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Removed unnecessary atomics while multi pages alloc/dealloc #308

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Removed unnecessary atomics while multi pages alloc/dealloc #308

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14 changes: 5 additions & 9 deletions include/mallocMC/creationPolicies/FlatterScatter/AccessBlock.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -431,7 +431,7 @@ namespace mallocMC::CreationPolicies::FlatterScatterAlloc
{
// At this point, we have acquired all the pages we need and nobody can mess with them anymore.
// We still have to set the chunk size correctly.
setChunkSizes(localAcc, firstIndex, numPagesNeeded, numBytes);
setChunkSizes(localAcc, firstIndex, numBytes);
result = &pages[firstIndex];
}
else
Expand Down Expand Up @@ -494,31 +494,26 @@ namespace mallocMC::CreationPolicies::FlatterScatterAlloc
}

/**
* @brief Set the chunk sizes of a contiguous array of pages.
* @brief Set the chunk size of a contiguous array of pages.
*
* This function assumes that all the pages are locked by the current thread and performs a hard set operation
* without checking the previous content.
*
* @param firstIndex Start index of the contiguous array of pages.
* @param numPagesNeeded The number of pages to set the chunk size on.
* @param numBytes Chunk size to be set in number of bytes.
*/
ALPAKA_FN_INLINE ALPAKA_FN_ACC auto setChunkSizes(
auto const& acc,
uint32_t const firstIndex,
uint32_t const numPagesNeeded,
uint32_t const numBytes) -> void
{
for(uint32_t numPagesAcquired = 0U; numPagesAcquired < numPagesNeeded; ++numPagesAcquired)
{
// At this point in the code, we have already locked all the pages. So, we literally don't care what
// other threads thought this chunk size would be because we are the only ones legitimately messing
// with this page. This chunk size may be non-zero because we could have taken over a page before it
// was properly cleaned up. That is okay for us because we're handing out uninitialised memory anyways.
// But it is very important to record the correct chunk size here, so the destroy method later on knows
// how to handle this memory.
alpaka::atomicExch(acc, &pageTable.chunkSizes[firstIndex + numPagesAcquired], numBytes);
}
alpaka::atomicExch(acc, &pageTable.chunkSizes[firstIndex], numBytes);
}

/**
Expand Down Expand Up @@ -836,6 +831,8 @@ namespace mallocMC::CreationPolicies::FlatterScatterAlloc
uint32_t const pageIndex,
uint32_t const chunkSize)
{
alpaka::atomicCas(acc, &pageTable.chunkSizes[pageIndex], chunkSize, 0U);

auto numPagesNeeded = ceilingDivision(chunkSize, pageSize);
for(uint32_t i = 0; i < numPagesNeeded; ++i)
{
Expand All @@ -848,7 +845,6 @@ namespace mallocMC::CreationPolicies::FlatterScatterAlloc
{
MyPageInterpretation{pages[myIndex], T_AlignmentPolicy::Properties::dataAlignment}.cleanupFull();
alpaka::mem_fence(acc, alpaka::memory_scope::Device{});
alpaka::atomicCas(acc, &pageTable.chunkSizes[myIndex], chunkSize, 0U);
}
alpaka::atomicSub(acc, &pageTable.fillingLevels[myIndex], +pageSize);
}
Expand Down
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