33 lines
1.7 KiB
Markdown
33 lines
1.7 KiB
Markdown
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# Using VMKit as BDWGC replacement
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VMKit provides a BDWGC API shim which allows you to use VMKit as a drop-in replacement for BDWGC. In most of the
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cases it runs out of the box without any changes and provides 2-3x performance improvement.
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# How to use
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- Build VMKit with `--features uncooperative` and then install `libvmkit.so` from `target/release` or `target/debug`
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in a way that pkg-config or build system of your project can find it.
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- Swap `-lgc` with `-lvmkit` in your build command.
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- Done.
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## Notes
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- VMKit does not support every feature of BDWGC, some shims are no-ops.
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- It is necessary to manually call `GC_init()` on program startup.
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- We do not redirect pthread functions, use `GC_pthread_*` functions directly
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or register your threads with `GC_register_my_thread()` and `GC_unregister_my_thread()`.
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- `GC_pthread_create` will wait for thread to be started and registered with GC.
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- Dynamic library support is minimal: we simply invoke `dl_iterate_phdr()` to scan loaded
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data segments for GC roots.
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- Finalization is not yet implemented, only disappearing links are provided.
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- Interior pointers are limited to displacement of 128 bytes from base pointer.
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## Implementation
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BDWGC shim is implemented in `src/bdwgc_shim.rs`. We simply map all APIs to `MemoryManager` methods
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or other functions from `mmtk` crate. Since MMTk requires all objects to provide `scan` method,
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each allocated object actually stores a vtable with necessary metadata to implement it. This vtable
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is stored in the object header, right behind the object payload.
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Thread suspension is implemented using `pthread_kill`, the mechanism is similar to that of BDWGC, but we
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rely on `sigsuspend` instead of spin waiting.
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