Struct wasmtime::Config

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pub struct Config { /* private fields */ }
Expand description

Global configuration options used to create an Engine and customize its behavior.

This structure exposed a builder-like interface and is primarily consumed by Engine::new().

The validation of Config is deferred until the engine is being built, thus a problematic config may cause Engine::new to fail.

Implementations

Creates a new configuration object with the default configuration specified.

Sets the target triple for the Config.

By default, the host target triple is used for the Config.

This method can be used to change the target triple.

Cranelift flags will not be inferred for the given target and any existing target-specific Cranelift flags will be cleared.

Errors

This method will error if the given target triple is not supported.

Whether or not to enable support for asynchronous functions in Wasmtime.

When enabled, the config can optionally define host functions with async. Instances created and functions called with this Config must be called through their asynchronous APIs, however. For example using Func::call will panic when used with this config.

Asynchronous Wasm

WebAssembly does not currently have a way to specify at the bytecode level what is and isn’t async. Host-defined functions, however, may be defined as async. WebAssembly imports always appear synchronous, which gives rise to a bit of an impedance mismatch here. To solve this Wasmtime supports “asynchronous configs” which enables calling these asynchronous functions in a way that looks synchronous to the executing WebAssembly code.

An asynchronous config must always invoke wasm code asynchronously, meaning we’ll always represent its computation as a Future. The poll method of the futures returned by Wasmtime will perform the actual work of calling the WebAssembly. Wasmtime won’t manage its own thread pools or similar, that’s left up to the embedder.

To implement futures in a way that WebAssembly sees asynchronous host functions as synchronous, all async Wasmtime futures will execute on a separately allocated native stack from the thread otherwise executing Wasmtime. This separate native stack can then be switched to and from. Using this whenever an async host function returns a future that resolves to Pending we switch away from the temporary stack back to the main stack and propagate the Pending status.

In general it’s encouraged that the integration with async and wasmtime is designed early on in your embedding of Wasmtime to ensure that it’s planned that WebAssembly executes in the right context of your application.

Execution in poll

The Future::poll method is the main driving force behind Rust’s futures. That method’s own documentation states “an implementation of poll should strive to return quickly, and should not block”. This, however, can be at odds with executing WebAssembly code as part of the poll method itself. If your WebAssembly is untrusted then this could allow the poll method to take arbitrarily long in the worst case, likely blocking all other asynchronous tasks.

To remedy this situation you have a a few possible ways to solve this:

  • The most efficient solution is to enable Config::epoch_interruption in conjunction with crate::Store::epoch_deadline_async_yield_and_update. Coupled with periodic calls to crate::Engine::increment_epoch this will cause executing WebAssembly to periodically yield back according to the epoch configuration settings. This enables Future::poll to take at most a certain amount of time according to epoch configuration settings and when increments happen. The benefit of this approach is that the instrumentation in compiled code is quite lightweight, but a downside can be that the scheduling is somewhat nondeterministic since increments are usually timer-based which are not always deterministic.

    Note that to prevent infinite execution of wasm it’s recommended to place a timeout on the entire future representing executing wasm code and the periodic yields with epochs should ensure that when the timeout is reached it’s appropriately recognized.

  • Alternatively you can enable the Config::consume_fuel method as well as crate::Store::out_of_fuel_async_yield When doing so this will configure Wasmtime futures to yield periodically while they’re executing WebAssembly code. After consuming the specified amount of fuel wasm futures will return Poll::Pending from their poll method, and will get automatically re-polled later. This enables the Future::poll method to take roughly a fixed amount of time since fuel is guaranteed to get consumed while wasm is executing. Unlike epoch-based preemption this is deterministic since wasm always consumes a fixed amount of fuel per-operation. The downside of this approach, however, is that the compiled code instrumentation is significantly more expensive than epoch checks.

    Note that to prevent infinite execution of wasm it’s recommended to place a timeout on the entire future representing executing wasm code and the periodic yields with epochs should ensure that when the timeout is reached it’s appropriately recognized.

In all cases special care needs to be taken when integrating asynchronous wasm into your application. You should carefully plan where WebAssembly will execute and what compute resources will be allotted to it. If Wasmtime doesn’t support exactly what you’d like just yet, please feel free to open an issue!

