[Mesa-dev] [PATCH v4] i965 : optimized bucket index calculation

[Ian Romanick]

On 11/08/2017 09:45 PM, aravindan.muthukumar at intel.com wrote:
> From: Aravindan Muthukumar <aravindan.muthukumar at intel.com>
> 
> Reducing Bucket index calculation to O(1).
> 
> This algorithm calculates the index using matrix method.
> Matrix arrangement is as below:
> Assuming PAGE_SIZE is 4096.
> 
>           1*4096   2*4096    3*4096    4*4096
>           5*4096   6*4096    7*4096    8*4096
>           10*4096  12*4096   14*4096   16*4096
>           20*4096  24*4096   28*4096   32*4096
>            ...      ...       ...       ...
>            ...      ...       ...       ...
>            ...      ...       ...   max_cache_size
> 
> From this matrix its clearly seen that every row
> follows the below way:
>           ...       ...       ...        n
>         n+(1/4)n  n+(1/2)n  n+(3/4)n    2n
> 
> Row is calculated as log2(size/PAGE_SIZE)
> Column is calculated as converting the difference
> between the elements to fit into power size of two
> and indexing it.
> 
> Final Index is (row*4)+(col-1)
> 
> Tested with Intel Mesa CI.
> 
> Improves performance of 3DMark on BXT by 0.705966% +/- 0.229767% (n=20)
> 
> v4: Review comments on style and code comments implemented (Ian).
> v3: Review comments implemented (Ian).
> v2: Review comments implemented (Jason).
> 
> Signed-off-by: Aravindan Muthukumar <aravindan.muthukumar at intel.com>
> Signed-off-by: Kedar Karanje <kedar.j.karanje at intel.com>

Since a fair amount of the code is now authored by me, I guess it's more
appropriate to add

Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>

I think 3 S-o-b and a R-b should be sufficient, so I have pushed it.

> Reviewed-by: Yogesh Marathe <yogesh.marathe at intel.com>
> ---
>  src/mesa/drivers/dri/i965/brw_bufmgr.c | 47 ++++++++++++++++++++++++++++------
>  1 file changed, 39 insertions(+), 8 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.c b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> index 17036b5..f21df5a 100644
> --- a/src/mesa/drivers/dri/i965/brw_bufmgr.c
> +++ b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> @@ -86,6 +86,8 @@
>  
>  #define memclear(s) memset(&s, 0, sizeof(s))
>  
> +#define PAGE_SIZE 4096
> +
>  #define FILE_DEBUG_FLAG DEBUG_BUFMGR
>  
>  static inline int
> @@ -180,19 +182,44 @@ bo_tile_pitch(struct brw_bufmgr *bufmgr, uint32_t pitch, uint32_t tiling)
>     return ALIGN(pitch, tile_width);
>  }
>  
> +/**
> + * This function finds the correct bucket fit for the input size.
> + * The function works with O(1) complexity when the requested size
> + * was queried instead of iterating the size through all the buckets.
> + */
>  static struct bo_cache_bucket *
>  bucket_for_size(struct brw_bufmgr *bufmgr, uint64_t size)
>  {
> -   int i;
> +   /* Calculating the pages and rounding up to the page size. */
> +   const unsigned pages = (size + PAGE_SIZE - 1) / PAGE_SIZE;
> +
> +   /* Row  Bucket sizes    clz((x-1) | 3)   Row    Column
> +    *        in pages                      stride   size
> +    *   0:   1  2  3  4 -> 30 30 30 30        4       1
> +    *   1:   5  6  7  8 -> 29 29 29 29        4       1
> +    *   2:  10 12 14 16 -> 28 28 28 28        8       2
> +    *   3:  20 24 28 32 -> 27 27 27 27       16       4
> +    */
> +   const unsigned row = 30 - __builtin_clz((pages - 1) | 3);
> +   const unsigned row_max_pages = 4 << row;
> +   
> +   /* The '& ~2' is the special case for row 1. In row 1, max pages /
> +    * 2 is 2, but the previous row maximum is zero (because there is
> +    * no previous row). All row maximum sizes are power of 2, so that
> +    * is the only case where that bit will be set.
> +    */
> +   const unsigned prev_row_max_pages = (row_max_pages / 2) & ~2;
> +   int col_size_log2 = row - 1;
> +   col_size_log2 += (col_size_log2 < 0);
>  
> -   for (i = 0; i < bufmgr->num_buckets; i++) {
> -      struct bo_cache_bucket *bucket = &bufmgr->cache_bucket[i];
> -      if (bucket->size >= size) {
> -         return bucket;
> -      }
> -   }
> +   const unsigned col = (pages - prev_row_max_pages +
> +                        ((1 << col_size_log2) - 1)) >> col_size_log2;
>  
> -   return NULL;
> +   /* Calculating the index based on the row and column. */
> +   const unsigned index = (row * 4) + (col - 1);
> +
> +   return (index < bufmgr->num_buckets) ?
> +          &bufmgr->cache_bucket[index] : NULL;
>  }
>  
>  int
> @@ -1254,6 +1281,10 @@ add_bucket(struct brw_bufmgr *bufmgr, int size)
>     list_inithead(&bufmgr->cache_bucket[i].head);
>     bufmgr->cache_bucket[i].size = size;
>     bufmgr->num_buckets++;
> +
> +   assert(bucket_for_size(bufmgr, size) == &bufmgr->cache_bucket[i]);
> +   assert(bucket_for_size(bufmgr, size - 2048) == &bufmgr->cache_bucket[i]);
> +   assert(bucket_for_size(bufmgr, size + 1) != &bufmgr->cache_bucket[i]);
>  }
>  
>  static void
>