Track first free atbs for multiple block sizes instead of just 1

py/gc.c

@@ -150,9 +150,13 @@ void gc_init(void *start, void *end) {
 #endif
 
     // Set first free ATB index to the start of the heap.
-    MP_STATE_MEM(gc_first_free_atb_index) = 0;
+    for (size_t i = 0; i < MICROPY_ATB_INDICES; i++) {
+        MP_STATE_MEM(gc_first_free_atb_index)[i] = 0;
+    }
+
     // Set last free ATB index to the end of the heap.
     MP_STATE_MEM(gc_last_free_atb_index) = MP_STATE_MEM(gc_alloc_table_byte_len) - 1;
+
     // Set the lowest long lived ptr to the end of the heap to start. This will be lowered as long
     // lived objects are allocated.
     MP_STATE_MEM(gc_lowest_long_lived_ptr) = (void*) PTR_FROM_BLOCK(MP_STATE_MEM(gc_alloc_table_byte_len * BLOCKS_PER_ATB));
@@ -387,7 +391,9 @@ void gc_collect_root(void **ptrs, size_t len) {
 void gc_collect_end(void) {
     gc_deal_with_stack_overflow();
     gc_sweep();
-    MP_STATE_MEM(gc_first_free_atb_index) = 0;
+    for (size_t i = 0; i < MICROPY_ATB_INDICES; i++) {
+        MP_STATE_MEM(gc_first_free_atb_index)[i] = 0;
+    }
     MP_STATE_MEM(gc_last_free_atb_index) = MP_STATE_MEM(gc_alloc_table_byte_len) - 1;
     MP_STATE_MEM(gc_lock_depth)--;
     GC_EXIT();
@@ -513,14 +519,16 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser, bool long_lived) {
     size_t crossover_block = BLOCK_FROM_PTR(MP_STATE_MEM(gc_lowest_long_lived_ptr));
     while (keep_looking) {
         int8_t direction = 1;
-        size_t start = MP_STATE_MEM(gc_first_free_atb_index);
+        size_t bucket = MIN(n_blocks, MICROPY_ATB_INDICES) - 1;
+        size_t first_free = MP_STATE_MEM(gc_first_free_atb_index)[bucket];
+        size_t start = first_free;
         if (long_lived) {
             direction = -1;
             start = MP_STATE_MEM(gc_last_free_atb_index);
         }
         n_free = 0;
         // look for a run of n_blocks available blocks
-        for (size_t i = start; keep_looking && MP_STATE_MEM(gc_first_free_atb_index) <= i && i <= MP_STATE_MEM(gc_last_free_atb_index); i += direction) {
+        for (size_t i = start; keep_looking && first_free <= i && i <= MP_STATE_MEM(gc_last_free_atb_index); i += direction) {
             byte a = MP_STATE_MEM(gc_alloc_table_start)[i];
             // Four ATB states are packed into a single byte.
             int j = 0;
@@ -565,22 +573,24 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser, bool long_lived) {
 
     // Found free space ending at found_block inclusive.
     // Also, set last free ATB index to block after last block we found, for start of
-    // next scan.  To reduce fragmentation, we only do this if we were looking
-    // for a single free block, which guarantees that there are no free blocks
-    // before this one.  Also, whenever we free or shrink a block we must check
-    // if this index needs adjusting (see gc_realloc and gc_free).
+    // next scan. Also, whenever we free or shrink a block we must check if this index needs
+    // adjusting (see gc_realloc and gc_free).
     if (!long_lived) {
         end_block = found_block;
         start_block = found_block - n_free + 1;
-        if (n_blocks == 1) {
-            MP_STATE_MEM(gc_first_free_atb_index) = (found_block + 1) / BLOCKS_PER_ATB;
+        if (n_blocks < MICROPY_ATB_INDICES) {
+            size_t next_free_atb = (found_block + n_blocks) / BLOCKS_PER_ATB;
+            // Update all atb indices for larger blocks too.
+            for (size_t i = n_blocks - 1; i < MICROPY_ATB_INDICES; i++) {
+                MP_STATE_MEM(gc_first_free_atb_index)[i] = next_free_atb;
+            }
         }
     } else {
         start_block = found_block;
         end_block = found_block + n_free - 1;
-        if (n_blocks == 1) {
-            MP_STATE_MEM(gc_last_free_atb_index) = (found_block - 1) / BLOCKS_PER_ATB;
-        }
+        // Always update the bounds of the long lived area because we assume it is contiguous. (It
+        // can still be reset by a sweep.)
+        MP_STATE_MEM(gc_last_free_atb_index) = (found_block - 1) / BLOCKS_PER_ATB;
     }
 
