More optimizations for calling into WebAssembly #2759
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This commit is an extension of bytecodealliance#2757 where the goal is to optimize entry into WebAssembly. Currently wasmtime has two stack-based cleanups when entering wasm, one for the externref activation table and another for stack limits getting reset. This commit fuses these two cleanups together into one and moves some code around which enables less captures for fewer closures and such to speed up calls in to wasm a bit more. Overall this drops the execution time from 88ns to 80ns locally for me. This also updates the atomic orderings when updating the stack limit from `SeqCst` to `Relaxed`. While `SeqCst` is a reasonable starting point the usage here should be safe to use `Relaxed` since we're not using the atomics to actually protect any memory, it's simply receiving signals from other threads.
The macOS implementation of traps recently changed to using mach ports for handlers instead of signal handlers. This means that a previously relied upon invariant, each thread fixes its own trap, was broken. The macOS implementation worked around this by maintaining a global map from thread id to thread local information, however, to solve the problem. This global map is quite slow though. It involves taking a lock and updating a hash map on all calls into WebAssembly. In my local testing this accounts for >70% of the overhead of calling into WebAssembly on macOS. Naturally it'd be great to remove this! This commit fixes this issue and removes the global lock/map that is updated on all calls into WebAssembly. The fix is to maintain a global map of wasm modules and their trap addresses in the `wasmtime` crate. Doing so is relatively simple since we're already tracking this information at the `Store` level. Once we've got a global map then the macOS implementation can use this from a foreign thread and everything works out. Locally this brings the overhead, on macOS specifically, of calling into wasm from 80ns to ~20ns.
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bnjbvr
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Mar 24, 2021
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| debug_assert!(!jmp_buf.is_null()); |
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There was a check in the unix implementation that the jmp_buf could be set to 1 (meaning a custom signal handler has run and the trap handler should return early). Is it not necessary anymore?
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Ah yeah I realized that it was no longer necessary since we don't run the custom trap handler on macOS, it's only run on Unix and Windows now.
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This continues #2757 a bit more by optimizing calls into WebAssembly. The first commit should affect all platforms, but is only a minor speedup. The second commit is a macOS-specific improvement but is a very large improvement for macOS. I believe after this all platforms should be again performing roughly on-par with one another.