Configures whether or not stacks used for async futures are reset to zero after usage.

When the async_support method is enabled for Wasmtime and the call_async variant of calling WebAssembly is used then Wasmtime will create a separate runtime execution stack for each future produced by call_async. When using the pooling instance allocator (InstanceAllocationStrategy::Pooling) this allocation will happen from a pool of stacks and additionally deallocation will simply release the stack back to the pool. During the deallocation process Wasmtime won’t by default reset the contents of the stack back to zero.

When this option is enabled it can be seen as a defense-in-depth mechanism to reset a stack back to zero. This is not required for correctness and can be a costly operation in highly concurrent environments due to modifications of the virtual address space requiring process-wide synchronization.

This option defaults to false.

Configures whether DWARF debug information will be emitted during compilation.

By default this option is false.

👎 Deprecated:

Backtraces will always be enabled in future Wasmtime releases; if this causes problems for you, please file an issue.

Configures whether backtraces exist in a Trap.

Enabled by default, this feature builds in support to generate backtraces at runtime for WebAssembly modules. This means that unwinding information is compiled into wasm modules and necessary runtime dependencies are enabled as well.

When disabled, wasm backtrace details are ignored, and crate::Trap::trace() will always return None.

Configures whether backtraces in Trap will parse debug info in the wasm file to have filename/line number information.

When enabled this will causes modules to retain debugging information found in wasm binaries. This debug information will be used when a trap happens to symbolicate each stack frame and attempt to print a filename/line number for each wasm frame in the stack trace.

By default this option is WasmBacktraceDetails::Environment, meaning that wasm will read WASMTIME_BACKTRACE_DETAILS to indicate whether details should be parsed.

Configures whether to generate native unwind information (e.g. .eh_frame on Linux).

This configuration option only exists to help third-party stack capturing mechanisms, such as the system’s unwinder or the backtrace crate, determine how to unwind through Wasm frames. It does not affect whether Wasmtime can capture Wasm backtraces or not, or whether Trap::trace returns Some or None.

Note that native unwind information is always generated when targeting Windows, since the Windows ABI requires it.

This option defaults to true.

Configures whether execution of WebAssembly will “consume fuel” to either halt or yield execution as desired.

This can be used to deterministically prevent infinitely-executing WebAssembly code by instrumenting generated code to consume fuel as it executes. When fuel runs out the behavior is defined by configuration within a Store, and by default a trap is raised.

Note that a Store starts with no fuel, so if you enable this option you’ll have to be sure to pour some fuel into Store before executing some code.

By default this option is false.

Enables epoch-based interruption.

When executing code in async mode, we sometimes want to implement a form of cooperative timeslicing: long-running Wasm guest code should periodically yield to the executor loop. This yielding could be implemented by using “fuel” (see consume_fuel). However, fuel instrumentation is somewhat expensive: it modifies the compiled form of the Wasm code so that it maintains a precise instruction count, frequently checking this count against the remaining fuel. If one does not need this precise count or deterministic interruptions, and only needs a periodic interrupt of some form, then It would be better to have a more lightweight mechanism.

Epoch-based interruption is that mechanism. There is a global “epoch”, which is a counter that divides time into arbitrary periods (or epochs). This counter lives on the Engine and can be incremented by calling Engine::increment_epoch. Epoch-based instrumentation works by setting a “deadline epoch”. The compiled code knows the deadline, and at certain points, checks the current epoch against that deadline. It will yield if the deadline has been reached.

The idea is that checking an infrequently-changing counter is cheaper than counting and frequently storing a precise metric (instructions executed) locally. The interruptions are not deterministic, but if the embedder increments the epoch in a periodic way (say, every regular timer tick by a thread or signal handler), then we can ensure that all async code will yield to the executor within a bounded time.

The deadline check cannot be avoided by malicious wasm code. It is safe to use epoch deadlines to limit the execution time of untrusted code.

The Store tracks the deadline, and controls what happens when the deadline is reached during execution. Several behaviors are possible:

Trapping is the default. The yielding behaviour may be used for the timeslicing behavior described above.

