[alsa-devel] [PATCH 1/2 v2] ASoC: soc-cache: block based rbtree compression

Dimitris Papastamos dp at opensource.wolfsonmicro.com
Mon May 2 16:42:56 CEST 2011

On Mon, May 02, 2011 at 04:29:01PM +0200, Takashi Iwai wrote:
> At Mon,  2 May 2011 13:28:28 +0100,
> Dimitris Papastamos wrote:
> > 
> > This patch prepares the ground for the actual rbtree optimization patch
> > which will save a pointer to the last accessed register block that was used
> > in either the read() or write() functions.
> > 
> > Each node manages a block of up to RBTREE_BLOCK_NUM registers.  There can be
> > no two nodes with overlapping blocks.  Currently there is no check in the code
> > to scream in case that ever happens.  Each block has a base and top register,
> > all others lie in between these registers.  Note that variable length blocks
> > aren't supported.  So if you have an interval [8, 15] and only some of those
> > registers actually exist on the device, the block will have the non-existent
> > registers as zero.  There is also no way of reporting that any of those
> > non-existent registers were accessed/modified.
> > 
> > The larger the block size, the more probable it is that one of the
> > managed registers is non-zero, and therefore the node will need to be
> > allocated at initialization time and waste space.
> > 
> > If register N is accessed and it is not part of any of the current
> > blocks in the rbtree, a new node is created with a base register
> > which is floor(N / RBTREE_BLOCK_NUM) * RBTREE_BLOCK_NUM and a top
> > register as base_register + RBTREE_BLOCK_NUM - 1.  All other registers
> > in the block are initialized as expected and the node is inserted into
> > the tree.
> > 
> > Signed-off-by: Dimitris Papastamos <dp at opensource.wolfsonmicro.com>
> Looking through the patch, I wonder whether it gives a performance
> gain enough for the additional complexity.  Did you measure somehow?

This patch might still provide a performance benefit by itself since by
increasing the block size we decrease the number of nodes in the rbtree
that need to be looked up everytime we touch the cache.


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