path: root/HACKING.txt

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---


Hacking on rototiller / libtil:


---


 Introduction:

    This document primarily attempts to describe how one goes about hacking
  on new rototiller modules.

    Initially only a bare minimum module creation is described.  This is a
  single-threaded, unconfigurable at runtime, simple module, requiring only
  the rendering function be implemented.

    Later more advanced topics like threaded rendering and runtime
  configurability will be covered.  These are completely optional and can
  safely be ignored until such facilities are desired.

    The creative process of developing a new module often starts with
  writing nothing more than a rendering function, later evolving to become
  more complex in pursuit of better performance via threaded rendering, and
  greater flexibility via runtime settings and/or taps.


---


 Getting started:

    After acquiring a copy of the source, adding a new module to rototiller
  consists of four steps:

  1. Giving the module a unique name, creating a directory using that name
     under src/modules.  This can be a temporary working name just to get
     started, what's important is that it not conflcit with any existing
     names.

  2. Implementing at least a ${new}_render_fragment() method for the module
     in a file placed in its directory at "src/modules/${new}/${new}.c".

  3. Integrating the module into the build system by adding its directory
     to the existing "configure.ac" and "src/modules/Makefile.am" files,
     creating its own "src/modules/${new}/Makefile.am" file, and adding
     it's produced archive to "src/Makefile.am" for linking.

  4. Wiring the module into libtil exposing it to the world by adding it
     to the modules[] array in "src/til.c".

    Most of these steps are self-explanatory after looking at the existing
  code/build-system files.  It's common (and encouraged) to bootstrap a new
  module by copying a "Makefile.am" and "${new}.c" file from one of the
  existing modules.

    There's also a "stub" branch provided in the git repository, adding a
  bare minimum module rendering a solid white canvas every frame.  This is
  intended for use as a clean slate for bootstrapping new modules, there's
  no harm in deriving new modules from either this "stub" branch, or
  existing modules.  The "stub" branch may lag behind the state of the
  world, so it's probably better to use an actual existing module.


---


 Configuring and building the source:

    Rototiller uses GNU Autotools for its build system.  Generally all
  that's required for building the source is the following sequence of
  shell commands:

      $ ./bootstrap
      $ mkdir build; cd build; ../configure
      $ make -j4

    The source is all C, so a C compiler is required.  Autotools is also
  required for `bootstrap` to succeed in generating the configure script
  and Makefile templates, `pkg-config` is used by configure, and a `make`
  program for executing the build.  On Debian-derived systems installing
  the "build-essential" meta-package and "libtool" should at least get
  things building successfully.

    To actually produce a `rototiller` binary usable for rendering visible
  output, libsdl2 and/or libdrm development packages will also be needed.
  Look at the `../configure` output for SDL and DRM lines to see which have
  been enabled.  If both report "no" then the build will only produce a
  libtil library for potential use in other frontends, and a rototiller
  binary only capable of invisible in-memory rendering useful for testing
  and benchmarking, but no visible output.  The SDL backend is preferred
  when available, and known to work on Linux, MacOS, and Windows(mingw).

    After successfully building rototiller, an executable will be at
  "src/rototiller" in the build tree.  If the steps above were followed
  verbatim, that would be at "build/src/rototiller".  This program is the
  primary CLI front-end for rototiller.


---


 Quickly testing modules via the CLI frontend:

    The included frontend supports both an interactive, text dialog style
  setup, and specifying those same settings via command-line arguments.  If
  run without any arguments, i.e. just running `build/src/rototiller`, a
  comprehensive, guided interactive setup will be performed for determining
  all needed module and video settings.  Pressing just <enter> at any time
  accepts the default, shown between the [] of the prompt.

    Prior to actually proceeding with a given setup, the configured setup
  to be used is always printed on stdout in a valid command-line argument
  syntax.  This may be copied and reused for an immediate, non-interactive
  reexecution with those settings.  One can specify '--go' to skip the wait
  for <enter> normally inserted while showing the resulting command-line.

    One can also partially specify any setup in the command-line arguments,
  resulting in a partial interactive setup for just the missing settings.

