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.
For LLDB:
(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.
Thanks,
Scott
#include <dlfcn.h>
#include <iostream>
#include <chrono>
#include <unistd.h>
int main() {
struct timespec start;
clock_gettime(CLOCK_MONOTONIC, &start);
// Used a stripped version of liblldb.so, as the unstripped version is 1.7GB
// and takes too long to be a useful reproduction case.
void* handle = dlopen("liblldb.so", RTLD_NOW | RTLD_GLOBAL);
struct timespec stop;
clock_gettime(CLOCK_MONOTONIC, &stop);
if (handle != 0) {
dlclose(handle);
}
long seconds, nseconds;
seconds = stop.tv_sec - start.tv_sec;
nseconds = stop.tv_nsec - start.tv_nsec;
float duration = seconds * 1000 + nseconds / 1000000.0f;
std::cout << "Handle: " << handle << std::endl;
std::cout << "Done loading. " << duration << "ms" << std::endl;
}
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