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Memory model and memory usage tunning

The memory model

The memory model of WAMR can be basically described as below:

Note:

  • global heap: the heap to allocate memory for runtime data structures, including wasm module, wasm module instance, exec env, wasm operand stack and so on. It is initialized by wasm_runtime_init or wasm_runtime_full_init. And for wasm_runtime_full_init, developer can specify the memory allocation mode with RuntimeInitArgs *init_args: allocate memory from a user defined byte buffer, from user defined allocation function, or from the platform's os_malloc function. Refer to wasm_export.h and Embedding WAMR guideline for more details. And developer can use wasm_runtime_malloc/wasm_runtime_free to allocate/free memory from/to the global heap.
  • wasm operand stack: the stack to store the operands required by wasm bytecodes as WebAssembly is based on a stack machine. If the exec_env is created by developer with wasm_runtime_create_exec_env, then its size is specified by wasm_runtime_create_exec_env, otherwise if the exec_env is created by runtime internally, e.g. by wasm_application_execute_main or wasm_application_execute_func, then the size is specified by wasm_runtime_instantiate.
  • linear memory: a contiguous, mutable array of raw bytes. It is created with an initial size but might be grown dynamically. For most compilers, e.g. wasi-sdk, emsdk, rustc or asc, normally it includes three parts, data area, auxiliary stack area and heap area. For wasi-sdk, the initial/max size can be specified with -Wl,--initial-memory=n1,--max-memory=n2, for emsdk, the initial/max size can be specified with -s INITIAL_MEMORY=n1 -s MAXIMUM_MEMORY=n2 -s ALLOW_MEMORY_GROWTH=1 or -s TOTAL_MEMORY=n, and for asc, they can be specified with --initialMemory and --maximumMemory flags.
  • aux stack: the auxiliary stack resides in linear memory to store some temporary data when calling wasm functions, for example, calling a wasm function with complex struct arguments. For wasi-sdk, the size can be specified with -z stack-size=n, for emsdk, the size can be specified with -s TOTAL_STACK=n.
  • app heap and libc heap: the heap to allocate memory for wasm app, note that app heap is created only when the malloc/free functions (or __new/__release functions for AssemblyScript) are not exported and runtime can not detect the libc heap. To export the malloc/free functions, for wasi-sdk and emsdk, developer can use -Wl,--export=malloc -Wl,--export=free options, for asc, developer can use --exportRuntime option. For app heap, the size is specified by wasm_runtime_instantiate. It is recommended to export the malloc/free functions and disable app heap in single thread mode, and for multi-threading, as the libc heap isn't thread-safe, it is recommended to remove the dlmalloc.o from libc.a for wasi-sdk and use -s MALLOC="none" for emsdk, refer to WAMR pthread library for more details. And developer can use wasm_runtime_module_malloc/wasm_runtime_module_free to allocate/free memory from/to app heap (or libc heap if malloc/free functions are exported).
  • __data_end global and __heap_base global: two globals exported by wasm application to indicate the end of data area and the base address of libc heap. For WAMR, it is recommended to export them as when there are no possible memory grow operations, runtime will truncate the linear memory into the size indicated by __heap_base, so as to reduce the footprint, or at least one page (64KB) is required by linear memory.

Tune the memory usage

Normally there are some methods to tune the memory usage:

  • set the global heap size with wasm_runtime_full_init
  • set the wasm operand stack size with wasm_runtime_create_exec_env or wasm_runtime_instantiate
  • set the linear memory size
  • set the auxiliary stack size
  • export malloc/free functions to use libc heap and disable app heap
  • set the app heap size with wasm_runtime_instantiate
  • use nostdlib mode, add -Wl,--strip-all: refer to How to reduce the footprint of building wasm app for more details
  • use XIP mode, refer to WAMR XIP (Execution In Place) feature introduction for more details