    When developing a new module it's common to specify the video settings,
  and just the module name under development, leaving the module's settings
  for interactive dialog during the experimentation process.  i.e.:

      $ build/src/rototiller --module=$new --video=sdl,fullscreen=off,size=640x480

    This way, if "$new" implements settings, only those unspecified will
  be asked for interactively.  Note this isn't coarse-grained, you can
  specify some of the settings while leaving the one you want to be asked
  for out, e.g.

      $ build/src/rototiller --module=$new,foo=bar --video=sdl,fullscreen=off,size=640x480

    If "$new" expects settings foo= and baz=, it rototiller would query
  about baz= while accepting the "bar" for "foo".


---


 The render function, a bare minimum module:

    All rendering in rototiller is performed using the CPU, in 24-bit "True
  color" pixel format, with 32-bits/4-bytes of space used per pixel.

    The surface for rendering into is described using a display-system
  agnostic "framebuffer fragment" structure type named "til_fb_fragment_t",
  defined in "src/til_fb.h" as:

      typedef struct til_fb_fragment_t {
        uint32_t *buf;           /* pointer to the first pixel in the fragment */
        unsigned x, y;           /* absolute coordinates of the upper left corner of this fragment within the logical frame */
        unsigned width, height;  /* actual width and height of this fragment */
        unsigned frame_width;    /* logical width of the frame this fragment is part of */
        unsigned frame_height;   /* logical height of the frame this fragment is part of */
        unsigned stride;         /* number of words from the end of one row to the start of the next */
        unsigned pitch;          /* number of words separating y from y + 1, including any padding */
        unsigned number;         /* this fragment's number as produced by fragmenting */
        unsigned cleared:1;      /* if this fragment has been cleared since last flip */
      } til_fb_fragment_t;

    For most modules these members are simply used as provided, and there's
  no need to manipulate them.  For simple non-threaded cases only the "buf"
  and "{width,height}" members are required, with "stride" or "pitch"
  becoming important for algorithms directly manipulating buf's memory
  contents to properly address rows of pixels since fragments may be
  discontiguous in buf at row boundaries for a variety of reasons.

    Particularly when using threaded rendering, the "frame_{width,height}"
  members become important as they describe a fragment's full-frame
  dimensions, while "{x,y,width,height}" describe the specific fragment
  within the frame being rendered by render_fragment().

    Note that even when not implementing a threaded module, the logical
  "frame_{width,height}" may differ from "{width,height}".  So all modules
  should take into consideration where the fragment they're rendering is
  placed within its logical frame.  This is because rototiller treats
  rendering modules as composable, and some modules may use other modules
  as a sort of paintbrush in a greater whole, which may require clipping
  the output by specifying a smaller fragment within a larger logical
  frame.  You can ignore this detail initially, it's usually not too
  complicated to correct later.

    The module_render() function prototype is declared within the
  "til_module_t" struct in "src/til.h" as:

    void (*render_fragment)(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned cpu, til_fb_fragment_t **fragment_ptr);

    Every module must provide a "til_module_t" instance having at least this
  "render_fragment" member initialized to its rendering function.  This is
  typically done using a global instance named using the module's name as
  the prefix.

    None of the other function pointer members in "til_module_t" are
  required, and the convention is to use designated initialization in
  assigning a module's "til_module_t" members ensuring zero-initialization
  of omitted members, i.e.:

      static void minimal_render_fragment(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned cpu, til_fb_fragment_t **fragment_ptr)
      {
        /* render into (*fragment_ptr)->buf */
      }

      til_module_t minimal_module = {
              .render_fragment = minimal_render_fragment,
              .name = "minimal",
              .description = "Minimal example module",
      }

    Note that the render_fragment() prototype has more arguments than just
  the "til_fb_fragment_t **fragment_ptr":

    til_module_context_t *context:

      For modules implementing a create_context() function, this will be
      the pointer returned by that function.  Intended for modules that
      require state persisted across frames rendered.  For those not
      implementing create_context(), this will be a libtil-provided context
      containing only the members of "til_module_context_t" found in
      "src/til_module_context.h".

    unsigned ticks:

      A convenient time-like counter the frontend advances during
      operation.  Instead of calling some kind of time function in every
      module which may become costly, "ticks" may be used to represent
      time.  This value is also adjusted to compensate for how many frames
      are queued for display, making it a more accurate reference for what
      tick is being rendered within the visuals timeline.

    unsigned cpu:

      An identifier representing which logical CPU # the render function is
      executing on.  This isn't interesting for simple single-threaded
      modules, but when implementing more advanced threaded renderers this
      may be useful for indexing per-cpu resources to avoid contention.

