc_num.sweep_time = sweep_time;
-
- int thrashing = 0; // maybe we should report this to the user or error out?
+
  size_t heap_size = jl_atomic_load_relaxed(&gc_heap_stats.heap_size);
  double target_allocs = 0.0;
  double min_interval = default_collect_interval;
@@ -3581,24 +3581,32 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
  double collect_smooth_factor = 0.5;
  double tuning_factor = 0.03;
  double alloc_mem = jl_gc_smooth(old_alloc_diff, alloc_diff, alloc_smooth_factor);
- double alloc_time = jl_gc_smooth(old_mut_time, mutator_time, alloc_smooth_factor);
+ double alloc_time = jl_gc_smooth(old_mut_time, mutator_time + sweep_time, alloc_smooth_factor); // Charge sweeping to the mutator
  double gc_mem = jl_gc_smooth(old_freed_diff, freed_diff, collect_smooth_factor);
- double gc_time = jl_gc_smooth(old_pause_time, pause, collect_smooth_factor);
+ double gc_time = jl_gc_smooth(old_pause_time, pause - sweep_time, collect_smooth_factor);
  old_alloc_diff = alloc_diff;
  old_mut_time = mutator_time;
  old_freed_diff = freed_diff;
  old_pause_time = pause;
- old_heap_size = heap_size;
- thrashing = gc_time > mutator_time * 98 ? 1 : 0;
+ old_heap_size = heap_size; // TODO: Update these values dynamically instead of just during the GC
+ if (gc_time > alloc_time * 95 && !(thrash_counter < 4))
+ thrash_counter += 1;
+ else if (thrash_counter > 0)
+ thrash_counter -= 1;
  if (alloc_mem != 0 && alloc_time != 0 && gc_mem != 0 && gc_time != 0 ) {
  double alloc_rate = alloc_mem/alloc_time;
  double gc_rate = gc_mem/gc_time;
  target_allocs = sqrt(((double)heap_size/min_interval * alloc_rate)/(gc_rate * tuning_factor)); // work on multiples of min interval
  }
  }
- if (target_allocs == 0.0 || thrashing) // If we are thrashing go back to default
- target_allocs = 2*sqrt((double)heap_size/min_interval);
+ if (thrashing == 0 && thrash_counter >= 3)
+ thrashing = 1;
+ else if (thrashing == 1 && thrash_counter <= 2)
+ thrashing = 0; // maybe we should report this to the user or error out?
 
+ int bad_result = (target_allocs*min_interval + heap_size) > 2 * jl_atomic_load_relaxed(&gc_heap_stats.heap_target); // Don't follow through on a bad decision
+ if (target_allocs == 0.0 || thrashing || bad_result) // If we are thrashing go back to default
+ target_allocs = 2*sqrt((double)heap_size/min_interval);
  uint64_t target_heap = (uint64_t)target_allocs*min_interval + heap_size;
  if (target_heap > max_total_memory && !thrashing) // Allow it to go over if we are thrashing if we die we die
  target_heap = max_total_memory;
@@ -3852,10 +3860,10 @@ void jl_gc_init(void)
  total_mem = uv_get_total_memory();
  uint64_t constrained_mem = uv_get_constrained_memory();
  if (constrained_mem > 0 && constrained_mem < total_mem)
- total_mem = constrained_mem;
+ jl_gc_set_max_memory(constrained_mem - 250*1024*1024); // LLVM + other libraries need some amount of memory
 #endif
  if (jl_options.heap_size_hint)
- jl_gc_set_max_memory(jl_options.heap_size_hint);
+ jl_gc_set_max_memory(jl_options.heap_size_hint - 250*1024*1024);
 
