Help in understanding -log_summary output

[Rafael Santos Coelho]

Hi all

Lately, I've been working on this article which covers the design principles
that guide the development of the PETSc library and also the main
functionalities it offers for the solution of PDEs. One of the things I've
pointed out are the profiling capabilities that PETSc includes.

I need some help to properly interpret the output from the '-log_summary'
command line option, especially its "phase summaries section". I've read the
documentation a few times but I still have difficulties in grasping the
meaning of the percentages of the statistics (T, R, L and M). For instance,
the following line

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Phase   Count      Time (sec)           Flops/sec
                            --- Global ---            --- Stage
---           Total
                       Max    Ratio         Max    Ratio      Mess     Avg
len    Reduct   %T %F %M %L %R %T %F %M %L %R Mflop/s
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(...)

--- Event Stage 5: KSPSolve 1
MatMult   56   1.199e+00 1.1     5.3e+07 1.0     1.1e+03 4.2e+03 0.0e+00
5   28    99   23  0   30   28   99   99    0    201

(...)


indicates that the 'MatMult' routine was called 56 times, a total of 1.1e+03
messages were sent, with an average length of 4.2e+03 bytes, and there were
no global reductions, right?

Now, what about the "--- Global ---" and "--- Stage ---" columns? What do
those percentages really say? Is it possible to estimate how much time the
overall program execution spent performing floating point operations and
inter-process communication by looking exclusively at the output?

Thanks in advance,

Rafael
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