     #ifdef LOG_HEAP_ACTIVITY
@@ -676,30 +686,37 @@ void gc_free(void *ptr) {
         }
         // get the GC block number corresponding to this pointer
         assert(VERIFY_PTR(ptr));
-        size_t block = BLOCK_FROM_PTR(ptr);
-        assert(ATB_GET_KIND(block) == AT_HEAD);
+        size_t start_block = BLOCK_FROM_PTR(ptr);
+        assert(ATB_GET_KIND(start_block) == AT_HEAD);
 
         #if MICROPY_ENABLE_FINALISER
-        FTB_CLEAR(block);
+        FTB_CLEAR(start_block);
         #endif
 
-        // set the last_free pointer to this block if it's earlier in the heap
-        if (block / BLOCKS_PER_ATB < MP_STATE_MEM(gc_first_free_atb_index)) {
-            MP_STATE_MEM(gc_first_free_atb_index) = block / BLOCKS_PER_ATB;
-        }
-        if (block / BLOCKS_PER_ATB > MP_STATE_MEM(gc_last_free_atb_index)) {
-            MP_STATE_MEM(gc_last_free_atb_index) = block / BLOCKS_PER_ATB;
-        }
-
         // free head and all of its tail blocks
-            #ifdef LOG_HEAP_ACTIVITY
-            gc_log_change(block, 0);
-            #endif
+        #ifdef LOG_HEAP_ACTIVITY
+        gc_log_change(start_block, 0);
+        #endif
+        size_t block = start_block;
         do {
             ATB_ANY_TO_FREE(block);
             block += 1;
         } while (ATB_GET_KIND(block) == AT_TAIL);
 
+        // Update the first free pointer for our size only. Not much calls gc_free directly so there
+        // is decent chance we'll want to allocate this size again. By only updating the specific
+        // size we don't risk something smaller fitting in.
+        size_t n_blocks = block - start_block;
+        size_t bucket = MIN(n_blocks, MICROPY_ATB_INDICES) - 1;
+        size_t new_free_atb = start_block / BLOCKS_PER_ATB;
+        if (new_free_atb < MP_STATE_MEM(gc_first_free_atb_index)[bucket]) {
+            MP_STATE_MEM(gc_first_free_atb_index)[bucket] = new_free_atb;
+        }
+        // set the last_free pointer to this block if it's earlier in the heap
+        if (new_free_atb > MP_STATE_MEM(gc_last_free_atb_index)) {
+            MP_STATE_MEM(gc_last_free_atb_index) = new_free_atb;
+        }
+
         GC_EXIT();
 
         #if EXTENSIVE_HEAP_PROFILING
@@ -870,11 +887,13 @@ void *gc_realloc(void *ptr_in, size_t n_bytes, bool allow_move) {
         }
 
