[RFC] Assign part of wasm address space to a shared heap to share memory among wasm modules with zero-copying about wasm-micro-runtime

[no1wudi]

@no1wudi yes, maybe we can add another option for wamrc to allow to call runtime API to do boundary check in AOT/JIT mode, but I am not sure whether it is good to make it as default mode for the shared-heap/mmap functionality, had better test the performance to see the result first?

The performance overhead does need to be tested. What I can confirm is that if boundary checks are implemented using an if-else if sequence in LLVM IR, it will significantly increase the code size. In some of our applications, the code size could double as a result.

[WenLY1]

Implementation tasks:

• add WAMR_BUILD_SHARED_HEAP and WASM_ENABLE_SHARED_HEAP

• add wamrc --enable-shared-heap

• consider the behavior of import memory in multi-module feature

• adapt address conversion/validation
  aot code boundary check
  wasm_runtime_invoke_native
  wasm_runtime_invoke_native_raw
  wasm_runtime_validate_app_addr
  wasm_runtime_addr_app_to_native
  wasm_runtime_addr_native_to_app
  wasm_runtime_validate_native_addr

• interpreter/fast-jit do boundary check
  classic interp mode
  fast interp mode
  jit

• Support setting shared heap’s size and shared heap’s alloc options in RuntimeInitArgs for wasm_runtime_full_init

• Add check in load module, default memory’s max_memory_size should be no larger than 4G-shared_heap_size, if not, reduce it to 4G-shared_heap_size

• Add API wasm_runtime_shared_malloc/wasm_runtime_shared_free

• App allocates memory from shared heap with API shared_malloc/shared_free
  example

            #include <stdio.h>  
            #include <stdlib.h>  
            extern void *shared_malloc(int size);  
            extern void shared_free(void *ptr);  
            int main()  
            {  
	            int *pa = (int *)malloc(4);  
	            *pa = 4;  
	            printf("pa value is %d\n", *pa);  
	            free(pa);  
	            int *ps = (int *)shared_malloc(4);  
	            *ps = 5;  
	            printf("ps value is %d\n", *ps);  
	            shared_free(ps);  
            } 

#3543

[yamt]

do you mean to have a single global shared memory, which all wasm modules with a linear memory on the system can fully access?
i guess it would be a bit more usuful if you can control it. eg. multiple shared memories, which the embedder can selectively associate to wasm instances.

semantically, is the shared region always treated as if it's a shared memory?
eg. does it prevent some possible optimizations like dead load/store eliminations?

[woodsmc]

Awesome. At the W3C in person event, Deepti presented on providing a mmap function. In her use case this is used to mmap hardware memory into the WASM VM's address space, and could perceivably physically call mmap on the host system. The remapping of linear memory to the mmap space was discussed as well, and idea was to use the lower memory address range as an easily re-mappable memory region with an efficient load / store implementation.

Is it worth while checking in with Deepti, to ensure there is no clashes, kinda wondering if we could end up with the lower chunk of linear memory made available to an mmap instruction, and the uppper chunk reserved from sharing between modules. It might be nice / important to be able to mmap host memory and share it between modules too?

[wenyongh]

do you mean to have a single global shared memory, which all wasm modules with a linear memory on the system can fully access? i guess it would be a bit more usuful if you can control it. eg. multiple shared memories, which the embedder can selectively associate to wasm instances.

Yes, in current discussion, it is supposed to only have a global shared heap, each wasm module can access it, and the shared heap is created during runtime initialization. Your idea sounds reasonable, but then runtime should create the shared heap lazily, the working flow may be like below:

• runtime initialization as normal
• runtime creates shared heap 1
• some instances associate to shared heap 1
• runtime creates shared heap 2
• some instances associate to shared heap 2
• ...

For performance consideration, I think we had better restrict that each wasm instance can only associate to one shared heap, or it will be too complex and might greatly impact performance. How do you think?

semantically, is the shared region always treated as if it's a shared memory? eg. does it prevent some possible optimizations like dead load/store eliminations?

