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ChunkMap: limit flat pagemap size

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Presently, our flat pagemap can be configured to take...

                   32-bit AS    48-bit AS
USE_SMALL_CHUNKS     16 KiB        1 GiB
default               4 KiB      256 MiB
USE_LARGE_CHUNKS    256   B       16 MiB

At 1 GiB, we're already past the 512 MiB threshold imposed when
src/test/func/memory/memory.cc, when configured to TEST_LIMITED, probes the
effect of rlimit.

Instead, restrict flat pagemaps to at most 256 MiB of AS by default (override
by defining SNMALLOC_MAX_FLATPAGEMAP_SIZE), which forces the USE_SMALL_CHUNKS &
48-bit AS configuration to use the tree-based version.

While here, rename USE_FLATPAGEMAP to CHUNKMAP_USE_FLATPAGEMAP.
This commit is contained in:
Nathaniel Filardo
2020-11-20 03:16:07 +00:00
committed by Matthew Parkinson
parent c1d5f48797
commit e9ed219fd8
1 changed files with 21 additions and 6 deletions
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27
src/mem/chunkmap.h
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@@ -56,15 +56,30 @@ namespace snmalloc
SUPERSLAB_BITS > CMMediumslab, "Large allocations may be too small");
#ifndef SNMALLOC_MAX_FLATPAGEMAP_SIZE
// Use flat map is under a single node.
# define SNMALLOC_MAX_FLATPAGEMAP_SIZE PAGEMAP_NODE_SIZE
/*
* Unless otherwise specified, use a flat pagemap for the chunkmap (1 byte per
* Superslab-sized and -aligned region of the address space) if either of the
* following hold:
*
* - the platform supports LazyCommit and the flat structure would occupy 256
* MiB or less. 256 MiB is more than adequate for 32-bit architectures and
* is the size of the flat pagemap for a 48-bit AS with the default chunk
* size or the USE_LARGE_CHUNKS chunksize (that is, configurations other
* than USE_SMALL_CHUNKS).
*
* - the platform does not support LazyCommit but the flat structure would
* occupy less than PAGEMAP_NODE_SIZE (i.e., the backing store for an
* internal tree node in the non-flat pagemap).
*/
# define SNMALLOC_MAX_FLATPAGEMAP_SIZE \
(pal_supports<LazyCommit> ? 256ULL * 1024 * 1024 : PAGEMAP_NODE_SIZE)
#endif
static constexpr bool USE_FLATPAGEMAP = pal_supports<LazyCommit> ||
(SNMALLOC_MAX_FLATPAGEMAP_SIZE >=
sizeof(FlatPagemap<SUPERSLAB_BITS, uint8_t>));
static constexpr bool CHUNKMAP_USE_FLATPAGEMAP =
SNMALLOC_MAX_FLATPAGEMAP_SIZE >=
sizeof(FlatPagemap<SUPERSLAB_BITS, uint8_t>);
using ChunkmapPagemap = std::conditional_t<
USE_FLATPAGEMAP,
CHUNKMAP_USE_FLATPAGEMAP,
FlatPagemap<SUPERSLAB_BITS, uint8_t>,
Pagemap<SUPERSLAB_BITS, uint8_t, 0>>;