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This document was last updated for go1.16 but probably still applies to older/newer versions for the most parts.
Stack Traces in Go
Stack traces play a critical role in Go profiling. So let's try to understand them to see how they might impact the overhead and accuracy of our profiles.
Introduction
All Go profilers work by collecting samples of stack traces and putting them into pprof profiles. Ignoring some details, a pprof profile is just a frequency table of stack traces like shown below:
| stack trace | count |
|---|---|
| main;foo | 5 |
| main;foo;bar | 3 |
| main;foobar | 4 |
Let's zoom in on the first stack trace in the table above: main;foo. A Go developer will usually be more familiar with seeing a stack trace like this as rendered by panic() or runtime.Stack() as shown below:
goroutine 1 [running]:
main.foo(...)
/path/to/go-profiler-notes/examples/stack-trace/main.go:9
main.main()
/path/to/go-profiler-notes/examples/stack-trace/main.go:5 +0x3a
This text format has been described elsewhere so we won't discuss the details of it here. Instead we'll dive deeper into the source of this data.
Goroutine Stack
As the name implies, stack traces originate from "the stack". Even so the details vary, most programming languages have a concept of a stack and use it to store things like local variables, arguments, return values and return addresses. Generating a stack trace usually involves navigating the stack in a process known as Unwinding that will be described in more detail later on.
Platforms like x86-64 define a stack layout and calling convention for C and encourage other programming languages to adopt it for interoperability. Go doesn't follow these conventions, and instead uses its own idiosyncratic calling convention. Future versions of Go (1.17?) will adopt a more traditional register-based convention that will improve performance . However compatibility with platform conventions is not planned as it would negatively impact goroutine scalability.
Even today, Go's stack layout is slightly different on different platforms. To keep things manageable, we'll assume that we're on x86-64 for the remainder of this note.
Stack Layout
Now let's take a closer look at the stack. Every goroutine has its own stack that is at least 2 KiB and grows from a high memory address towards lower memory addresses. This can be a bit confusing and is mostly a historical convention from a time when memory was so limited that one had to worry about the stack colliding with other memory regions used by the program.
There is a lot going on in the picture above, but for now let's focus on the things highlighted in red. To get a stack trace, the first thing we need is the current program counter (pc) which identifies the function that is currently being executed. This is found in a CPU register called rip (instruction pointer register) that points to another region of memory that holds the executable machine code of our program. If you're not familiar with registers, you can think of them as special CPU variables that are incredibly fast to access.
The next step is to find the program counters of all the callers of the current function, i.e. all the return address (pc) values that are also highlighted in red. There are various techniques for doing this, which are described in the Unwinding section. The end result is a list of program counters that represent a stack trace. In fact, it's exactly the same list you can get from runtime.Callers() within your program. Last but not least, these pc values are translated into human readable file/line/function names as described in the Symbolization section below.
Real Example
Looking at pretty pictures can be a good way to get a high level understanding of the stack, but it has its limits. Sometimes you need to look at the raw bits & bytes in order to get a full understanding. If you're not interested in that, feel free to skip ahead to the next section.
To take a look at the stack, we'll use delve which is a wonderful debugger for Go. In order to inspect the stack, I wrote a script called stackannotate.star that can used to print the annotated stack for a simple example program:
$ dlv debug ./examples/stackannotate/main.go
Type 'help' for list of commands.
(dlv) source delve/stackannotate.star
(dlv) continue examples/stackannotate/main.go:19
Breakpoint 1 set at 0x1067d94 for main.bar() ./examples/stackannotate/main.go:19
> main.bar() ./examples/stackannotate/main.go:19 (hits goroutine(1):1 total:1) (PC: 0x1067d94)
14: }
15:
16: func bar(a int, b int) int {
17: s := 3
18: for i := 0; i < 100; i++ {
=> 19: s += a * b
20: }
21: return s
22: }
(dlv) stackannotate
regs addr offset value explanation
c00004c7e8 0 0 ?
c00004c7e0 -8 0 ?
c00004c7e8 -16 0 ?
c00004c7e0 -24 0 ?
c00004c7d8 -32 1064ac1 return addr to runtime.goexit
c00004c7d0 -40 0 frame pointer for runtime.main
c00004c7c8 -48 1082a28 ?
c00004c7c0 -56 c00004c7ae ?
c00004c7b8 -64 c000000180 var g *runtime.g
c00004c7b0 -72 0 ?
c00004c7a8 -80 100000000000000 var needUnlock bool
c00004c7a0 -88 0 ?
c00004c798 -96 c00001c060 ?
c00004c790 -104 0 ?
c00004c788 -112 c00001c060 ?
c00004c780 -120 1035683 return addr to runtime.main
c00004c778 -128 c00004c7d0 frame pointer for main.main
c00004c770 -136 c00001c0b8 ?