Yes, the shared region is always mapped to the shared heap, and my suggestion is we always use software boundary check for it since we have to add extra check for which region the wasm addr belongs to, so it should be able to prevent dead load elimination optimization.

[yamt]

but then runtime should create the shared heap lazily, the working flow may be like below

yes.

if you want, you can still create the first shared heap on the runtime initialization.
i don't think it simplifies things much though.

I think we had better restrict that each wasm instance can only associate to one shared heap

i agree.

[wenyongh]

Awesome. At the W3C in person event, Deepti presented on providing a mmap function. In her use case this is used to mmap hardware memory into the WASM VM's address space, and could perceivably physically call mmap on the host system. The remapping of linear memory to the mmap space was discussed as well, and idea was to use the lower memory address range as an easily re-mappable memory region with an efficient load / store implementation.

Thanks @woodsmc. Not know how mmap function is used, does it mean that the wasm app can call the mmap function, and runtime maps the mmapped memory to the lower range of wasm memory address space and then change the behavior of wasm load/store accordingly? And can wasm app call mmap function multiple times? If yes, it may impact performance a lot. And not sure why map to lower range of wasm address space: (1) IIUC, the 0 of wasm addr is reserved for the check for C NULL pointer by clang, clang reserves a space from 0 and doesn't put the app's global data at 0, (2) if map to the lower range, then wasm app should reserve a relatively larger space for it, it may be not so convenient for toolchains/developers, at least for clang, developer should add --global-base=n option.

Is it worth while checking in with Deepti, to ensure there is no clashes, kinda wondering if we could end up with the lower chunk of linear memory made available to an mmap instruction, and the uppper chunk reserved from sharing between modules. It might be nice / important to be able to mmap host memory and share it between modules too?

Yes, it would be great if we discuss more with Deepti. I think we should be able to support both the mmap and the shared heap if needed since one uses the lower range of wasm addr space and the other uses the higher range, but maybe we don't need to support mmap when the shared heap is enabled, since runtime can also use mmap function to allocate the memory for shared heap or even provide callback for developer to allocate the memory.

[no1wudi]

I guess that implementing shared memory using either the shared heap or mmap methods will make memory boundary checks more complex. Therefore, I have an idea about boundary checks in #3548.

Perhaps it could serve as the basis for implementing shared memory functionality. What do you think?

[wenyongh]

@no1wudi yes, maybe we can add another option for wamrc to allow to call runtime API to do boundary check in AOT/JIT mode, but I am not sure whether it is good to make it as default mode for the shared-heap/mmap functionality, had better test the performance to see the result first?

[ayakoakasaka]

Yes, in current discussion, it is supposed to only have a global shared heap, each wasm module can access it, and the shared heap is created during runtime initialization. Your idea sounds reasonable, but then runtime should create the shared heap lazily, the working flow may be like below:
runtime initialization as normal
runtime creates shared heap 1
some instances associate to shared heap 1
runtime creates shared heap 2
some instances associate to shared heap 2

Could this association conform to the principles of a component model in the future? Being able to restrict the accessible area per component (or similar concept) would support a variety of use cases.

[wenyongh]

Yes, in current discussion, it is supposed to only have a global shared heap, each wasm module can access it, and the shared heap is created during runtime initialization. Your idea sounds reasonable, but then runtime should create the shared heap lazily, the working flow may be like below:
runtime initialization as normal
runtime creates shared heap 1
some instances associate to shared heap 1
runtime creates shared heap 2
some instances associate to shared heap 2

Could this association conform to the principles of a component model in the future? Being able to restrict the accessible area per component (or similar concept) would support a variety of use cases.

Yes, when component mode is implemented in the future, I think we can also associate part or all of the instances inside the component to a shared heap, and for the latter, we may add an API for the component to associate all its instances to a shared heap. It depends on the requirement.