b73ac83c37
As first suggested by Jan on the Zulip here [1], a cheap and effective way to obtain copy-on-write semantics of a "backing image" for a Wasm memory is to mmap a file with `MAP_PRIVATE`. The `memfd` mechanism provided by the Linux kernel allows us to create anonymous, in-memory-only files that we can use for this mapping, so we can construct the image contents on-the-fly then effectively create a CoW overlay. Furthermore, and importantly, `madvise(MADV_DONTNEED, ...)` will discard the CoW overlay, returning the mapping to its original state. By itself this is almost enough for a very fast instantiation-termination loop of the same image over and over, without changing the address space mapping at all (which is expensive). The only missing bit is how to implement heap *growth*. But here memfds can help us again: if we create another anonymous file and map it where the extended parts of the heap would go, we can take advantage of the fact that a `mmap()` mapping can be *larger than the file itself*, with accesses beyond the end generating a `SIGBUS`, and the fact that we can cheaply resize the file with `ftruncate`, even after a mapping exists. So we can map the "heap extension" file once with the maximum memory-slot size and grow the memfd itself as `memory.grow` operations occur. The above CoW technique and heap-growth technique together allow us a fastpath of `madvise()` and `ftruncate()` only when we re-instantiate the same module over and over, as long as we can reuse the same slot. This fastpath avoids all whole-process address-space locks in the Linux kernel, which should mean it is highly scalable. It also avoids the cost of copying data on read, as the `uffd` heap backend does when servicing pagefaults; the kernel's own optimized CoW logic (same as used by all file mmaps) is used instead. [1] https://bytecodealliance.zulipchat.com/#narrow/stream/206238-general/topic/Copy.20on.20write.20based.20instance.20reuse/near/266657772
65 lines
1.8 KiB
TOML
65 lines
1.8 KiB
TOML
[package]
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name = "wasmtime-runtime"
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version = "0.33.0"
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authors = ["The Wasmtime Project Developers"]
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description = "Runtime library support for Wasmtime"
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documentation = "https://docs.rs/wasmtime-runtime"
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license = "Apache-2.0 WITH LLVM-exception"
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categories = ["wasm"]
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keywords = ["webassembly", "wasm"]
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repository = "https://github.com/bytecodealliance/wasmtime"
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edition = "2018"
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[dependencies]
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wasmtime-environ = { path = "../environ", version = "=0.33.0" }
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wasmtime-fiber = { path = "../fiber", version = "=0.33.0", optional = true }
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region = "2.1.0"
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libc = { version = "0.2.112", default-features = false }
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log = "0.4.8"
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memoffset = "0.6.0"
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indexmap = "1.0.2"
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thiserror = "1.0.4"
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more-asserts = "0.2.1"
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cfg-if = "1.0"
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backtrace = "0.3.61"
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lazy_static = "1.3.0"
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rand = "0.8.3"
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anyhow = "1.0.38"
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[target.'cfg(target_os = "macos")'.dependencies]
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mach = "0.3.2"
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[target.'cfg(unix)'.dependencies]
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rustix = "0.31.0"
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[target.'cfg(target_os = "windows")'.dependencies]
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winapi = { version = "0.3.7", features = ["winbase", "memoryapi", "errhandlingapi", "handleapi"] }
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[target.'cfg(target_os = "linux")'.dependencies]
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userfaultfd = { version = "0.4.1", optional = true }
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memfd = { version = "0.4.1", optional = true }
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[build-dependencies]
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cc = "1.0"
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[badges]
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maintenance = { status = "actively-developed" }
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[features]
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default = []
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async = ["wasmtime-fiber"]
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# Enables support for the pooling instance allocator
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pooling-allocator = []
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# Enables support for userfaultfd in the pooling allocator when building on Linux
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uffd = ["userfaultfd", "pooling-allocator"]
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# Enables trap handling using POSIX signals instead of Mach exceptions on MacOS.
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# It is useful for applications that do not bind their own exception ports and
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# need portable signal handling.
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posix-signals-on-macos = []
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memfd-allocator = ["pooling-allocator", "memfd"]
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