This feature is available with or without async support. However, without async support, the timeslicing behaviour is not available. This means epoch-based interruption can only serve as a simple external-interruption mechanism.

An initial deadline must be set before executing code by calling Store::set_epoch_deadline. If this deadline is not configured then wasm will immediately trap.

When to use fuel vs. epochs

In general, epoch-based interruption results in faster execution. This difference is sometimes significant: in some measurements, up to 2-3x. This is because epoch-based interruption does less work: it only watches for a global rarely-changing counter to increment, rather than keeping a local frequently-changing counter and comparing it to a deadline.

Fuel, in contrast, should be used when deterministic yielding or trapping is needed. For example, if it is required that the same function call with the same starting state will always either complete or trap with an out-of-fuel error, deterministically, then fuel with a fixed bound should be used.

See Also

Configures the maximum amount of stack space available for executing WebAssembly code.

WebAssembly has well-defined semantics on stack overflow. This is intended to be a knob which can help configure how much stack space wasm execution is allowed to consume. Note that the number here is not super-precise, but rather wasm will take at most “pretty close to this much” stack space.

If a wasm call (or series of nested wasm calls) take more stack space than the size specified then a stack overflow trap will be raised.

Caveat: this knob only limits the stack space consumed by wasm code. More importantly, it does not ensure that this much stack space is available on the calling thread stack. Exhausting the thread stack typically leads to an abort of the process.

Here are some examples of how that could happen:

  • Let’s assume this option is set to 2 MiB and then a thread that has a stack with 512 KiB left.

    If wasm code consumes more than 512 KiB then the process will be aborted.

  • Assuming the same conditions, but this time wasm code does not consume any stack but calls into a host function. The host function consumes more than 512 KiB of stack space. The process will be aborted.

There’s another gotcha related to recursive calling into wasm: the stack space consumed by a host function is counted towards this limit. The host functions are not prevented from consuming more than this limit. However, if the host function that used more than this limit and called back into wasm, then the execution will trap immediatelly because of stack overflow.

When the async feature is enabled, this value cannot exceed the async_stack_size option. Be careful not to set this value too close to async_stack_size as doing so may limit how much stack space is available for host functions.

By default this option is 512 KiB.

Errors

The Engine::new method will fail if the size specified here is either 0 or larger than the Config::async_stack_size configuration.

Configures the size of the stacks used for asynchronous execution.

This setting configures the size of the stacks that are allocated for asynchronous execution. The value cannot be less than max_wasm_stack.

The amount of stack space guaranteed for host functions is async_stack_size - max_wasm_stack, so take care not to set these two values close to one another; doing so may cause host functions to overflow the stack and abort the process.

By default this option is 2 MiB.

Errors

The Engine::new method will fail if the value for this option is smaller than the Config::max_wasm_stack option.

Configures whether the WebAssembly threads proposal will be enabled for compilation.

The WebAssembly threads proposal is not currently fully standardized and is undergoing development. Additionally the support in wasmtime itself is still being worked on. Support for this feature can be enabled through this method for appropriate wasm modules.

This feature gates items such as shared memories and atomic instructions. Note that the threads feature depends on the bulk memory feature, which is enabled by default.

This is false by default.

Note: Wasmtime does not implement everything for the wasm threads spec at this time, so bugs, panics, and possibly segfaults should be expected. This should not be enabled in a production setting right now.

Errors

The validation of this feature are deferred until the engine is being built, and thus may cause Engine::new fail if the bulk_memory feature is disabled.

Configures whether the WebAssembly reference types proposal will be enabled for compilation.

This feature gates items such as the externref and funcref types as well as allowing a module to define multiple tables.

Note that the reference types proposal depends on the bulk memory proposal.

This feature is true by default.

Errors

The validation of this feature are deferred until the engine is being built, and thus may cause Engine::new fail if the bulk_memory feature is disabled.

Configures whether the WebAssembly SIMD proposal will be enabled for compilation.

The WebAssembly SIMD proposal. This feature gates items such as the v128 type and all of its operators being in a module. Note that this does not enable the relaxed simd proposal as that is not implemented in Wasmtime at this time.

On x86_64 platforms note that enabling this feature requires SSE 4.2 and below to be available on the target platform. Compilation will fail if the compile target does not include SSE 4.2.

