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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 still
retaining relatively fast allocation.  This patch works because there is
always going to be a single block allocated now and then, which advances
the gc_last_free_atb_index pointer often enough so that the whole heap
doesn't need scanning.

Should address issue #836.
This commit is contained in:
Damien George 2014-08-28 10:18:40 +01:00
parent a97e091d4e
commit b796e3d848
1 changed files with 7 additions and 9 deletions
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16
py/gc.c
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@ -386,13 +386,6 @@ void *gc_alloc(mp_uint_t n_bytes, bool has_finaliser) {
if (ATB_2_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 2; goto found; } } else { n_free = 0; }
if (ATB_3_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 3; goto found; } } else { n_free = 0; }
}
for (i = 0; i < gc_last_free_atb_index; i++) {
byte a = gc_alloc_table_start[i];
if (ATB_0_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 0; goto found; } } else { n_free = 0; }
if (ATB_1_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 1; goto found; } } else { n_free = 0; }
if (ATB_2_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 2; goto found; } } else { n_free = 0; }
if (ATB_3_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 3; goto found; } } else { n_free = 0; }
}
// nothing found!
if (collected) {
@ -409,8 +402,13 @@ found:
end_block = i;
start_block = i - n_free + 1;
// set last free ATB index to last block we found, for start of next scan
gc_last_free_atb_index = i / BLOCKS_PER_ATB;
// 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.
if (n_free == 1) {
gc_last_free_atb_index = (i + 1) / BLOCKS_PER_ATB;
}
// mark first block as used head
ATB_FREE_TO_HEAD(start_block);