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memory_usage.md

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Memory usage estimation for a module

This document aims to provide information useful to make a rough estimation of necessary memory to execute a WASM module.

Instead of trying to cover every possible configurations, the following configuration is assumed in this document:

  • Module is built with wasi-sdk
  • Module is loaded with wasm_runtime_load
  • AOT is used
  • WASI is used
  • libc heap is used
  • app heap is not used
  • The pthread implementation in wasi-libc, which is based on wasi-threads (WASM_ENABLE_LIB_WASI_THREADS) might be used
  • The another pthread implementation (WASM_ENABLE_LIB_PTHREAD) is not used

Module

The memory to store the module binary is allocated by the embedder and passed to wasm_runtime_load. While WAMR owns the buffer, WAMR might make in-place modifications to its contents.

Loaded module and its instances

Many of data structures for module and instances are allocated from the global heap. (aka. wasm_runtime_malloc)

AOT code section

Memory to load AOT machine code section.

Because this memory needs to be executable, depending on platforms, it's allocated from a separate allocator. For example, mmap and mprotect are used on POSIX-like platforms.

Linear memory

A WASM linear memory is either shared or non-shared.

A WASM linear memory has min and max sizes. (They correspond to wasm-ld's --init-memory and --max-memory options.) They are in the number of WASM pages, each of which is of 65536 bytes. The max is optional for non-shared memory. When omitted, it effectively means unlimited.

The linear memory is allocated via os_mmap and os_mem_commit/os_mprotect.

The min size of memory is allocated on instantiation. It can later grow up to the max size via the memory.grow instruction.

Libc heap

The libc heap is the last (highest address) part of linear memory, which might be dynamically grown with memory.grow instruction, when necessary to serve memory allocations within the module.

App heap

Not used for the above mentioned configuration.

You can safely disable the app heap creation by specifying 0 for the heap_size argument of wasm_runtime_instantiate. (It's automatically disabled if malloc/free are exported from the module.)

WASM stack

Operand stack is not used for AOT.

However, a small amount of WASM stack is used for call frames when certain features are enabled. (WASM_ENABLE_DUMP_CALL_STACK or WASM_ENABLE_PERF_PROFILING)

It's allocated from the global heap.

You can specify its size with the stack_size argument of wasm_runtime_instantiate and wasm_runtime_create_exec_env. (1 is the minimum because 0 means the default.)

AUX stack (aka. C shadow stack)

For the main thread, it's a part of the linear memory, between __data_end and __heap_base symbols. You can control the size of this stack with wasm-ld's -z stack-size option.

For threads created by pthread_create, libc allocates the stack for them dynamically from the libc heap. The size of this stack is inherited from the main thread's one unless overwritten with pthread_attr_setstacksize etc.

WAMR tries to detect overflow/underflow when updating the stack pointer global. For threads created by pthread_create, the detection mechanism is disabled as of writing this.

Native stack

The stack of the host environment thread which runs WAMR.

For threads created by pthread_create, WAMR automatically creates host threads to run those WASM threads. The stack size of these host threads are controlled by a build-time configuration. (APP_THREAD_STACK_SIZE_DEFAULT)

In some configurations, runtime overflow can be detected using hardware traps. (OS_ENABLE_HW_BOUND_CHECK)

In some configurations, explicit overflow detection logic can be emitted into AOT modules themselves. (cf. os_thread_get_stack_boundary, check_stack_boundary, wamrc --stack-bounds-checks=1/0)

Memory profiling

You can collect and dump detailed information about memory usage by actually running a module with the WASM_ENABLE_MEMORY_PROFILING build-time option.