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

[Takashi Iwai]

At Mon, 2 May 2011 15:40:20 +0100,
Dimitris Papastamos wrote:
> 
> 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 itself does not provide a performance benefit.  The next
> patch in line which caches the last used block will provide a
> performance benefit.

Yeah, sure, it was a question to this series of patches :)

>  I have planted some cache hit/miss counters in my
> debug code to measure this.  Ideally it'd be nice for the codec
> drivers/machine drivers to be able to tune the block size at init time.

For such a performance improvement change, always the objective
measurement is preferred.  If only hit/miss matters, a hash table with
a reasonable size can be often a better option (and much simpler).


thanks,

Takashi