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Update comments in emmalloc.c regarding root regions and sbrk(). #20784

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Merged
merged 1 commit into from
Nov 28, 2023
Merged

Update comments in emmalloc.c regarding root regions and sbrk(). #20784

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21 changes: 13 additions & 8 deletions system/lib/emmalloc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@
*
* - sbrk() is used to claim new memory (sbrk handles geometric/linear
* - overallocation growth)
* - sbrk() can be used by other code outside emmalloc.
* - sbrk() can also be called by other code, not reserved to emmalloc only.
* - sbrk() is very fast in most cases (internal wasm call).
* - sbrk() returns pointers with an alignment of alignof(max_align_t)
*
Expand All @@ -27,6 +27,8 @@
* merged.
* - Memory allocation takes constant time, unless the alloc needs to sbrk()
* or memory is very close to being exhausted.
* - Free and used regions are managed inside "root regions", which are slabs
* of memory acquired via calls to sbrk().
*
* Debugging:
*
Expand Down Expand Up @@ -486,10 +488,11 @@ static bool claim_more_memory(size_t numBytes)
validate_memory_regions();
#endif

// Make sure we send sbrk requests of aligned sizes. If we do not do that then
// it will not return contiguous regions, which leads to use creating more
// root regions below, inefficiently. (Note that we assume our alignment is
// identical to sbrk's, see the assumptions at the start of this file.)
// Make sure we always send sbrk requests with the same alignment that sbrk()
// allocates memory at. Otherwise we will not properly interpret returned memory
// to form a seamlessly contiguous region with earlier root regions, which would
// lead to inefficiently treating the sbrk()ed region to be a new disjoint root
// region.
numBytes = (size_t)ALIGN_UP(numBytes, MALLOC_ALIGNMENT);

// Claim memory via sbrk
Expand Down Expand Up @@ -517,7 +520,7 @@ static bool claim_more_memory(size_t numBytes)
if (startPtr == previousSbrkEndAddress)
{
#ifdef EMMALLOC_VERBOSE
MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: expanding previous'));
MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: sbrk() returned a region contiguous to last root region, expanding the existing root region'));
#endif
Region *prevEndSentinel = prev_region((Region*)startPtr);
assert(debug_region_is_consistent(prevEndSentinel));
Expand All @@ -544,9 +547,11 @@ static bool claim_more_memory(size_t numBytes)
}
else
{
// Create a root region at the start of the heap block
// Unfortunately some other user has sbrk()ed to acquire a slab of memory for themselves, and now the sbrk()ed
// memory we got is not contiguous with our previous managed root regions.
// So create a new root region at the start of the sbrk()ed heap block.
#ifdef EMMALLOC_VERBOSE
MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: creating new root region'));
MAIN_THREAD_ASYNC_EM_ASM(err('claim_more_memory: sbrk() returned a disjoint region to last root region, some external code must have sbrk()ed outside emmalloc(). Creating a new root region'));
#endif
create_used_region(startPtr, sizeof(Region));

Expand Down