This is true by default.

Configures whether the WebAssembly bulk memory operations proposal will be enabled for compilation.

This feature gates items such as the memory.copy instruction, passive data/table segments, etc, being in a module.

This is true by default.

Feature reference_types, which is also true by default, requires this feature to be enabled. Thus disabling this feature must also disable reference_types as well using wasm_reference_types.

Errors

Disabling this feature without disabling reference_types will cause Engine::new to fail.

Configures whether the WebAssembly multi-value proposal will be enabled for compilation.

This feature gates functions and blocks returning multiple values in a module, for example.

This is true by default.

Configures whether the WebAssembly multi-memory proposal will be enabled for compilation.

This feature gates modules having more than one linear memory declaration or import.

This is false by default.

Configures whether the WebAssembly memory64 proposal will be enabled for compilation.

Note that this the upstream specification is not finalized and Wasmtime may also have bugs for this feature since it hasn’t been exercised much.

This is false by default.

Configures which compilation strategy will be used for wasm modules.

This method can be used to configure which compiler is used for wasm modules, and for more documentation consult the Strategy enumeration and its documentation.

The default value for this is Strategy::Auto.

Creates a default profiler based on the profiling strategy chosen.

Profiler creation calls the type’s default initializer where the purpose is really just to put in place the type used for profiling.

Some ProfilingStrategy require specific platforms or particular feature to be enabled, such as ProfilingStrategy::JitDump requires the jitdump feature.

Errors

The validation of this field is deferred until the engine is being built, and thus may cause Engine::new fail if the required feature is disabled, or the platform is not supported.

Configures whether the debug verifier of Cranelift is enabled or not.

When Cranelift is used as a code generation backend this will configure it to have the enable_verifier flag which will enable a number of debug checks inside of Cranelift. This is largely only useful for the developers of wasmtime itself.

The default value for this is false

Configures the Cranelift code generator optimization level.

When the Cranelift code generator is used you can configure the optimization level used for generated code in a few various ways. For more information see the documentation of OptLevel.

The default value for this is OptLevel::None.

Configures whether Cranelift should perform a NaN-canonicalization pass.

When Cranelift is used as a code generation backend this will configure it to replace NaNs with a single canonical value. This is useful for users requiring entirely deterministic WebAssembly computation. This is not required by the WebAssembly spec, so it is not enabled by default.

The default value for this is false

Allows setting a Cranelift boolean flag or preset. This allows fine-tuning of Cranelift settings.

Since Cranelift flags may be unstable, this method should not be considered to be stable either; other Config functions should be preferred for stability.

Safety

This is marked as unsafe, because setting the wrong flag might break invariants, resulting in execution hazards.

Errors

The validation of the flags are deferred until the engine is being built, and thus may cause Engine::new fail if the flag’s name does not exist, or the value is not appropriate for the flag type.

Allows settings another Cranelift flag defined by a flag name and value. This allows fine-tuning of Cranelift settings.

Since Cranelift flags may be unstable, this method should not be considered to be stable either; other Config functions should be preferred for stability.

Safety

This is marked as unsafe, because setting the wrong flag might break invariants, resulting in execution hazards.

Errors

The validation of the flags are deferred until the engine is being built, and thus may cause Engine::new fail if the flag’s name does not exist, or incompatible with other settings.

For example, feature wasm_backtrace will set unwind_info to true, but if it’s manually set to false then it will fail.

Loads cache configuration specified at path.

This method will read the file specified by path on the filesystem and attempt to load cache configuration from it. This method can also fail due to I/O errors, misconfiguration, syntax errors, etc. For expected syntax in the configuration file see the documentation online.

By default cache configuration is not enabled or loaded.

This method is only available when the cache feature of this crate is enabled.

Errors

This method can fail due to any error that happens when loading the file pointed to by path and attempting to load the cache configuration.

Loads cache configuration from the system default path.

This commit is the same as Config::cache_config_load except that it does not take a path argument and instead loads the default configuration present on the system. This is located, for example, on Unix at $HOME/.config/wasmtime/config.toml and is typically created with the wasmtime config new command.

By default cache configuration is not enabled or loaded.

