@TechReport{	  it:2011-029,
  author	= {Alberto Ros and Stefanos Kaxiras},
  title		= {{VIPS}: Simple Directory-Less Broadcast-Less Cache
		  Coherence Protocol},
  institution	= {Department of Information Technology, Uppsala University},
  department	= {Division of Computer Systems},
  year		= {2011},
  number	= {2011-029},
  month		= nov,
  abstract	= {Coherence in multicores introduces complexity and overhead
		  (directory, state bits) in exchange for local caching,
		  while being ``invisible'' to the memory consistency model.
		  In this paper we show that a much simpler
		  (directory-less/broadcast-less) multicore coherence
		  provides almost the same performance without the complexity
		  and overhead of a directory protocol. Motivated by recent
		  efforts to simplify coherence for disciplined parallelism,
		  we propose a hardware approach that does not require any
		  application guidance. The cornerstone of our approach is a
		  run-time, application-transparent, division of data into
		  private and shared at a page-level granularity. This allows
		  us to implement a dynamic write-policy (write-back for
		  private, write-through for shared), simplifying the
		  protocol to just two stable states. Self-invalidation of
		  the shared data at synchronization points allows us to
		  remove the directory (and invalidations) completely, with
		  just a data-race-free guarantee (at the write-through
		  granularity) from software. Allowing multiple simultaneous
		  writers and merging their writes, relaxes the DRF guarantee
		  to a word granularity and optimizes traffic. This leads to
		  our main result: a virtually costless coherence that uses
		  the same simple protocol for both shared, DRF data and
		  private data (differentiating only in the timing of when to
		  put data back in the last-level cache) while at the same
		  time approaching the performance (within 3\%) of a complex
		  directory protocol. }
}