         // set the last_free pointer to end of this block if it's earlier in the heap
-        if ((block + new_blocks) / BLOCKS_PER_ATB < MP_STATE_MEM(gc_first_free_atb_index)) {
-            MP_STATE_MEM(gc_first_free_atb_index) = (block + new_blocks) / BLOCKS_PER_ATB;
+        size_t new_free_atb = (block + new_blocks) / BLOCKS_PER_ATB;
+        size_t bucket = MIN(n_blocks - new_blocks, MICROPY_ATB_INDICES) - 1;
+        if (new_free_atb < MP_STATE_MEM(gc_first_free_atb_index)[bucket]) {
+            MP_STATE_MEM(gc_first_free_atb_index)[bucket] = new_free_atb;
         }
-        if ((block + new_blocks) / BLOCKS_PER_ATB > MP_STATE_MEM(gc_last_free_atb_index)) {
-            MP_STATE_MEM(gc_last_free_atb_index) = (block + new_blocks) / BLOCKS_PER_ATB;
+        if (new_free_atb > MP_STATE_MEM(gc_last_free_atb_index)) {
+            MP_STATE_MEM(gc_last_free_atb_index) = new_free_atb;
         }
 
         GC_EXIT();

py/mpconfig.h

@@ -244,6 +244,14 @@
 #define alloca(x) m_malloc(x)
 #endif
 
+// Number of atb indices to cache. Allocations of fewer blocks will be faster
+// because the search will be accelerated by the index cache. This only applies
+// to short lived allocations because we assume the long lived allocations are
+// contiguous.
+#ifndef MICROPY_ATB_INDICES
+#define MICROPY_ATB_INDICES (8)
+#endif
+
 /*****************************************************************************/
 /* MicroPython emitters                                                     */
 

py/mpstate.h

@@ -92,7 +92,7 @@ typedef struct _mp_state_mem_t {
     size_t gc_alloc_threshold;
     #endif
 
-    size_t gc_first_free_atb_index;
+    size_t gc_first_free_atb_index[MICROPY_ATB_INDICES];
     size_t gc_last_free_atb_index;
 
     #if MICROPY_PY_GC_COLLECT_RETVAL

tools/gc_activity.md

@@ -13,7 +13,7 @@ correct port. GDB is usually :3333 and JLink is :2331.
 Now, run gdb from your port directory:
 
 ```
-arm-none-eabi-gdb -x ../tools/output_gc_until_repl.txt build-metro_m0_express/firmware.elf
+arm-none-eabi-gdb -x ../../tools/output_gc_until_repl.txt build-metro_m0_express/firmware.elf
 ```
 
