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modules/checkers w/fill_module=$module requires a consistent
mapping of cpu to fragnum since it creates a per-cpu
til_module_context_t for the fill_module.
The existing implementation for threaded rendering maximizes
performance by letting *any* scheduled to run thread advance
fragnum atomically and render the acquired fragnum
indiscriminately. A side effect of this is any given frame, even
rendered by the same module, will have a random mapping of
cpus/threads to fragnums.
With this change, the simple til_module_t.prepare_frame() API of
returning a bare fragmenter function is changed to instead return
a "frame plan" in til_frame_plan_t. Right now til_frame_plan_t
just contains the same fragmenter as before, but also has a
.cpu_affinity member for setting if the frame requires a stable
relationship of cpu/thread to fragnum.
Setting .cpu_affinity should be avoided if unnecessary, and that
is the default if you don't mention .cpu_affinity at all when
initializing the plan in the ergonomic manner w/designated
initializers. This is because the way .cpu_affinity is
implemented will leave threads spinning while they poll for
*their* next fragnum using atomic intrinsics. There's probably
some room for improvement here, but this is good enough for now
to get things working and correct.
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- modules now allocate their contexts using
til_module_context_new() instead of [cm]alloc().
- modules simply embed til_module_context_t at the start of their
respective private context structs, if they do anything with
contexts
- modules that do nothing with contexts (lack a create_context()
method), will now *always* get a til_module_context_t supplied
to their other methods regardless of their create_context()
presence. So even if you don't have a create_context(), your
prepare_frame() and/or render_fragment() methods can still
access seed and n_cpus from within the til_module_context_t
passed in as context, *always*.
- modules that *do* have a create_context() method, implying they
have their own private context type, will have to cast the
til_module_context_t supplied to the other methods to their
private context type. By embedding the til_module_context_t at
the *start* of their private context struct, a simple cast is
all that's needed. If it's placed somewhere else, more
annoying container_of() style macros are needed - this is
strongly discouraged, just put it at the start of struct.
- til_module_create_context() now takes n_cpus, which may be set
to 0 for automatically assigning the number of threads in its
place. Any non-zero value is treated as an explicit n_cpus,
primarily intended for setting it to 1 for single-threaded
contexts necessary when embedded within an already-threaded
composite module.
- modules like montage which open-coded a single-threaded render
are now using the same til_module_render_fragment() as
everything else, since til_module_create_context() is accepting
n_cpus.
- til_module_create_context() now produces a real type, not void
*, that is til_module_context_t *. All the other module
context functions now operate on this type, and since
til_module_context_t.module tracks the module this context
relates to, those functions no longer require both the module
and context be passed in. This is especially helpful for
compositing modules which do a lot of module context creation
and destruction; the module handle is now only needed to create
the contexts. Everything else operating on that context only
needs the single context pointer, not module+context pairs,
which was unnecessarily annoying.
- if your module's context can be destroyed with a simple free(),
without any deeper knowledge or freeing of nested pointers, you
can now simply omit destroy_context() altogether. When
destroy_context() is missing, til_module_context_free() will
automatically use libc's free() on the pointer returned from
your create_context() (or on the pointer that was automatically
created if you omitted create_context() too, for the
bare til_module_context_t that got created on your behalf
anyways).
For the most part, these changes don't affect module creation.
In some ways this eases module creation by making it more
convenient access seed and n_cpus if you had no further
requirement for a context struct.
In other ways it's slightly annoying to have to do type-casts
when you're working with your own context type, since before it
was all void* and didn't require casts when assigning to your
typed context variables.
The elimination for requiring a destroy_context() method in
simple free() of private context scenarios removes some
boilerplate in simple cases.
I think it's a wash for module writers, or maybe a slight win for
the simple cases.
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