    For simple modules these can all be safely ignored, "ticks" does tend
  to be useful for even simple modules however.

    Rendering functions shouldn't make assumptions about the contents of
  "(*fragment_ptr)->buf", in part because rototiller will always use
  multiple buffers for rendering which may be recycled in any order.
  Additionally, it's possible a given fragment will be further manipulated
  in composited scenarios.  Consequently it's important that every
  render_fragment() function fully render the region described by the
  fragment.

    There tends to be two classes of rendering algorithms; those that
  always produce a color for every pixel available in the output, and those
  producing sparse output akin to an overlay.

    In the latter case it's common to require bulk-clearing the fragment
  before the algorithm draws its sparse overlay-like contents onto the
  canvas.  To facilitate potential compositing of such modules, the
  "til_fb_fragment_t" structure contains a "cleared" member used to indicate
  if a given fragment's buf contents have been fully initialized yet for
  the current frame.  When "cleared" is already set, the bulk clearing
  operation should be skipped, allowing the existing contents to serve as
  the logically blank canvas.

    A convenience helper for such modules is provided named
  til_fb_fragment_clear().  Simply call this at the start of the
  render_fragment() function, and the conditional cleared vs. non-cleared
  details will be handled automatically.  Otherwise see the implementation
  in "src/til_fb.h" to see what's appropriate to DIY.  To clarify, modules
  implementing algorithms that naturally always write every pixel in the
  fragment may completely ignore this aspect, and need not set the cleared
  member; that's handled automatically.


---


 Stateful rendering:

    It's common to require some state persisting from one frame to the
  next.  Achieving this is a simple matter of providing create_context()
  and destroy_context() functions when initializing til_module_t, i.e.:

      typedef struct minimal_context_t {
        til_module_context_t  til_module_context; /* must be at start of struct */

        int stateful_variables;
      } minimal_context_t;

      static til_module_context_t * minimal_create_context(const til_module_t *module, til_stream_t *stream, unsigned seed, unsigned ticks, unsigned n_cpus, til_setup_t *setup)
      {
        minimal_context_t *ctxt;

        ctxt = til_module_context_new(module, sizeof(minimal_context_t), stream, seed, ticks, n_cpus, setup);
        if (!ctxt)
          return NULL;

        /* this can include more elaborate initialization of minimal_context_t as needed */
        ctxt->stateful_variables = 42;

        return &ctxt->til_module_context;
      }

      static void minimal_destroy_context(til_module_context_t *context)
      {
        /* Note that if all you need in your destroy_context is to free the
         * top-level til_module_context_t, you can omit destroy_context(),
         * libtil will do the bare free() for you.
         *
         * But if you have additional allocations requiring a "deep" free,
         * then you must provide a destroy_context() and you're responsible
         * for freeing the top-level context as well:
         */
        free(context);
      }

      static void minimal_render_fragment(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned cpu, til_fb_fragment_t **fragment_ptr)
      {
        minimal_context_t *ctxt = (minimal_context_t *)context;

        /* render into (*fragment_ptr)->buf utilizing/updating ctxt->stateful_variables */
      }

      til_module_t minimal_module = {
              .create_context = minimal_create_context,
              .destroy_context = minimal_destroy_context,
              .render_fragment = minimal_render_fragment,
              .name = "minimal",
              .description = "Minimal example module",
      }

      static til_module_context_t * minimal_create_context(const til_module_t *module, til_stream_t *stream, unsigned seed, unsigned ticks, unsigned n_cpus, til_setup_t *setup)

    Note that the create_context() function prototype includes some
  arguments:

    til_module_t *module:

        The til_module_t this create_context came from, it should match the
      til_module_t of your module.

    til_stream_t *stream:

        The stream this context is participating in.  Module contexts and
      their taps are made discoverable from other modules via the stream.
      You can ignore this for now.

    unsigned seed:

        The PRNG seed to be used for seeding any PRNGs used by the module.
      It's very common to need random numbers, but rototiller strives to
      make results reproducible, a component of that is controlling the
      seeds used.  Note the --seed= CLI argument.

