gprof
gprof is the GNU profiler, suitable for function-level profiling code compiled with C, C++ & Fortran compiled with the GNU compilers. If already compiling your code with GCC/G++, it’s relatively easy to setup and use. As a command-line profiler output is provided in a plaintext format, this can be challenging to interpret for more nuanced and subtle bottlenecks.
QuickStart
1. In order to profile with gprof, you should pass the flag -pg at compile time.
e.g.
# Compile hello_world.c with -pg
gcc -pg -o hello_world hello_world.c
# or
# Configure the cmake project to build with -pg
cmake -DCMAKE_C_FLAGS=-pg -DCMAKE_CXX_FLAGS=-pg -DCMAKE_EXE_LINKER_FLAGS=-pg -DCMAKE_SHARED_LINKER_FLAGS=-pg ..
2. Now you can execute the program compiled with -pg like normal, this will output a profiling dump of the run to gmon.out. The program must exit successfully.
./hello_world
3. Finally, you should pass both gmon.out and the program to gprof, to produce the human-readable output.
# > analysis.txt, pipes the output of the command to file
gprof hello_world gmon.out > analysis.txt
Interpreting Output
The output file analysis.txt can be opened with a text editor, it contains several sections.
Flat Profile
This first section provides a table that identifies functions in order of the proportion of execution time they occupied.
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls s/call s/call name
13.75 16.38 16.38 125676749836 0.00 0.00 tVect::operator+(tVect const&) const
12.60 31.40 15.02 94039864750 0.00 0.00 tVect::operator*(double const&) const
10.02 43.34 11.94 65081331 0.00 0.00 elmt::correct(double const*, double const*)
8.54 53.52 10.18 4068178008 0.00 0.00 DEM::particleParticleCollision(particle const*...
7.62 62.60 9.08 2392019595 0.00 0.00 elmt::predict(double const*, double const*)
5.66 69.34 6.74 41323211762 0.00 0.00 tVect::operator-(tVect const&) const
5.45 75.84 6.50 36306445222 0.00 0.00 quaternion::operator*(double const&) const
4.74 81.49 5.65 7264658795 0.00 0.00 project(tVect, quaternion)
4.60 86.97 5.48 32022205839 0.00 0.00 quaternion::operator+(quaternion const&) const
3.61 91.27 4.30 2380119 0.00 0.00 DEM::evaluateForces()
2.25 93.95 2.68 16726846623 0.00 0.00 tVect::cross(tVect const&) const
...
After the table, a key to the columns is provided.
Flat Profile Key
% the percentage of the total running time of the
time program used by this function.
cumulative a running sum of the number of seconds accounted
seconds for by this function and those listed above it.
self the number of seconds accounted for by this
seconds function alone. This is the major sort for this
listing.
calls the number of times this function was invoked, if
this function is profiled, else blank.
self the average number of milliseconds spent in this
ms/call function per call, if this function is profiled,
else blank.
total the average number of milliseconds spent in this
ms/call function and its descendents per call, if this
function is profiled, else blank.
name the name of the function. This is the minor sort
for this listing. The index shows the location of
the function in the gprof listing. If the index is
in parenthesis it shows where it would appear in
the gprof listing if it were to be printed.
Call Graph
Following the flat profile, is the call graph. This further breaks down the information from the flat profile into the unique calling hierarchies. This is much longer and can be harder to interpret, however it may provide useful context if a function highlighted by the flat profile is called from many locations.
...
-----------------------------------------------
0.00 0.00 1/4000003 LB::latticeBolzmannInit(std::vector<cylinder, std::allocator<cylinder> >&, std::vector<wall, std::allocator<wall> >&, std::vector<particle, std::allocator<particle> >&, std::vector<object, std::allocator<object> >&, bool, bool) [20]
0.01 1.39 2000001/4000003 LB::latticeBolzmannStep(std::vector<elmt, std::allocator<elmt> >&, std::vector<particle, std::allocator<particle> >&, std::vector<wall, std::allocator<wall> >&, std::vector<object, std::allocator<object> >&) [5]
0.01 1.39 2000001/4000003 LB::latticeBoltzmannFreeSurfaceStep() [6]
[4] 55.9 0.02 2.78 4000003 LB::cleanLists() [4]
0.97 1.81 12000009/16000013 void std::__heap_select<std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, __gnu_cxx::__ops::_Iter_less_iter>(std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, __gnu_cxx::__ops::_Iter_less_iter) [3]
-----------------------------------------------
0.00 2.18 2000001/2000001 goCycle(IO&, DEM&, LB&) [2]
[5] 43.6 0.00 2.18 2000001 LB::latticeBolzmannStep(std::vector<elmt, std::allocator<elmt> >&, std::vector<particle, std::allocator<particle> >&, std::vector<wall, std::allocator<wall> >&, std::vector<object, std::allocator<object> >&) [5]
0.01 1.39 2000001/4000003 LB::cleanLists() [4]
0.16 0.30 2000001/16000013 void std::__heap_select<std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, __gnu_cxx::__ops::_Iter_less_iter>(std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, std::reverse_iterator<__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > > >, __gnu_cxx::__ops::_Iter_less_iter) [3]
0.32 0.00 2000001/2000001 LB::streaming(std::vector<wall, std::allocator<wall> >&, std::vector<object, std::allocator<object> >&) [11]
-----------------------------------------------
...
Again it is followed by an explanation for understanding the output.
Call Graph Help Text
This table describes the call tree of the program, and was sorted by
the total amount of time spent in each function and its children.
Each entry in this table consists of several lines. The line with the
index number at the left hand margin lists the current function.
The lines above it list the functions that called this function,
and the lines below it list the functions this one called.
This line lists:
index A unique number given to each element of the table.
Index numbers are sorted numerically.
The index number is printed next to every function name so
it is easier to look up where the function is in the table.
% time This is the percentage of the `total' time that was spent
in this function and its children. Note that due to
different viewpoints, functions excluded by options, etc,
these numbers will NOT add up to 100%.
self This is the total amount of time spent in this function.
children This is the total amount of time propagated into this
function by its children.
called This is the number of times the function was called.
If the function called itself recursively, the number
only includes non-recursive calls, and is followed by
a `+' and the number of recursive calls.
name The name of the current function. The index number is
printed after it. If the function is a member of a
cycle, the cycle number is printed between the
function's name and the index number.
For the function's parents, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the function into this parent.
children This is the amount of time that was propagated from
the function's children into this parent.
called This is the number of times this parent called the
function `/' the total number of times the function
was called. Recursive calls to the function are not
included in the number after the `/'.
name This is the name of the parent. The parent's index
number is printed after it. If the parent is a
member of a cycle, the cycle number is printed between
the name and the index number.
If the parents of the function cannot be determined, the word
`<spontaneous>' is printed in the `name' field, and all the other
fields are blank.
For the function's children, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the child into the function.
children This is the amount of time that was propagated from the
child's children to the function.
called This is the number of times the function called
this child `/' the total number of times the child
was called. Recursive calls by the child are not
listed in the number after the `/'.
name This is the name of the child. The child's index
number is printed after it. If the child is a
member of a cycle, the cycle number is printed
between the name and the index number.
If there are any cycles (circles) in the call graph, there is an
entry for the cycle-as-a-whole. This entry shows who called the
cycle (as parents) and the members of the cycle (as children.)
The `+' recursive calls entry shows the number of function calls that
were internal to the cycle, and the calls entry for each member shows,
for that member, how many times it was called from other members of
the cycle.
Limitations
Whilst gprof will profile code that includes OpenMP, it produces spurious timing information if OpenMP execution is present.