extra_information.txt

i7. HotFloorplan:

a) As mentioned above, one of the inputs to HotFloorplan is a
   floorplan description file that has information about the
   blocks to be floorplanned. The maximum aspect ratio constraint is set
   to 1:3 except when a block has a bigger aspect ratio in the
   base floorplan. In that case, the block's aspect
   ratio itself forms the upper limit. Also,
   the connectivity section of 'ev6.desc' currently lists 13
   major interconnects that we thought were important at the
   architecture level. They are assumed to be of equal wire
   density. Also, the wire_density field of the floorplan
   description can be used to assign weights for the different
   wires according to their contribution to performance. In
   'ev6.desc', the assignment of these weights is uniform
   across all wires.
b) HotFloorplan includes a first-order wire-delay model for
   wires in the global and intermediate metal layers. It is
   a simple stand-alone model that converts wire length to
   wire delay and can be used in any tool, whether the thermal
   model, floorplan model, or any other simulator. It is
   adapted from Otten and Brayton, DAC'98, and related work.
   The interfaces for the model are provided 'wire.[ch]'. An
   appropriate process technology node can be chosen by using
   the TECHNODE #define in 'wire.h'. After that, the model is
   a simple call to the function 'wirelength2delay'.
c) HotFloorplan can be configured through many command line options.
   The 'hotspot.config' file clearly lists them and what they do.
   Among the options, following merit further discussion:
   i) Annealing parameters:
      The meaning of each of these can be easily obtained from any
      standard text on VLSI CAD that deals with simulated
      annealing (e.g.: Sarrafzadeh and Wong, "An Introduction to
      VLSI Physical Design", McGraw-Hill 1996). The original
      DAC '86 paper by Wong and Liu on floorplanning is a very good
      starting point.
  ii) Compaction ratio:
      During floorplanning, empty spaces can arise in a floorplan.
      Sometimes, these spaces are just an artifact of the discrete
      nature of the aspect ratio function of the individual
      blocks. Typically, about 50% of the maximum possible number
      of dead spaces is due to this artifact. Such dead spaces
      are very tiny and hence can be safely ignored without any
      impact on the thermal model. In fact, ignoring such blocks
      reduces the size of the problem input to the thermal solver.
      Since the thermal model's solver algorithm is a higher order
      polynomial function in the number of functional blocks,
      ignoring the tiny blocks improves the performance of the
      floorplanner significantly. The 'compact_ratio' option
      sets a threshold on the area of such tiny blocks. The
      default is 0.5% of the area of the encompassing 'parent
      rectangle'. So, dead spaces that are smaller than 0.5% of
      the parent rectangle are ignored during floorplanning. This
      results in a maximum error in core area of about 0.46% for
      our experiments. This setting, combined with the 'n_orients'
      setting that determines the magnitude of discreteness of
      the blocks' aspect ratio function, can be tuned to manage
      the trade-off between the speed and accuracy of the
      floorplanner. It is to be noted that in HotFloorplan, the
      empty spaces that are not compacted are named in the form
      _0, _1 and so on. It can be seen that the 'output.flp'
      generated from the steps discussed above has blocks named
      in that form.
 iii) Weights for the metric (objective function):
      HotFloorplan uses an objective function that is of the form
      lambdaA * A + lambdaT * T + lambdaW * W where A, T and W are
      the area, temperature and wire length respectively. The
      weights lambdaA lambdaT and lambdaW can be set through
      configuration parameters. Please note that for HotFloorplan,
      A is in the order or hundreds of mm, T is in the order of
      hundreds of Kelvin and W is in the order of tens of mm. In
      combining them to a single metric, the weights have to be
      assigned in a manner that not only takes into account the
      desired importance of the variables A, T and W but also
      compensates for the mismatch in their units of measurement.
      The default weights in HotFloorplan are chosen based on this
      principle. If one is interested in objective functions that
      are not in the above-mentioned form, it is quite easy to
      make the change in HotFloorplan. The design of the simulated
      annealing algorithm is such that any arbitrary metric (objective
      function) can be incorporated. In HotFloorplan, this is done
      just by changing the return statement of the
      'flp_evaluate_metric' function at the beginning of 'flp.c' into
      an appropriate objective function of A, T and W.