    unsigned ticks:

        Same as render_fragment; a time-like counter.  This is provided to
      the create_context() function in the event that some ticks-derived
      state must be initialized continuously with the ticks value
      subsequently passed to render_fragment().
      This is often ignored.

    unsigned n_cpus:

        This is the number of logical CPUs the context is permitted to
      utilize in rendering, which is potentially relevant for threaded
      renderers.  The "unsigned cpu" parameter supplied to
      render_fragment() will always be < this n_cpus value, and the two are
      intended to complement eachother.  When creating the context, one may
      allocate per-cpu cache-aligned space of n_cpus quantity.  Then the
      render_fragment() function would address this per-cpu space using the
      cpu parameter as an index into the n_cpus sized allocation.
      This is often ignored.

    til_setup_t *setup:

        For modules implementing runtime-configuration by providing a
      setup() function in their til_module_t initializer, this will contain
      the pointer returned in res_setup by their setup() function.
      Unless implementing runtime configuration, this would be ignored.

    As mentioned above in describing the rendering function, this is
  entirely optional.  One may create 100% valid modules implementing only
  the render_fragment() function.


---


 Runtime-configurable rendering:

    For myriad reasons ranging from debugging and creative experimentation,
  to aesthetic variety, it's important to support runtime configuration of
  modules.

    Everything configurable that is potentially interesting to a viewer is
  best exposed via runtime settings, as opposed to hidden behind
  compile-time constants like #defines or magic numbers in the source.

    It's implied that when adding runtime configuration to a module, it
  will also involve stateful rendering as described in the previous
  section.  This isn't absolutely required, but without an allocated
  context to apply the runtime-configuration to, the configuration will be
  applied in some global fashion.  Any modules to be merged upstream
  shouldn't apply their configuration globally if at all avoidable.

    Adding runtime configuration requires implementing a setup() function
  for a given module.  This setup() function is then provided when
  initializing til_module_t.  Building upon the previous minimal example
  from stateful rendering:

      typedef struct minimal_setup_t {
        til_setup_t til_setup; /* must be at start of struct */

        int foobar;
      } minimal_setup_t;

      typedef struct minimal_context_t {
        til_module_context_t til_module_context; /* must be at start of struct */

        int stateful_variables;
      } minimal_context_t;

      static til_module_context_t * minimal_create_context(const til_module_t *module, til_stream_t *stream, unsigned seed, unsigned ticks, unsigned n_cpus, til_setup_t *setup)
      {
        minimal_context_t *ctxt;

        ctxt = til_module_context_new(module, sizeof(minimal_context_t), stream, seed, ticks, n_cpus, setup);
        if (!ctxt)
          return NULL;

        /* seed the stateful_variables from the runtime-provided setup */
        ctxt->stateful_variables = ((minimal_setup_t *)setup)->foobar;

        return &ctxt->til_module_context;
      }

      static void minimal_destroy_context(til_module_context_t *context)
      {
        free(context);
      }

      static void minimal_render_fragment(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned cpu, til_fb_fragment_t **fragment_ptr)
      {
        minimal_context_t *ctxt = (minimal_context_t *)context;

        /* render into (*fragment_ptr)->buf utilizing/updating ctxt->stateful_variables */
      }

      static int minimal_setup(const til_settings_t *settings, til_setting_t **res_setting, const til_setting_desc_t **res_desc, til_setup_t **res_setup);

      til_module_t minimal_module = {
              .create_context = minimal_create_context,
              .destroy_context = minimal_destroy_context,
              .render_fragment = minimal_render_fragment,
              .setup = minimal_setup,
              .name = "minimal",
              .description = "Minimal example module",
      }

      static int minimal_setup(const til_settings_t *settings, til_setting_t **res_setting, const til_setting_desc_t **res_desc, til_setup_t **res_setup)
      {
        const char    *values[] = {
              "off",
              "on",
              NULL
            };
        til_setting_t *foobar;
        int           r;

        r = til_settings_get_and_describe_setting(settings,
                  &(til_setting_desc_t){
                    .name = "Foobar configurable setting",
                    .key = "foobar",
                    .regex = "^(off|on)",
                    .preferred = values[0],
                    .values = values,
                    .annotations = NULL
                  },
                  &foobar,
                  res_setting,
                  res_desc);
        if (r)
          return r;

        if (res_setup) {
          minimal_setup_t  *setup;

          setup = til_setup_new(settings, sizeof(*setup), NULL, &minimal_module);
          if (!setup)
            return -ENOMEM;

          if (!strcasecmp(foobar, "on"))
            setup->foobar = 1;

