Re: Oprofile vs. perf vs. gperf

On Thu, 2015-06-04 at 07:10 +0000, Mahmood N wrote:
> Hi,
> What is the difference among Oprofile, perf-tools and gperf-tools? I
> didn't find an study comparing them. Which one is mostly accurate for
> AMD processors?

Hi,

I don't know much about gprof, so maybe someone should add some more
information...

The aim of OProfile is counting, profiling (sampling), code annotation
and callgraph generation, all that based on PMU events generated by CPU.
Historically, it supported some software TIMER interrupts, but now,
since opcontrol legacy mode has been replaced by operf/ocount tools, the
oprofile's source of events is PMU only. OProfile now uses kernel
backend for it, it's the same backend as perf uses.

Perf tool came later as a part of kernel. Before that oprofile worked in
the legacy mode (opcontrol), so some daemon and kernel module had to be
enabled before the profiling may have started, etc. When perf came with
the kernel backend, oprofile switched to using it as well.

Which one is more accurate? Hard to say, in my opinion they should be
almost at the same level.

Perf can use PMU as well, but it is not the main aim of it. So it has
some events predefined (see `perf list hw hwcache` command) and the rest
has to be specified by raw numbers like "-e r410e". So that means, that
perf can use all the events oprofile can, but many of them are
unfriendly to use and you just have to go through your CPU vendor's
documentation to see what numbers to put there.

Perf can also use so called "tracepoint" events (see `perf list
tracepoint` (you have to be root to run it)), which are mapped to some
kernel trace points. You can also probe functions (similar to
systemtap), trace syscalls (like strace), do some NUMA benchmarking and
many other things.

Compare the following:

ocount ls
perf stat -e cycles ls

The numbers should be similar. If you see differences there, it's not
because perf or oprofile are imprecise compared to the other tool, but
the reason is rather that in each run is a bit different and CPU can
pre-cache something, etc.

As I understand, gprof is a bit different tool which needs to be used
also at the compilation time, so the application is instrumented by it
before it is built.

When you are profiling with `operf` or `perf record` you don't need
anything special, just if you have the debuginfo, you can see the
symbols in the report, otherwise you will see addresses...

I hope I have explained it a bit.

Regards,
Michael

>  
> Regards,
> Mahmood
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