Debugging

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This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

What-does-it-spit-out debugging

print statements

For simple bugs this is simple, effective, though you should later either take them out or turn them into filterable logging instead.


Logging

Live debugging

From the IDE

Terms

  • Step into: Go into the called function's workings. (does the same as 'over' if not a call)
  • Step over: Perform evaluation/call, but don't go into it, and don't display ("I trust this, give me the next thing at the current level")
  • Step out: "I've seen enough of this function's guts, go (back) to its caller."

Note: while 'over' may suggest skipping code execution, none of the options skip execution - it just means skipping its debugger display.


Debug and performance inspection tools

strace

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

(On solaris, see truss)


finds syscalls a program makes, prints to stderr.

Great for debugging when programs don't say enough, and for workload characterization for things that seem slow.

strace can run a command for you, or attach to a running process via its PID (-p)

For things that fork off, you'll want -f

To compare time spent in each syscall, use
-c
.


You can filter what syscalls are reported using their names, and with a few existing categorizations:

  • file: any file-related calls (access, state, read, write, etc.)
  • desc: file descriptor related
  • process: process management, e.g. fork, wait,l exec
  • network
  • signal
  • ipc
  • memory


For example:

# only mention open() calls
strace -eopen ls     
# see whether there are a bulk of stat()s done (for how many, use -c)
strace -estat,fstat,lstat,newfstatat  find /tmp
# see what progrms are invoked
strace -f -eprocess  service apache2 status 
# summarize time spent in each syscall
strace -c  ls -l /proc


# comparing e.g. ls on small and large dirs
strace -c ls -l /data/largedir
For example, say you notice that
ls -l
seems to do a lot of lgetxattr()s.
-c
would show that for large directories getdents dwarfs everything, and in general it's nearly free assuming that you've just lstat()ted.

ltrace

An execution shim that shows calls into shared libraries.

https://en.wikipedia.org/wiki/Ltrace

pcstat

Page Cache statistics

For individual files, find how much is in the page cache

See also :



echo l > /proc/sysrq-trigger

...gives the backtrace for what's on each CPU.

Writes it to dmesg, which often also goes to something like kern.log, syslog / messages

kernel profiler sources

kprobes

(kernel functions)

kernel tracepoints

dtrace

userspace profiler sources

uprobes

(userspace functions)

USDT (Userland Statically Defined Tracing)

A way of embedding dtrace probes into an app


LTTng userspace

profiler tools / frontends

(Note that some basic things can be gotten from /proc)


dtrace (solaris, also freebsd, linux, osx, smartos)

Very cool tools originating on Solaris.

The rest are ports, some close to the Solaris version, some further away.


perf (linux)

a.k.a. perf_events


https://perf.wiki.kernel.org/index.php/Main_Page

http://www.brendangregg.com/perf.html


systemtap (linux)

systemtap seems to imitate dtrace decently


https://en.wikipedia.org/wiki/SystemTap

https://sourceware.org/systemtap/

https://sourceware.org/systemtap/tutorial.pdf

ftrace

BPF (linux, bsd, more?)

extended Berkeley Packet Filter (eBPF, also just BPF) originated in network packet filtering, but grew so flexible that it is also very useful for system tracing (so now needs a new name).

The thing that allows it to be safe, flexible, and fast is that it compiles user requests into sandboxed bytecode in the kernel.



https://en.wikipedia.org/wiki/Berkeley_Packet_Filter

LTTng

OProfile

https://en.wikipedia.org/wiki/OProfile

Unsorted

Instruments (osx)

https://en.wikipedia.org/wiki/Instruments_(software)

Xperf (windows)

https://blogs.msdn.microsoft.com/ntdebugging/2008/04/03/windows-performance-toolkit-xperf/


flame graphs

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

For context (though I think they came later): Firefox and Chrome has Flame Charts, which basically just stack over time. Since this is usually shown at a scale useful to find code that slow down the framerate, and JS is single-threaded, this tends to look fairly clean.


CPU flame graphs instead are primarily geared to show how common a particular stack is.

While it does not show a timeline (x axis is sorted alphabetically), or really collect amount of time spent, the amount of times the sample mentions the specific function is still a good indication of where most time is spent (particularly when things are unusually slow, which often means one function is dominating).

It's regularly used with statistical profiling, which takes samples frequently enough (though ideally not in step with anything else), which for specific functions is more approximate than cycle-accurate profiling , but can be a more real-world-accurate view of the general execution due to the profiling having minimal impact itself (on CPU, but also cache).


The file format is basically just lines of (verify)

stackframe;stackframe;stackframe count

Notes:

  • the count is technically optional, but can make these files a lot shorter.
  • a second count column is used for the differential variant
  • variants use some annotation on the function name
  • See flamegraph.pl's comments for more detail


It was recently popularized by Brendan Gregg, who specializes in digging into lower levels of system so there are various relevant tools for kernel use, like syscalls or network efficiency - basically scripts that use dtrace, perf, SystemTap, etc.

Various runtimes can also output things useful for these.

There is sometimes value to producing them from code, e.g. get a better idea of database use with entries like stack;SQL milliseconds


Variations on the theme:

  • Off-CPU flame graphs - intercept mainly the syscalls related to why something is not scheduled for CPU, such as general scheduling, IO,
  • Hot/Cold flame graphs - combine CPU and Off-CPU, to get an indication of how a program is scheduled
  • Memory flame graph flame graphs - intercept mainly the syscalls related to memory allocation
  • differential flame graphs - mainly meant to indicate performance regressions/improvements; takes two CPU flame graphs, and show changes in time spent
since color is based on newstackvalue-oldstackvalue, this has issues like renames making things disappear


The original tool is [1], and see also [2], and there are various that imitate it, like [https://www.npmjs.com/package/stackvis stackvis]

http://www.brendangregg.com/flamegraphs.html



Unsorted

http://dtrace.org/blogs/brendan/

NVidia profiler tools

Post-mortem debugging

Debug symbols

GNU debugger