The flame graph is displayed abnormally

[ktxdgtx]

We introduced Photon and intended to capture flame graphs using perf to analyze some performance issues. However, the flame graph stacks captured look quite strange. There are many "unknown" stacks, and even when the stacks are visible, the call relationships seem odd.

unknown stack:

stack looks so odd

[beef9999]

For a photon thread created by photon::thread_create/photon::thread_create11, the top-most function in its call stack will have the function name "unknown".

Likewise, for the std::thread, the top function is usually clone

[beef9999]

As to the high proportion of [unknown], can you first try perf top -p <pid> with your program, and make sure you have build photon with debug-info(cmake)

[ktxdgtx]

As to the high proportion of [unknown], can you first try perf top -p <pid> with your program, and make sure you have build photon with debug-info(cmake)

Thanks, we compile photon with the compilation parameter -g -ggdb3 -fno-omit-frame-pointer.

Using perf top, I can see a more normal performance dashboard. However, is there a way to generate a relatively normal flame graph? This is because the more commonly method we use to analyze performance issues is perf record + FlameGraph.

[Coldwings]

The problem is caused by Thread11 using bottom of stack storing function and parameters, make it looks not like a formally thread stack.

It might have some solution (like add some separate spaces between paras and stack frame). We will try to fix that.

[dusteye]

The problem is caused by Thread11 using bottom of stack storing function and parameters, make it looks not like a formally thread stack.

It might have some solution (like add some separate spaces between paras and stack frame). We will try to fix that.

Does ‘Thread11’ here refer to the thread created via thread_create11 ?

[lihuiba]

Does ‘Thread11’ here refer to the thread created via thread_create11 ?

Yes, but I don't think that's the reason. We are still investigating the problem.

[dusteye]

Does ‘Thread11’ here refer to the thread created via thread_create11 ?

Yes, but I don't think that's the reason. We are still investigating the problem.

Agree, we do not use thread_create11 and produce the same problem, and boost fiber works just fine.

[lihuiba]

Boost.Context also has a 0x0000000000000000 in ?? () frame in gdb, which I believe is the [unkonwn] in perf.

Breakpoint 1, 0x000000000040bcd0 in jump_fcontext ()
(gdb) bt
#0  0x000000000040bcd0 in jump_fcontext ()
#1  0x000000000040318f in void boost::context::detail::fiber_entry<boost::context::detail::fiber_record<boost::context::fiber, boost::context::basic_fixedsize_stack<boost::context::stack_traits>, test_move()::{lambda(boost::context::fiber&&)#1}> >(boost::context::detail::transfer_t) ()
#2  0x000000000040bcbf in make_fcontext ()
#3  0x0000000000000000 in ?? ()
(gdb)

What is the flame graph like?

[Coldwings]

Does ‘Thread11’ here refer to the thread created via thread_create11 ?

Yes, but I don't think that's the reason. We are still investigating the problem.

Agree, we do not use thread_create11 and produce the same problem, and boost fiber works just fine.

Sorry for the misleading.
I have checked out implementation and no problem building the stack bottom like you said.

The problem is not about the existence of [unknown] trace, but all-stacked unknown frames like a huge stack.
boost.fibers also shows [unknown] frame, but only one frame in the flame plot.

[Coldwings]

Turns out that when recording by perf, use

perf record --call-graph dwarf ....

works well and able to record correct calling stack.
the perf tools use -g resolving call-graph using fp(frame pointer), but leads to weird result.

Once use dwarf mode to resolve call-graph, it will get such flame graph which is correct.

It is a temporary resolve. We will search for better solution works well when using -g or --call-graph fp

[Coldwings]

In PR #937 , we have tried to make stack bottom with well set frame pointer.
Now with -fno-omit-frame-pointer, perf tool with -g able to generate correct flame graph without [unknown] frames or stacked [unknown] parts. Just like shown below

(The [unknown] parts in this graph are symbols from libgtest)

It should worked in all versions from 0.6, still, we do not want to making critical changes for stable versions.

@ktxdgtx @dusteye
You can also try this patch on your working branch.

[dusteye]

In PR #937 , we have tried to make stack bottom with well set frame pointer. Now with -fno-omit-frame-pointer, perf tool with -g able to generate correct flame graph without [unknown] frames or stacked [unknown] parts. Just like shown below

(The [unknown] parts in this graph are symbols from libgtest)

It should worked in all versions from 0.6, still, we do not want to making critical changes for stable versions.

@ktxdgtx @dusteye You can also try this patch on your working branch.

Great! I'll try it.