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Commitb796e3d

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py: Reduce fragmentation of GC heap.
Recent speed up of GC allocation made the GC have a fragmented heap.This patch restores "original fragmentation behaviour" whilst stillretaining relatively fast allocation. This patch works because there isalways going to be a single block allocated now and then, which advancesthe gc_last_free_atb_index pointer often enough so that the whole heapdoesn't need scanning.Should address issueadafruit#836.
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‎py/gc.c‎

Lines changed: 7 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -386,13 +386,6 @@ void *gc_alloc(mp_uint_t n_bytes, bool has_finaliser) {
386386
if (ATB_2_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+2; gotofound; } }else {n_free=0; }
387387
if (ATB_3_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+3; gotofound; } }else {n_free=0; }
388388
}
389-
for (i=0;i<gc_last_free_atb_index;i++) {
390-
bytea=gc_alloc_table_start[i];
391-
if (ATB_0_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+0; gotofound; } }else {n_free=0; }
392-
if (ATB_1_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+1; gotofound; } }else {n_free=0; }
393-
if (ATB_2_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+2; gotofound; } }else {n_free=0; }
394-
if (ATB_3_IS_FREE(a)) {if (++n_free >=n_blocks) {i=i*BLOCKS_PER_ATB+3; gotofound; } }else {n_free=0; }
395-
}
396389

397390
// nothing found!
398391
if (collected) {
@@ -409,8 +402,13 @@ void *gc_alloc(mp_uint_t n_bytes, bool has_finaliser) {
409402
end_block=i;
410403
start_block=i-n_free+1;
411404

412-
// set last free ATB index to last block we found, for start of next scan
413-
gc_last_free_atb_index=i /BLOCKS_PER_ATB;
405+
// Set last free ATB index to block after last block we found, for start of
406+
// next scan. To reduce fragmentation, we only do this if we were looking
407+
// for a single free block, which guarantees that there are no free blocks
408+
// before this one.
409+
if (n_free==1) {
410+
gc_last_free_atb_index= (i+1) /BLOCKS_PER_ATB;
411+
}
414412

415413
// mark first block as used head
416414
ATB_FREE_TO_HEAD(start_block);

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