RE: RFC: Low-overhead profiling for LuaJIT 2.1
- From: Dan Tull <dtull@xxxxxxxxx>
- To: "luajit@xxxxxxxxxxxxx" <luajit@xxxxxxxxxxxxx>
- Date: Fri, 21 Jun 2013 07:37:28 -0700
Dan Eloff [dan.eloff@xxxxxxxxx] wrote:
> I have a slightly fringe view of profiling. Usually all I want in a profiler
> is a debugger
> with a pause capability and the ability to inspect the stack of the paused
> program.
> ...
> This answer on SO about what's wrong with gprof is somewhat enlightening:
> http://stackoverflow.com/questions/1777556/alternatives-to-gprof/1779343#1779343
This is what led me to build the sampling profiler before working on my
tracepoints
prototype. The sampling profiler has worked well enough I haven't gone back to
the
tracepoints work yet. I occasionally wish I had something like it, but not
often.
The key change I had to make to Lua was to add a low overhead trigger hook so
that
having sampling enabled doesn't distort performance.
Dan Eloff [dan.eloff@xxxxxxxxx] wrote:
> I haven't found a good way to apply this technique to Lua. It's simple to
> pause the
> interpreter with gdb, but I can't make any sense ...
> A Lua-based debugger resolves that, but doesn't let you pause inside C
> frames...
I've picked out the Lua stack frames in a native debugger, but it's a bit of a
pain
and you have to be careful since the VM state may not be entirely valid.
I'd really like to do something along these lines:
http://dtrace.org/blogs/brendan/2011/12/16/flame-graphs/
https://github.com/agentzh/nginx-systemtap-toolkit/blob/master/ngx-sample-lua-bt
However, I don't know of a way to do it across the platforms I need to support
(Mac and
Windows and iOS). The dtrace techniques used for the above involves ustack
helpers
which I gather are broken on OS X. I haven't even dug into Windows yet.
Dan Tull
Adobe
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