[Mesa-dev] RFC: parallel shader compiles in mesa

[Ilia Mirkin]

AFAIK, nothing in the spec prevents shader compiles to happen in the
background. Further, ARB_parallel_shader_compile provides a way to
explicitly support this by providing a non-blocking query of current
compile status. I take this to mean that any "regular" touch of the
shader/programs will wait for the background compile to finish. Here's
what I was thinking for implement this:

I started by writing a threadpool with adjustable thread counts. I
figure this will be shared by all contexts and default to the # of
cores - 1, and be adjusted to the current context's # of threads
whenever work is submitted.

Both glCompileShader (-> gl_shader) and glLinkProgram (->
gl_shader_program) will be made to compile asynchronously. Basically
instead of running compile_shader and link_program directly, I'm going
to submit them as work functions. Any lookup of the relevant objects
will cause that lookup to wait for the relevant work to complete
(except for the GL_COMPLETION_STATUS_ARB query). This will be enforced
by the lookup_shader(_program)(_err) functions, and an additional
helper will be made to wrap direct accesses.

There's an additional problem that it'd be ideal if you could
glCompileShader and then glLinkProgram without the glLinkProgram call
waiting for the glCompileShader call to complete. I'm not keen on
creating some complex dependency system for jobs, so I was thinking of
something simple -- a predicate that determines whether work is
doable, which is run before doing the work. If it's not, it's put into
a separate list. Whenever any job completes, that side list is
re-added to the main work list again. This will cause a little bit of
extra work to be done, but I don't think it's too bad.

I'm sure there will be a lot of specific issues that I'll have to
tackle in making compile_shader/link_program asynchronous, but they
seem to be mostly self-contained already. Does this overall approach
sound reasonable?

Thanks,

  -ilia