This method is only available when the cache feature of this crate is enabled.

Errors

This method can fail due to any error that happens when loading the default system configuration. Note that it is not an error if the default config file does not exist, in which case the default settings for an enabled cache are applied.

Sets a custom memory creator.

Custom memory creators are used when creating host Memory objects or when creating instance linear memories for the on-demand instance allocation strategy.

Sets the instance allocation strategy to use.

When using the pooling instance allocation strategy, all linear memories will be created as “static” and the Config::static_memory_maximum_size and Config::static_memory_guard_size options will be used to configure the virtual memory allocations of linear memories.

Configures the maximum size, in bytes, where a linear memory is considered static, above which it’ll be considered dynamic.

Note: this value has important performance ramifications, be sure to understand what this value does before tweaking it and benchmarking.

This function configures the threshold for wasm memories whether they’re implemented as a dynamically relocatable chunk of memory or a statically located chunk of memory. The max_size parameter here is the size, in bytes, where if the maximum size of a linear memory is below max_size then it will be statically allocated with enough space to never have to move. If the maximum size of a linear memory is larger than max_size then wasm memory will be dynamically located and may move in memory through growth operations.

Specifying a max_size of 0 means that all memories will be dynamic and may be relocated through memory.grow. Also note that if any wasm memory’s maximum size is below max_size then it will still reserve max_size bytes in the virtual memory space.

Static vs Dynamic Memory

Linear memories represent contiguous arrays of bytes, but they can also be grown through the API and wasm instructions. When memory is grown if space hasn’t been preallocated then growth may involve relocating the base pointer in memory. Memories in Wasmtime are classified in two different ways:

  • static - these memories preallocate all space necessary they’ll ever need, meaning that the base pointer of these memories is never moved. Static memories may take more virtual memory space because of pre-reserving space for memories.

  • dynamic - these memories are not preallocated and may move during growth operations. Dynamic memories consume less virtual memory space because they don’t need to preallocate space for future growth.

Static memories can be optimized better in JIT code because once the base address is loaded in a function it’s known that we never need to reload it because it never changes, memory.grow is generally a pretty fast operation because the wasm memory is never relocated, and under some conditions bounds checks can be elided on memory accesses.

Dynamic memories can’t be quite as heavily optimized because the base address may need to be reloaded more often, they may require relocating lots of data on memory.grow, and dynamic memories require unconditional bounds checks on all memory accesses.

Should you use static or dynamic memory?

In general you probably don’t need to change the value of this property. The defaults here are optimized for each target platform to consume a reasonable amount of physical memory while also generating speedy machine code.

One of the main reasons you may want to configure this today is if your environment can’t reserve virtual memory space for each wasm linear memory. On 64-bit platforms wasm memories require a 6GB reservation by default, and system limits may prevent this in some scenarios. In this case you may wish to force memories to be allocated dynamically meaning that the virtual memory footprint of creating a wasm memory should be exactly what’s used by the wasm itself.

For 32-bit memories a static memory must contain at least 4GB of reserved address space plus a guard page to elide any bounds checks at all. Smaller static memories will use similar bounds checks as dynamic memories.

Default

The default value for this property depends on the host platform. For 64-bit platforms there’s lots of address space available, so the default configured here is 4GB. WebAssembly linear memories currently max out at 4GB which means that on 64-bit platforms Wasmtime by default always uses a static memory. This, coupled with a sufficiently sized guard region, should produce the fastest JIT code on 64-bit platforms, but does require a large address space reservation for each wasm memory.

For 32-bit platforms this value defaults to 1GB. This means that wasm memories whose maximum size is less than 1GB will be allocated statically, otherwise they’ll be considered dynamic.

Static Memory and Pooled Instance Allocation

When using the pooling instance allocator memories are considered to always be static memories, they are never dynamic. This setting configures the size of linear memory to reserve for each memory in the pooling allocator.

Indicates that the “static” style of memory should always be used.

This configuration option enables selecting the “static” option for all linear memories created within this Config. This means that all memories will be allocated up-front and will never move. Additionally this means that all memories are synthetically limited by the Config::static_memory_maximum_size option, irregardless of what the actual maximum size is on the memory’s original type.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size.