c00004c768 -144 0 var i int
c00004c760 -152 0 var n int
c00004c758 -160 0 arg ~r1 int
c00004c750 -168 1 arg a int
c00004c748 -176 1067c8c return addr to main.main
c00004c740 -184 c00004c778 frame pointer for main.foo
c00004c738 -192 c00004c778 ?
c00004c730 -200 0 arg ~r2 int
c00004c728 -208 2 arg b int
c00004c720 -216 1 arg a int
c00004c718 -224 1067d3d return addr to main.foo
bp --> c00004c710 -232 c00004c740 frame pointer for main.bar
c00004c708 -240 0 var i int
sp --> c00004c700 -248 3 var s int
The script isn't perfect and there are some addresses on the stack that it's unable to automatically annotate for now (contributions welcome!). But generally speaking, you should be able to use it to check your understanding against the abstract stack drawing that was presented earlier.
If you want to try it out yourself, perhaps modify the example program to spawn main.foo() as a goroutine and observe how that impacts the stack.
Unwinding
Unwinding (or stack walking) is the process of collecting all the return addresses (see red elements in Stack Layout) from the stack. Together with the current instruction pointer register (rip) they form a list of program counter (pc) values that can be turned into a human readable stack trace via Symbolization.
The Go runtime, including the builtin profilers, exclusively use .gopclntab for unwinding. However, we'll start with describing Frame Pointer unwinding first, because it is much easier to understand and might become the default in the future.
Frame Pointer
Frame pointer unwinding is the simple process of following the base pointer register (rbp) to the first frame pointer on the stack which points to the next frame pointer and so on. In other words, it is following the orange lines in the Stack Layout graphic. For each visited frame pointer, the return address (pc) sitting 8 bytes above the frame pointer is collected along the way.
The main downside to frame pointers is that they add some performance overhead to every function call during normal program execution. Because of this compilers such as gcc offer options such as -fomit-frame-pointers to omit them for better performance. However, it's a devil's bargain: It gives you small performance win right away, but it reduces your ability to debug and diagnose performance issues in the future. Because of this the general advice is:
Always compile with frame pointers. Omitting frame pointers is an evil compiler optimization that breaks debuggers, and sadly, is often the default. – Brendan Gregg
Despite this Go used to omit frame pointers which caused interoperability issues with third party debuggers and profilers such as Linux perf. Fortunately the Go developers recognized the issue and since Go 1.7 frame pointers are always included for 64bit binaries . And unlike gcc, the Go compiler offers no option to disable frame pointers, which is a good thing in my opinion.
Anyway, if all of this is too abstract for you, and you'd like to see some code, here is an alternative runtime.Callers() implementation that uses frame pointer unwinding instead of .gopclntab. The simplicity should speak for itself when compared to the alternative methods described below. It should also be clear that frame pointer unwinding has O(N) time complexity where N is the number of stack frames that need to be traversed.
.gopclntab
Since frame pointers are only available on 64bit platforms, Go has to use a more complicated approach to enable stack tracing in a cross-platform manner for now. The nitty-gritty of this has been described by Russ Cox in his Go 1.2 Runtime Symbol Information document, but I'll try to summarize the key idea.
Simply speaking, the idea is to take the current program count (pc) of a goroutine and look it up in the sorted <pc, func> table in the .gopclntab section embedded in the binary. If you want, you can get access to this table using the debug/elf or (debug/macho) package yourself.
DWARF
An unwinder that is several hundred lines long is simply not even remotely interesting to me. – Linus Torvalds
... so as long as the Go compiler emits valid DWARF symbols, then anything is possible. Erm, by "anything" I mean, specificially, "unwinding". Many things are still not possible, even with DWARF (and more frequently than you might think, many things are not possible precisely because DWARF exists-- true love, for instance) – David Sanchez (my colleague, who knows more about DWARF than any sane person probably should)
ORC, LBR, etc.
Symbolization
To be written ...
Overhead
To be written ...
- Frame Pointers: 2% for Go, Linux 5-10%
In fact, until recently, the Linux Kernel rejected anything other than frame pointers for stack unwinding despite causing up to 5-10% slowdown for some workloads. Go reported the overhead as 2% in the Go 1.7 release notes.
File size.
Accuracy
To be written ...
Function Inlining
Frame Pointer Race Condition
To be written ...
Goroutine Stack Truncation
To be written ...
cgo
To be written ...
pprof Labels
To be written ...
History
In order to support 3rd profilers such as perf the Go 1.7 (2016-08-15) release started to enable frame pointers by default for 64bit binaries.
Disclaimers
I'm felixge and work at Datadog on Continuous Profiling for Go. You should check it out. We're also hiring : ).
The information on this page is believed to be correct, but no warranty is provided. Feedback is welcome!