Debugging and Profiling

Debugging

As Faasm functions are compiled to WebAssembly and executed using WAVM,any general tips that apply in WAVM also apply to Faasm.

The instructions below assume that you're building Faasm locally as per the local devinstructions and that Faasm's built executables are on your PATH.

Symbols

To understand the output of gdb we will need the function symbols available, which we can do with

inv disas.symbols <user> <func>

This will output the mapping from things like functionDef123 to the names of functions as theyappear in the source.

The output should be at wasm/<user>/<function>/function.symbols.

GDB

You can use gdb to debug wasm functions with the simple_runner CMake target, e.g.

# Normally
simple_runner <user> <func>
# GDB
gdb --args simple_runner <user> <func>

Because the function itself is loaded with the LLVM JIT libraries, GDB doesn't have the symbolsup front, but if we know the function from the symbols output we can set a breakpoint pending sharedlibrary load.

break functionDef1234

You can then use normal gdb functionality, albeit with a lack of source information, e.g. view backtracesinspect stack frames etc.

Profiling

WAVM uses the LLVM JIT libraries to load and execute code at runtime. This can be connected to perf eventsso that this JITed code will properly be reported, but it requires a special build of LLVM to run.

To set things up you need to do the following:

• Run the perf.yml Ansible playbook to set up perf
• Create a build of LLVM with perf support by running ./bin/build/llvm_perf (this takes ages)
• Modify the top-level CMakeLists.txt to set FAASM_CUSTOM_LLVM to 1
• Rebuild the target you want to profile

Once this is done you can use perf as described here, i.e.:

# Do the profiling, e.g.
perf record -k 1 simple_runner <user> <function> 500
# OR
perf record -k 1 poly_bench poly_heat-3d 0 10
# Inject the JIT dumps into
perf inject -i perf.data -j -o perf.data.jitted
# View the report
perf report -i perf.data.jitted

Note that if the perf notifier isn't working, check that the code isn't getting excluded by thepre-processor by looking at the WAVM LLVMModule.cpp file.