[lldb-dev] Dlopen extremely slow while LLDB is attached

Scott Funkenhauser via lldb-dev lldb-dev at lists.llvm.org
Tue Apr 24 12:26:43 PDT 2018

Hey guys,

I'm trying to track down an issue I'm seeing where dlopen takes
significantly longer to execute when LLDB is attached vs GDB (I've attached
a simple program that I used to reproduce the issue).
I was wondering if anybody had any idea what might be contributing to the
additional execution time?

Running without any debugger attached:
$ ./lldb-load-sample
Handle: 0x555555768c80
Done loading. 848.27ms
$ ./lldb-load-sample
Handle: 0x555555768c80
Done loading. 19.6047ms

I noticed that the first run was significantly slower than any subsequent
runs. Most likely due to some caching in Linux.

(lldb) file lldb-load-sample
Current executable set to 'lldb-load-sample' (x86_64).
(lldb) run
Process 82804 launched: '/lldb-load-sample' (x86_64)
Handle: 0x555555768c80
Done loading. 5742.78ms
Process 82804 exited with status = 0 (0x00000000)
(lldb) run
Process 83454 launched: '/lldb-load-sample' (x86_64)
Handle: 0x555555768c80
Done loading. 19.4184ms
Process 83454 exited with status = 0 (0x00000000)

I noticed that subsequent runs were much faster (most likely due to some
caching in Linux / LLDB), but that isn't relevant in my situation. Exiting
LLDB and starting a new LLDB process still has an extremely long first run
(In this case ~5.5s). There are other real world cases (initializing Vulkan
which does a bunch of dlopens) where this can add 10s of seconds really
slowing down iteration time.

For GDB:
(gdb) file lldb-load-sample
Reading symbols from a.out...done.
(gdb) run
Starting program: /lldb-load-sample
Handle: 0x555555768c80
Done loading. 79.7276ms
[Inferior 1 (process 85063) exited normally]
(gdb) run
Starting program: /lldb-load-sample
Handle: 0x555555768c80
Done loading. 80.325ms
[Inferior 1 (process 85063) exited normally]

As you can see the first run is slightly slower than running without a
debugger attached, but it's not enough to be noticeable.

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