Configures the size, in bytes, of the guard region used at the end of a static memory’s address space reservation.

Note: this value has important performance ramifications, be sure to understand what this value does before tweaking it and benchmarking.

All WebAssembly loads/stores are bounds-checked and generate a trap if they’re out-of-bounds. Loads and stores are often very performance critical, so we want the bounds check to be as fast as possible! Accelerating these memory accesses is the motivation for a guard after a memory allocation.

Memories (both static and dynamic) can be configured with a guard at the end of them which consists of unmapped virtual memory. This unmapped memory will trigger a memory access violation (e.g. segfault) if accessed. This allows JIT code to elide bounds checks if it can prove that an access, if out of bounds, would hit the guard region. This means that having such a guard of unmapped memory can remove the need for bounds checks in JIT code.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size.

How big should the guard be?

In general, like with configuring static_memory_maximum_size, you probably don’t want to change this value from the defaults. Otherwise, though, the size of the guard region affects the number of bounds checks needed for generated wasm code. More specifically, loads/stores with immediate offsets will generate bounds checks based on how big the guard page is.

For 32-bit memories a 4GB static memory is required to even start removing bounds checks. A 4GB guard size will guarantee that the module has zero bounds checks for memory accesses. A 2GB guard size will eliminate all bounds checks with an immediate offset less than 2GB. A guard size of zero means that all memory accesses will still have bounds checks.

Default

The default value for this property is 2GB on 64-bit platforms. This allows eliminating almost all bounds checks on loads/stores with an immediate offset of less than 2GB. On 32-bit platforms this defaults to 64KB.

Errors

The Engine::new method will return an error if this option is smaller than the value configured for Config::dynamic_memory_guard_size.

Configures the size, in bytes, of the guard region used at the end of a dynamic memory’s address space reservation.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size

For more information about what a guard is, see the documentation on Config::static_memory_guard_size.

Note that the size of the guard region for dynamic memories is not super critical for performance. Making it reasonably-sized can improve generated code slightly, but for maximum performance you’ll want to lean towards static memories rather than dynamic anyway.

Also note that the dynamic memory guard size must be smaller than the static memory guard size, so if a large dynamic memory guard is specified then the static memory guard size will also be automatically increased.

Default

This value defaults to 64KB.

Errors

The Engine::new method will return an error if this option is larger than the value configured for Config::static_memory_guard_size.

Configures the size, in bytes, of the extra virtual memory space reserved after a “dynamic” memory for growing into.

For the difference between static and dynamic memories, see the Config::static_memory_maximum_size

Dynamic memories can be relocated in the process’s virtual address space on growth and do not always reserve their entire space up-front. This means that a growth of the memory may require movement in the address space, which in the worst case can copy a large number of bytes from one region to another.

This setting configures how many bytes are reserved after the initial reservation for a dynamic memory for growing into. A value of 0 here means that no extra bytes are reserved and all calls to memory.grow will need to relocate the wasm linear memory (copying all the bytes). A value of 1 megabyte, however, means that memory.grow can allocate up to a megabyte of extra memory before the memory needs to be moved in linear memory.

Note that this is a currently simple heuristic for optimizing the growth of dynamic memories, primarily implemented for the memory64 proposal where all memories are currently “dynamic”. This is unlikely to be a one-size-fits-all style approach and if you’re an embedder running into issues with dynamic memories and growth and are interested in having other growth strategies available here please feel free to open an issue on the Wasmtime repository!

Default

For 64-bit platforms this defaults to 2GB, and for 32-bit platforms this defaults to 1MB.

Indicates whether a guard region is present before allocations of linear memory.

Guard regions before linear memories are never used during normal operation of WebAssembly modules, even if they have out-of-bounds loads. The only purpose for a preceding guard region in linear memory is extra protection against possible bugs in code generators like Cranelift. This setting does not affect performance in any way, but will result in larger virtual memory reservations for linear memories (it won’t actually ever use more memory, just use more of the address space).

The size of the guard region before linear memory is the same as the guard size that comes after linear memory, which is configured by Config::static_memory_guard_size and Config::dynamic_memory_guard_size.

Default

This value defaults to true.

