# QDirStat Debugging Tips
## Problem: Directory reading is too fast to debug anything
### Symptom
Only the very first time, reading a large directory like / takes long enough to
experiment with anything in the tree widget. Any subsequent time, it's just too
fast - one or two seconds, and *bam* it's finished already.
### Reason
The Linux kernel has become incredibly good at caching directory information,
and today's PCs have so much RAM that the kernel tends to use a large amount of
it as cache - files, inodes, dentries (directories). Once a directory is read,
it remains in the cache for a long time, so the speedup upon a subsequent read
is enormous.
### Fix / Workaround
Drop the kernel caches (as root):
su -
echo 3 > /proc/sys/vm/drop_caches
or
echo 3 | sudo tee /proc/sys/vm/drop_caches
(`sudo echo 3 > /proc/sys/vm/drop_caches` would NOT work because your non-root
shell would do the I/O redirection, so it would not have sufficient privileges)
### Reference
https://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system
## Profiling QDirStat with Valgrind / KCachegrind
### Prerequisites
Install vallgrind and kcachgrind:
sudo apt install valgrind kcachegrind
### Profile
Start QDirStat with the valgrind profiler:
valgrind --tool=callgrind --dump-instr=yes --simulate-cache=yes --collect-jumps=yes qdirstat ~
Don't use a huge directory since the profiling slows down everything
considerably!
The results go to a file `callgrind.out.*` in that directory. Visualize with
kcachegrind
### Reference
https://developer.mantidproject.org/ProfilingWithValgrind.html
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