[Mesa-dev] [PATCH v3] i965 : optimized bucket index calculation

[aravindan.muthukumar at intel.com]

From: Aravindan Muthukumar <aravindan.muthukumar at intel.com>

Now the complexity has been reduced to O(1)

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)

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>
Reviewed-by: Yogesh Marathe <yogesh.marathe at intel.com>
---
 src/mesa/drivers/dri/i965/brw_bufmgr.c | 38 +++++++++++++++++++++++++++-------
 1 file changed, 30 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..9a423da 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 +182,35 @@ 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 int pages = (size + PAGE_SIZE - 1) / PAGE_SIZE;
 
-   for (i = 0; i < bufmgr->num_buckets; i++) {
-      struct bo_cache_bucket *bucket = &bufmgr->cache_bucket[i];
-      if (bucket->size >= size) {
-         return bucket;
-      }
-   }
+   /* Finding the row number based on the calculated pages. */
+   const unsigned int rows = 30 - __builtin_clz((pages - 1) | 3);
 
-   return NULL;
+   const unsigned int row_max_pages = 4 << rows;
+   const unsigned int prev_row_max_pages = (row_max_pages / 2) & ~2;
+
+   /* Finding the column number using column interval. */
+   int col_size_log2 = rows - 1;
+   col_size_log2 += (col_size_log2 < 0);
+
+   const unsigned int col = ( (pages - prev_row_max_pages +
+                        ( (1 << col_size_log2) - 1) ) >> col_size_log2 );
+
+   /* Calculating the index based on the row and column. */
+   const unsigned int index = (rows * 4) + (col - 1);
+
+   return (index < bufmgr->num_buckets) ?
+          &bufmgr->cache_bucket[index] : NULL;
 }
 
 int
@@ -1254,6 +1272,10 @@ 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
-- 
2.7.4