          *res_setup = setup;
        }

        return 0;
      }


    In the above example the minimal module now has a "foobar" boolean
  style setting supporting the values "on" and "off".  It may be specified
  at runtime to rototiller (or any other frontend) via the command-line
  argument:

      "--module=minimal,foobar=on"

    And if the "foobar=on" setting were omitted from the command-line, in
  rototiller's CLI frontend an interactive setup dialog would occur, i.e:

      Foobar configurable setting:
       0: off
       1:  on
      Enter a value 0-1 [0 (off)]:

    Much can be said on the subject of settings, this introduction should
  be enough to get started.  Use the existing modules as a reference on how
  to implement settings.  The sparkler module in particular has one of the
  more complicated setup() functions involving dependencies where some
  settings become expected and described only if others are enabled,
  without too much complexity.

    None of the frontends currently enforce the regex, but it's best to
  always populate it with something valid as enforcement will become
  implemented at some point in the future.  Modules should be able to
  largely assume the input is valid at least in terms of passing the regex,
  but if it's too onerous to populate don't sweat it until front-ends start
  actually enforcing them.

    Note how the minimal_setup_t instance returned by setup() in res_setup
  is subsequently supplied to minimal_create_context() in its setup
  parameter.  In the previous "Stateful rendering" example, this setup
  parameter was ignored, but it would still be non-NULL with the bare
  til_setup_t populated, look at src/til_setup.h for what's there.  But
  here we used it to retrieve the "foobar" value wired up by the
  minimal_setup() function supplied as minimal_module.setup().


---


 Threaded rendering:

    Rototiller deliberately abstains from utilizing any GPU hardware-
  acceleration for rendering.  Instead, all rendering is done using the CPU
  programmed simply in C, without incurring a bunch of GPU API complexity
  like OpenGL/Direct3D or any need to manage GPU resources.

    Modern systems tend to have multiple CPU cores, enabling parallel
  execution similar to how GPUs employ multiple execution units for
  parallel rendering of pixels.  With some care and little effort
  rototiller modules may exploit these additional CPU resources.

    Rototiller takes care of the low-level minutia surrounding creating
  threads and efficiently scheduling rendering across them for every frame.
  The way modules integrate into this threaded rendering machinery is by
  implementing a prepare_frame() function that gets called at the start of
  every frame in a single-threaded fashion, followed by parallel execution
  of the module's render_fragment() function from potentially many threads.

    The prepare_frame() function prototype is declared within the
  "til_module_t" struct in "src/til.h" as:

      void (*prepare_frame)(til_module_context_t *context, til_stream_t *stream, unsigned ticks, til_fb_fragment_t **fragment_ptr, til_frame_plan_t *res_frame_plan);

    The context, stream, ticks, n_cpus, and fragment_ptr parameters here
  are semantically identical to their use in the other til_module_t
  functions explained previously in this document.

    What's special here is the res_frame_plan parameter.  This is where
  your module is expected to provide a fragmenter function rototiller will
  call repeatedly while breaking up the frame's fragment being rendered
  into smaller subfragments for passing to the module's render_fragment()
  in place of the larger frame's whole fragment.

    This effectively gives modules control over the order, quantity, size,
  and shape, of individually rendered subfragments.  Logically speaking,
  one may view the fragments described by the fragmenter function returned
  in res_fragmenter as the potentially parallel units of work dispatched to
  the rendering threads.

    The fragmenter function's prototype is declared in the
  "til_fragmenter_t" typedef, also in "src/til.h":

      typedef int (*til_fragmenter_t)(til_module_context_t *context, const til_fb_fragment_t *fragment, unsigned number, til_fb_fragment_t *res_fragment);

    While rototiller renders a frame using the provided fragmenter, it
  repeatedly calls the fragmenter with an increasing number parameter until
  the fragmenter returns 0.  The fragmenter is expected to return 1 when it
  described another fragment for the supplied number in *res_fragment.  For
  a given frame being rendered this way, the context and fragment
  parameters will be uniform throughout the frame.  The produced fragment
  in *res_fragment is expected to describe a subset of the provided
  fragment.

