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

[Jason Ekstrand]

In general, I'm very concerned about how this handles rounding behavior.
Almost everywhere, you round down when what you want to do is round up.
Also, as I said on IRC, I'd like to see some asserts in add_bucket so that
we are sure this calculation is correct.  In particular, I'd like to see

assert(bucket_for_size(size) == &bufmgr->cache_bucket[i]);
assert(bucket_for_size(size - 2048) == &bufmgr->cache_bucket[i]);
assert(bucket_for_size(size + 1) != &bufmgr->cache_bucket[i]);

We need to check on both sides of size to be 100% sure we're doing our
rounding correctly.

On Fri, Sep 8, 2017 at 1:11 AM, <aravindan.muthukumar at intel.com> wrote:

> From: Aravindan Muthukumar <aravindan.muthukumar at intel.com>
>
> Avoiding the loop which was running with O(n) complexity.
> Now the complexity has been reduced to O(1)
>
> Tested with piglit.
> Slight performance improvement (~1%) in 3d mark.
>

Which 3dmark test?  Also, what's the error in that 1%?


> Change-Id: Id099f1cd24ad5b691a69070eda79b8f4e9be39a6
> 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 | 48
> +++++++++++++++++++++++++++++-----
>  1 file changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.c
> b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> index 5b4e784..18cb166 100644
> --- a/src/mesa/drivers/dri/i965/brw_bufmgr.c
> +++ b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> @@ -87,6 +87,11 @@
>
>  #define memclear(s) memset(&s, 0, sizeof(s))
>
> +/* Macros for BO cache size */
> +#define CACHE_PAGE_SIZE    4096
>

Just call this PAGE_SIZE


> +#define PAGE_SIZE_SHIFT    12
> +#define BO_CACHE_PAGE_SIZE (4 * CACHE_PAGE_SIZE)
>

I think I'd rather we just use 4 * PAGE_SIZE explicitly than have this
extra #define.  I think it's making things harder to read and not easier.


> +
>  #define FILE_DEBUG_FLAG DEBUG_BUFMGR
>
>  static inline int
> @@ -181,19 +186,48 @@ bo_tile_pitch(struct brw_bufmgr *bufmgr, uint32_t
> pitch, uint32_t tiling)
>     return ALIGN(pitch, tile_width);
>  }
>
> +/*
> + * This functions is to find the correct bucket fit for the input size.
> + * This 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;
> +   struct bo_cache_bucket *bucket = NULL;
> +   int x=0,index = -1;
> +   int row, col=0;
>
> -   for (i = 0; i < bufmgr->num_buckets; i++) {
> -      struct bo_cache_bucket *bucket = &bufmgr->cache_bucket[i];
> -      if (bucket->size >= size) {
> -         return bucket;
> -      }
> +   /* condition for size less  than 4*4096 (4KB) page size */
> +   if(size < BO_CACHE_PAGE_SIZE){
>

This should be "<="


> +      index = (size>>PAGE_SIZE_SHIFT)+((size%(1<<PAGE_SIZE_SHIFT)?1:0)
> )-1;
>

I agree with tapani, that this can easily be an early return.

I think we can also make this calculation a lot more clear:

index = DIV_ROUND_UP(size, PAGE_SIZE) - 1;


>     }
> +   else{
> +      /* When the size is more than 4*4096, the logic follows a matrix
> method
> +       * where the index will be searched using Arithmetico-Geometric
> progression.
> +       * So the given size will be divided by 4096 & the index will be
> traced out.
> +       */
> +      x = size>>PAGE_SIZE_SHIFT;
>

This rounds down not up like you want.


> -   return NULL;
> +      /* Find the row using Geometric Progression. The highest bit set
> will give
> +       * the row number. num = a * r^(n-1) where num = size a = 4 r = 2
> +       */
> +      row = 31 - __builtin_clz(x>>1);
> +
> +     /* Find the column using AP but using the row value
> +      * calculated using GP.
> +      */
> +      col =((x-(1<<(row+1)))/(1<<(row-1)))+1;
> +      col += (size%(1<<PAGE_SIZE_SHIFT<<(row-1)))?1:0;
> +
> +      /* Finding the index value using calculated row and col number */
> +      index = ((row-1)<<2) + col + 2;
>

I think this can probably also be a lot simpler.  How about something like
this:

pages = DIV_ROUND_UP(size, PAGE_SIZE);
/* Steal this from anv_allocator.c */
pages_log2 = ilog2_round_up(pages);
row = pages_log2 - 1;
col = DIV_ROUND_UP(pages, (1 << (pages_log2 - 2)));
index = row * 4 + col;

+   }
> +
> +   /* Checking the error condition */
> +   bucket = (index >= 0 && index < bufmgr->num_buckets)?(&bufmgr-
> >cache_bucket[index]):NULL;
> +   return bucket;
>  }
>
>  int
> --
> 2.7.4
>
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