[Mesa-dev] [PATCH 09/10] vbo: disable the minmax cache when the hit rate is low

[Nicolai Hähnle]

From: Nicolai Hähnle <nicolai.haehnle at amd.com>

When applications stream their index buffers, the caches for those BOs become
useless and add overhead, so we want to disable them. The tricky part is
coming up with the right heuristic for *when* to disable them.

The first question is which hit rate to aim for. Since I'm not aware of any
interesting borderline applications that do something like "draw two or three
times for each upload", I just kept it simple.

The second question is how soon we should give up on the caching. Applications
might have a warm-up phase where they fill a buffer gradually but then keep
reusing it. For this reason, I count the number of indices that hit and miss
(instead of the number of calls that hit or miss), since comparing that to
the size of the buffer makes sense.
---
 src/mesa/main/mtypes.h          |  2 ++
 src/mesa/vbo/vbo_minmax_index.c | 34 ++++++++++++++++++++++++++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
index cfa11e8..68522b6 100644
--- a/src/mesa/main/mtypes.h
+++ b/src/mesa/main/mtypes.h
@@ -1286,6 +1286,8 @@ struct gl_buffer_object
 
    /** Memoization of min/max index computations for static index buffers */
    struct hash_table *MinMaxCache;
+   unsigned MinMaxCacheHitIndices;
+   unsigned MinMaxCacheMissIndices;
    bool MinMaxCacheDirty;
 };
 
diff --git a/src/mesa/vbo/vbo_minmax_index.c b/src/mesa/vbo/vbo_minmax_index.c
index 47b0d9c..0f75a87 100644
--- a/src/mesa/vbo/vbo_minmax_index.c
+++ b/src/mesa/vbo/vbo_minmax_index.c
@@ -117,9 +117,24 @@ vbo_get_minmax_cached(struct gl_buffer_object *bufferObj,
    mtx_lock(&bufferObj->Mutex);
 
    if (bufferObj->MinMaxCacheDirty) {
+      /* Disable the cache permanently for this BO if the number of hits
+       * is asymptotically less than the number of misses. This happens when
+       * applications use the BO for streaming.
+       *
+       * However, some initial optimism allows applications that interleave
+       * draw calls with glBufferSubData during warmup.
+       */
+      unsigned optimism = bufferObj->Size;
+      if (bufferObj->MinMaxCacheMissIndices > optimism &&
+          bufferObj->MinMaxCacheHitIndices < bufferObj->MinMaxCacheMissIndices - optimism) {
+         bufferObj->UsageHistory |= USAGE_DISABLE_MINMAX_CACHE;
+         vbo_delete_minmax_cache(bufferObj);
+         goto out_disable;
+      }
+
       _mesa_hash_table_clear(bufferObj->MinMaxCache, vbo_minmax_cache_delete_entry);
       bufferObj->MinMaxCacheDirty = false;
-      goto out;
+      goto out_invalidate;
    }
 
    key.type = type;
@@ -134,7 +149,22 @@ vbo_get_minmax_cached(struct gl_buffer_object *bufferObj,
       found = GL_TRUE;
    }
 
-out:
+out_invalidate:
+   if (found) {
+      /* The hit counter saturates so that we don't accidently disable the
+       * cache in a long-running program.
+       */
+      unsigned new_hit_count = bufferObj->MinMaxCacheHitIndices + count;
+
+      if (new_hit_count >= bufferObj->MinMaxCacheHitIndices)
+         bufferObj->MinMaxCacheHitIndices = new_hit_count;
+      else
+         bufferObj->MinMaxCacheHitIndices = ~(unsigned)0;
+   } else {
+      bufferObj->MinMaxCacheMissIndices += count;
+   }
+
+out_disable:
    mtx_unlock(&bufferObj->Mutex);
    return found;
 }
-- 
2.5.0