Move heap ownership info from chunk to pagemap #4368
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Prior to this commit, the chunk kept track of the heap ownership info in the `chunk->actor` field. This worked but was not ideal as noted in the `ponyint_heap_owner` function that this is a case of false sharing where the chunk owner needs the chunk but all other actors only need the chunk owner only. This led to lots of pointer chasing as the pagemap was used to retrieve the chunk pointerand then the chunk had to be loaded to retrieve the owning actor pointer. This commit removes the `chunk->actor` field further slimming down both the `small_chunk` and `large_chunk` so both now fit within 32 bytes (on 64 bit architectures). The chunk ownership information is now kept in the lowest level of the pagemap next to the chunk information. This removes the previous pointer chasing because the chunk owning actor pointer is retrieved at the same time as the chunk pointer without needing to load the chunk itself. This is a fairly standard tradeoff of memory for performance where we're now storing more data in the pagemap to minimize pointer chasing. The expectation is that this will have a net positive impact on performance but no benchmarks have been run to validate this assumption.
ponylang-main
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SeanTAllen
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Hi @dipinhora, The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do. Release notes are added by creating a uniquely named file in the The basic format of the release notes (using markdown) should be: Thanks. |
| return chunk; | ||
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| chunk_t* ponyint_pagemap_get(const void* addr, pony_actor_t** actor) |
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I think this should get a bit of documentation that actor is being set. It took me a bit to figure that out.
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added a comment.. let me know if it's not clear enough..
| return chunk; | ||
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| chunk_t* ponyint_pagemap_get(const void* addr, pony_actor_t** actor) |
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I think noting why we have a return chunk_t and and out parameter actor rather than two out parameters, would be good documentation for this function.
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| pony_assert(ix <= (uintptr_t)level[PAGEMAP_LEVELS - 1].size); | ||
| pony_assert(ix/2 <= (uintptr_t)level[PAGEMAP_LEVELS - 1].mask); | ||
| next_node = &(((PONY_ATOMIC_RVALUE(void*)*)node)[ix]); | ||
| atomic_store_explicit(next_node, actor, memory_order_release); |
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note from sync: please document your understanding of the atomics and their safety here as you stated in the sync call.
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added a comment.. let me know if it's not clear enough..
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release notes added |
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With the small change to fix the release notes format (we only do newlines for new paragraphs), I'm good with this. @jemc, can you also give another review with an eye towards comments and "documentation"?
ponylang-main
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This reverts commit a9343c5.
This reverts commit a1a1446.
Prior to this commit, the chunk kept track of the heap ownership info in the
chunk->actorfield. This worked but was not ideal as noted in theponyint_heap_ownerfunction that this is a case of false sharing where the chunk owner needs the chunk but all other actors only need the chunk owner only. This led to lots of pointer chasing as the pagemap was used to retrieve the chunk pointerand then the chunk had to be loaded to retrieve the owning actor pointer.This commit removes the
chunk->actorfield further slimming down both thesmall_chunkandlarge_chunkso both now fit within 32 bytes (on 64 bit architectures). The chunk ownership information is now kept in the lowest level of the pagemap next to the chunk information. This removes the previous pointer chasing because the chunk owning actor pointer is retrieved at the same time as the chunk pointer without needing to load the chunk itself. This is a fairly standard tradeoff of memory for performance where we're now storing more data in the pagemap to minimize pointer chasing. The expectation is that this will have a net positive impact on performance but no benchmarks have been run to validate this assumption.