    Some low-level fragmenting helpers have been provided in
  "src/til_fb.[ch]":

      int til_fb_fragment_slice_single(const til_fb_fragment_t *fragment, unsigned n_fragments, unsigned num, til_fb_fragment_t *res_fragment);
      int til_fb_fragment_tile_single(const til_fb_fragment_t *fragment, unsigned tile_size, unsigned num, til_fb_fragment_t *res_fragment);

    Threaded modules to simply call one of these in their
  fragmenter function, i.e. in the "ray" module:

      static int ray_fragmenter(til_module_context_t *context, const til_fb_fragment_t *fragment, unsigned number, til_fb_fragment_t *res_fragment)
      {
        return til_fb_fragment_tile_single(fragment, 64, number, res_fragment);
      }

    This results in tiling the frame into 64x64 tiles which are then passed
  to the module's render_fragment().  The other helper,
  til_fb_fragment_slice_single(), instead slices up the input fragment into
  n_fragments horizontal slices.  Which is preferable depends on the
  rendering algorithm.  Use of these helpers is optional and provided just
  for convenience, modules are free to do whatever they wish here.

    Some higher-level fragmenters helpers have also been provided in
  "src/til.[ch]":

      int til_fragmenter_slice_per_cpu(til_module_context_t *context, const til_fb_fragment_t *fragment, unsigned number, til_fb_fragment_t *res_fragment);
      int til_fragmenter_slice_per_cpu_x16(til_module_context_t *context, const til_fb_fragment_t *fragment, unsigned number, til_fb_fragment_t *res_fragment);
      int til_fragmenter_tile64(til_module_context_t *context, const til_fb_fragment_t *fragment, unsigned number, til_fb_fragment_t *res_fragment);

    These may be provided directly as the fragmenter, rather than having to
  write your own.  Most of the time, one of these suffices.

    Building upon the first minimal example from above, here's an example
  adding threaded (tiled) rendering, with the higher-level helper:

      static void minimal_prepare_frame(til_module_context_t *context, til_stream_t *stream, unsigned ticks, til_fb_fragment_t **fragment_ptr, til_frame_plan_t *res_frame_plan)
      {
	*res_frame_plan = (til_frame_plan_t){ .fragmenter = til_fragmenter_tile64 };
      }

      static void minimal_render_fragment(til_module_context_t *context, til_stream_t *stream, unsigned ticks, unsigned cpu, til_fb_fragment_t **fragment_ptr)
      {
        /* render into (*fragment_ptr)->buf, which will be a 64x64 tile within the frame (modulo clipping) */

        /* Note:
         *  (*fragment_ptr)->frame_{width,height} reflect the logical
         *  frame dimensions of the whole-frame fragment provided to
         *  prepare_frame()
         *
         *  (*fragment_ptr)->{x,y,width,height} describe this fragment's
         *  tile position and size within the logical frame, which
         *  fragment->buf points at the upper left corner of.
         */
      }

      til_module_t minimal_module = {
              .prepare_frame = minimal_prepare_frame,
              .render_fragment = minimal_render_fragment,
              .name = "minimal",
              .description = "Minimal threaded example module",
      }


    That's all one must do to achieve threaded rendering.  Note however
  this places new constraints on what's safe to do from within the module's
  render_fragment() function.

    When using threaded rendering, any varying state accessed via
  render_fragment() must either be per-cpu or synchronized using a mutex
  or atomics.  For performance reasons, the per-cpu option is strongly
  preferred, as it avoids the need for any synchronization/atomics.

    Both the create_context() and prepare_frame() functions receive an
  n_cpus parameter primarily for the purpose of preparing
  per-thread/per-cpu resources that may then be trivially indexed using the
  cpu parameter supplied to render_fragment().  When preparing such
  per-thread resources, care must be taken to avoid sharing of cache
  lines.  A trivial (though wasteful) way to achieve this is to simply
  page-align the per-cpu allocation.  With more intimate knowledge of the
  cache line size (64 bytes is very common), one can be more frugal.  See
  the "snow" module for a simple example of using per-cpu state for lockless
  threaded stateful rendering (PRNG seed state per cpu).