 This will take a little time while it breaks, backtraces and continues for every

tools/gc_activity_between_collects.py

@@ -0,0 +1,141 @@
+import sys
+import json
+
+# Map start block to current allocation info.
+current_heap = {}
+allocation_history = []
+root = {}
+
+def change_root(trace, size):
+    level = root
+    for frame in reversed(trace):
+        file_location = frame[1]
+        if file_location not in level:
+            level[file_location] = {"blocks": 0,
+                                    "file": file_location,
+                                    "function": frame[2],
+                                    "subcalls": {}}
+        level[file_location]["blocks"] += size
+        level = level[file_location]["subcalls"]
+
+total_actions = 0
+non_single_block_streak = 0
+max_nsbs = 0
+last_action = None
+last_total_actions = 0
+count = 0
+actions = {}
+last_ticks_ms = 0
+ticks_ms = 0
+block_sizes = {}
+allocation_sources = {}
+with open(sys.argv[1], "r") as f:
+    for line in f:
+        if not line.strip():
+            break
+    for line in f:
+        action = None
+        if line.startswith("Breakpoint 2"):
+            break
+        next(f) # throw away breakpoint code line
+        # print(next(f)) # first frame
+        block = 0
+        size = 0
+        trace = []
+        for line in f:
+            # print(line.strip())
+            if line[0] == "#":
+                frame = line.strip().split()
+                if frame[1].startswith("0x"):
+                    trace.append((frame[1], frame[-1], frame[3]))
+                else:
+                    trace.append(("0x0", frame[-1], frame[1]))
+            elif line[0] == "$":
+                #print(line.strip().split()[-1])
+                block = int(line.strip().split()[-1][2:], 16)
+                next_line = next(f)
+                size = int(next_line.strip().split()[-1][2:], 16)
+                # next_line = next(f)
+                # ticks_ms = int(next_line.strip().split()[-1][2:], 16)
+            if not line.strip():
+                break
+
+        action = "unknown"
+        if block not in current_heap:
+            current_heap[block] = {"start_block": block, "size": size, "start_trace": trace, "start_time": total_actions}
+            action = "alloc"
+            if size == 1:
+                max_nsbs = max(max_nsbs, non_single_block_streak)
+                non_single_block_streak = 0
+            else:
+                non_single_block_streak += 1
+            #change_root(trace, size)
+            if size not in block_sizes:
+                block_sizes[size] = 0
+            source = trace[-1][-1]
+            if source not in allocation_sources:
+                print(trace)
+                allocation_sources[source] = 0
+            allocation_sources[source] += 1
+            block_sizes[size] += 1
+        else:
+            alloc = current_heap[block]
+            alloc["end_trace"] = trace
+            alloc["end_time"] = total_actions
+            change_root(alloc["start_trace"], -1 * alloc["size"])
+            if size > 0:
+                action = "realloc"
+                current_heap[block] = {"start_block": block, "size": size, "start_trace": trace, "start_time": total_actions}
+                #change_root(trace, size)
+            else:
+                action = "free"
+                if trace[0][2] == "gc_sweep":
+                    action = "sweep"
+                    non_single_block_streak = 0
+                if (trace[3][2] == "py_gc_collect" or (trace[3][2] == "gc_deinit" and count > 1)) and last_action != "sweep":
+                    print(ticks_ms - last_ticks_ms, total_actions - last_total_actions, "gc.collect", max_nsbs)
+                    print(actions)
+                    print(block_sizes)
+                    print(allocation_sources)
+                    actions = {}
+                    block_sizes = {}
+                    allocation_sources = {}
+                    if count % 2 == 0:
+                        print()
+                    count += 1
+                    last_total_actions = total_actions
+                    last_ticks_ms = ticks_ms
+                    max_nsbs = 0
+                del current_heap[block]
+            alloc["end_cause"] = action
+            allocation_history.append(alloc)
+        if action not in actions:
+            actions[action] = 0
+        actions[action] += 1
+        last_action = action
+        #print(total_actions, non_single_block_streak, action, block, size)
+        total_actions += 1
+print(actions)
+print(max_nsbs)
+print()
+
+for alloc in current_heap.values():
+    alloc["end_trace"] = ""
+    alloc["end_time"] = total_actions
+    allocation_history.append(alloc)
+
+def print_frame(frame, indent=0):
+    for key in sorted(frame):
+        if not frame[key]["blocks"] or key.startswith("../py/malloc.c") or key.startswith("../py/gc.c"):
+            continue
+        print(" " * (indent - 1), key, frame[key]["function"], frame[key]["blocks"], "blocks")
+        print_frame(frame[key]["subcalls"], indent + 2)
+
+# print_frame(root)
+# total_blocks = 0
+# for key in sorted(root):
+#     total_blocks += root[key]["blocks"]
+# print(total_blocks, "total blocks")
+
+# with open("allocation_history.json", "w") as f:
+#     json.dump(allocation_history, f)

tools/output_gc_until_repl.txt

@@ -10,16 +10,23 @@ set logging on
 set remote hardware-breakpoint-limit 4
 
 # gc log
-break gc.c:103
+break gc.c:106
 commands
-backtrace
 p/x start_block
 p/x length
-append binary memory ram.bin &_srelocate &_estack
+p/x ticks_ms
+# backtrace output redirect is currently broken in gdb so we use up instead.
+# https://sourceware.org/bugzilla/show_bug.cgi?id=23439
+# backtrace
+up
+up
+up
+up
+# append binary memory ram.bin &_srelocate &_estack
 continue
 end
 
-break main.c:179
+break main.c:251
 
 continue