Configure the version information used in serialized and deserialzied crate::Modules. This effects the behavior of crate::Module::serialize(), as well as crate::Module::deserialize() and related functions.

The default strategy is to use the wasmtime crate’s Cargo package version.

Configure wether wasmtime should compile a module using multiple threads.

Disabling this will result in a single thread being used to compile the wasm bytecode.

By default parallel compilation is enabled.

Configures whether compiled artifacts will contain information to map native program addresses back to the original wasm module.

This configuration option is true by default and, if enables, generates the appropriate tables in compiled modules to map from native address back to wasm source addresses. This is used for displaying wasm program counters in backtraces as well as generating filenames/line numbers if so configured as well (and the original wasm module has DWARF debugging information present).

Configures whether copy-on-write memory-mapped data is used to initialize a linear memory.

Initializing linear memory via a copy-on-write mapping can drastically improve instantiation costs of a WebAssembly module because copying memory is deferred. Additionally if a page of memory is only ever read from WebAssembly and never written too then the same underlying page of data will be reused between all instantiations of a module meaning that if a module is instantiated many times this can lower the overall memory required needed to run that module.

This feature is only applicable when a WebAssembly module meets specific criteria to be initialized in this fashion, such as:

  • Only memories defined in the module can be initialized this way.
  • Data segments for memory must use statically known offsets.
  • Data segments for memory must all be in-bounds.

Modules which do not meet these criteria will fall back to initialization of linear memory based on copying memory.

This feature of Wasmtime is also platform-specific:

  • Linux - this feature is supported for all instances of Module. Modules backed by an existing mmap (such as those created by Module::deserialize_file) will reuse that mmap to cow-initialize memory. Other instance of Module may use the memfd_create syscall to create an initialization image to mmap.
  • Unix (not Linux) - this feature is only supported when loading modules from a precompiled file via Module::deserialize_file where there is a file descriptor to use to map data into the process. Note that the module must have been compiled with this setting enabled as well.
  • Windows - there is no support for this feature at this time. Memory initialization will always copy bytes.

By default this option is enabled.

A configuration option to force the usage of memfd_create on Linux to be used as the backing source for a module’s initial memory image.

When Config::memory_init_cow is enabled, which is enabled by default, module memory initialization images are taken from a module’s original mmap if possible. If a precompiled module was loaded from disk this means that the disk’s file is used as an mmap source for the initial linear memory contents. This option can be used to force, on Linux, that instead of using the original file on disk a new in-memory file is created with memfd_create to hold the contents of the initial image.

This option can be used to avoid possibly loading the contents of memory from disk through a page fault. Instead with memfd_create the contents of memory are always in RAM, meaning that even page faults which initially populate a wasm linear memory will only work with RAM instead of ever hitting the disk that the original precompiled module is stored on.

This option is disabled by default.

Configures the “guaranteed dense image size” for copy-on-write initialized memories.

When using the Config::memory_init_cow feature to initialize memory efficiently (which is enabled by default), compiled modules contain an image of the module’s initial heap. If the module has a fairly sparse initial heap, with just a few data segments at very different offsets, this could result in a large region of zero bytes in the image. In other words, it’s not very memory-efficient.

We normally use a heuristic to avoid this: if less than half of the initialized range (first non-zero to last non-zero byte) of any memory in the module has pages with nonzero bytes, then we avoid creating a memory image for the entire module.

However, if the embedder always needs the instantiation-time efficiency of copy-on-write initialization, and is otherwise carefully controlling parameters of the modules (for example, by limiting the maximum heap size of the modules), then it may be desirable to ensure a memory image is created even if this could go against the heuristic above. Thus, we add another condition: there is a size of initialized data region up to which we always allow a memory image. The embedder can set this to a known maximum heap size if they desire to always get the benefits of copy-on-write images.

In the future we may implement a “best of both worlds” solution where we have a dense image up to some limit, and then support a sparse list of initializers beyond that; this would get most of the benefit of copy-on-write and pay the incremental cost of eager initialization only for those bits of memory that are out-of-bounds. However, for now, an embedder desiring fast instantiation should ensure that this setting is as large as the maximum module initial memory content size.

By default this value is 16 MiB.

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