  jl_gc_mark_sp_t sp = {NULL, NULL, NULL, NULL};
  gc_mark_loop(NULL, sp);
@@ -3959,7 +3967,26 @@ JL_DLLEXPORT void *jl_gc_counted_realloc_with_old_size(void *p, size_t old, size
  jl_atomic_load_relaxed(&ptls->gc_num.allocd) + (sz - old));
  jl_atomic_store_relaxed(&ptls->gc_num.realloc,
  jl_atomic_load_relaxed(&ptls->gc_num.realloc) + 1);
- jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, sz-old);
+
+ int64_t diff = sz - old;
+ if (diff < 0) {
+ uint64_t free_acc = jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
+ if (free_acc + diff < 16*1024)
+ jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + (-diff));
+ else {
+ jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + (-diff)));
+ jl_atomic_store_relaxed(&ptls->gc_num.free_acc, 0);
+ }
+ }
+ else {
+ uint64_t alloc_acc = jl_atomic_load_relaxed(&ptls->gc_num.alloc_acc);
+ if (alloc_acc + diff < 16*1024)
+ jl_atomic_store_relaxed(&ptls->gc_num.alloc_acc, alloc_acc + diff);
+ else {
+ jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, alloc_acc + diff);
+ jl_atomic_store_relaxed(&ptls->gc_num.alloc_acc, 0);
+ }
+ }
  }
  return realloc(p, sz);
 }
@@ -4076,7 +4103,27 @@ static void *gc_managed_realloc_(jl_ptls_t ptls, void *d, size_t sz, size_t olds
  jl_atomic_load_relaxed(&ptls->gc_num.allocd) + (allocsz - oldsz));
  jl_atomic_store_relaxed(&ptls->gc_num.realloc,
  jl_atomic_load_relaxed(&ptls->gc_num.realloc) + 1);
- jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, allocsz-oldsz);
+
+ int64_t diff = allocsz - oldsz;
+ if (diff < 0) {
+ uint64_t free_acc = jl_atomic_load_relaxed(&ptls->gc_num.free_acc);
+ if (free_acc + diff < 16*1024)
+ jl_atomic_store_relaxed(&ptls->gc_num.free_acc, free_acc + (-diff));
+ else {
+ jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, -(free_acc + (-diff)));
+ jl_atomic_store_relaxed(&ptls->gc_num.free_acc, 0);
+ }
+ }
+ else {
+ uint64_t alloc_acc = jl_atomic_load_relaxed(&ptls->gc_num.alloc_acc);
+ if (alloc_acc + diff < 16*1024)
+ jl_atomic_store_relaxed(&ptls->gc_num.alloc_acc, alloc_acc + diff);
+ else {
+ jl_atomic_fetch_add_relaxed(&gc_heap_stats.heap_size, alloc_acc + diff);
+ jl_atomic_store_relaxed(&ptls->gc_num.alloc_acc, 0);
+ }
+ }
+
  int last_errno = errno;
 #ifdef _OS_WINDOWS_
  DWORD last_error = GetLastError();

test/cmdlineargs.jl

@@ -858,6 +858,6 @@ end
  @test lines[3] == "foo"
  @test lines[4] == "bar"
  end
-#heap-size-hint
-@test readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=500M -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`) == "524288000"
+#heap-size-hint, we reserve 250 MB for non GC memory (llvm, etc.)
+@test readchomp(`$(Base.julia_cmd()) --startup-file=no --heap-size-hint=500M -e "println(@ccall jl_gc_get_max_memory()::UInt64)"`) == "$((500-250)*1024*1024)"
 end

test/testenv.jl

@@ -37,6 +37,13 @@ if !@isdefined(testenv_defined)
 
  function addprocs_with_testenv(X; rr_allowed=true, kwargs...)
  exename = rr_allowed ? `$rr_exename $test_exename` : test_exename
+<<<<<<< HEAD
+=======
+ if X isa Integer
+ heap_size=round(Int,(Sys.free_memory()/(1024^2)/(X+1)))
+ push!(test_exeflags.exec, "--heap-size-hint=$(heap_size)M")
+ end
+>>>>>>> ab94fadc9f (Add fallback if we have make a weird GC decision. (#50682))
  addprocs(X; exename=exename, exeflags=test_exeflags, kwargs...)
  end