On Thu, Feb 11, 2021 at 5:40 PM John Bates <[email protected]> wrote:
>
<snip>
> Runtime Characteristics
>
> ~500KB additional binary size. Even with using only the basic features of
> perfetto, it will increase the binary size of mesa by about 500KB.
IMHO, that size is negligible.. looking at freedreno, a mesa build
*only* enabling freedreno is already ~6MB.. distros typically use
"megadriver" (ie. all the drivers linked into a single .so with hard
links for the different ${driver}_dri.so), which on my fedora laptop
is ~21M. Maybe if anything is relevant it is how much of that
actually gets paged into RAM from disk, but I think 500K isn't a thing
to worry about too much.
> Background thread. Perfetto uses a background thread for communication with
> the system tracing daemon (traced) to advertise trace data and get
> notification of trace start/stop.
Mesa already tends to have plenty of threads.. some of that depends on
the driver, I think currently radeonsi is the threading king, but
there are several other drivers working on threaded_context and async
compile thread pool.
It is worth mentioning that, AFAIU, perfetto can operate in
self-server mode, which seems like it would be useful for distros
which do not have the system daemon. I'm not sure if we lose that
with percetto?
> Runtime overhead when disabled is designed to be optimal with one predicted
> branch, typically a few CPU cycles per event. While enabled, the overhead can
> be around 1 us per event.
>
> Integration Challenges
>
> The perfetto SDK is C++ and designed around macros, lambdas, inline
> templates, etc. There are ongoing discussions on providing an official
> perfetto C API, but it is not yet clear when this will land on the perfetto
> roadmap.
> The perfetto SDK is an amalgamated .h and .cc that adds up to 100K lines of
> code.
> Anything that includes perfetto.h takes a long time to compile.
> The current Perfetto SDK design is incompatible with being a shared library
> behind a C API.
So, C++ on it's own isn't a showstopper, mesa has plenty of C++ code.
But maybe we should verify that MSVC is happy with it, otherwise we
need to take a bit more care in some parts of the codebase.
As far as compile time, I wonder if we can regenerate the .cc/.h with
only the gpu trace parts? But I wouldn't expect the .h to be
something widely included. For example, for gpu timeline traces in
freedreno, I'm expecting it to look like a freedreno_perfetto.cc with
extern "C" {} around the callbacks that would hook into the
u_tracepoint tracepoints. That one file would pull in the perfetto
.h, and we'd just not build that file if perfetto was disabled.
Overall having to add our own extern C wrappers in some places doesn't
seem like the *end* of the world.. a bit annoying, but we might end up
doing that regardless if other folks want the ability to hook in
something other than perfetto?
<snip>
> Mesa Integration Alternatives
I'm kind of leaning towards the "just slurp in the .cc/.h" approach..
that is mostly because I expect to initially just add some basic gpu
timeline tracepoints, but over time iterate on adding more.. it would
be nice to not have to depend on a newer version of an external
library at each step. That is ofc only my $0.02..
BR,
-R
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