[Mesa-dev] [PATCH 1/2] winsys/amdgpu: adjust IB size based on buffer wait time

[Grigori Goronzy]

Small IBs help to reduce stalls for workloads that require a lot of
synchronization. On the other hand, if there is no notable
synchronization, we can use a large IB size to slightly improve
performance in some cases.

This introduces tuning of the IB size based on feedback on the average
buffer wait time. The average wait time is tracked with exponential
smoothing.
---
 src/gallium/winsys/amdgpu/drm/amdgpu_bo.c     |  6 +++++-
 src/gallium/winsys/amdgpu/drm/amdgpu_cs.c     | 17 ++++++++++++-----
 src/gallium/winsys/amdgpu/drm/amdgpu_cs.h     |  2 ++
 src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h |  9 +++++++++
 4 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c b/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
index 036301e..4b8554d 100644
--- a/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
+++ b/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
@@ -195,8 +195,10 @@ static void *amdgpu_bo_map(struct pb_buffer *buf,
                return NULL;
             }
          }
+         amdgpu_winsys_update_buffer_wait_avg(bo->ws, 0);
       } else {
          uint64_t time = os_time_get_nano();
+         uint64_t duration;
 
          if (!(usage & PIPE_TRANSFER_WRITE)) {
             /* Mapping for read.
@@ -221,7 +223,9 @@ static void *amdgpu_bo_map(struct pb_buffer *buf,
                            RADEON_USAGE_READWRITE);
          }
 
-         bo->ws->buffer_wait_time += os_time_get_nano() - time;
+         duration = os_time_get_nano() - time;
+         bo->ws->buffer_wait_time += duration;
+         amdgpu_winsys_update_buffer_wait_avg(bo->ws, duration);
       }
    }
 
diff --git a/src/gallium/winsys/amdgpu/drm/amdgpu_cs.c b/src/gallium/winsys/amdgpu/drm/amdgpu_cs.c
index 69902c4..b9a7c5b 100644
--- a/src/gallium/winsys/amdgpu/drm/amdgpu_cs.c
+++ b/src/gallium/winsys/amdgpu/drm/amdgpu_cs.c
@@ -201,10 +201,7 @@ amdgpu_ctx_query_reset_status(struct radeon_winsys_ctx *rwctx)
 static bool amdgpu_get_new_ib(struct radeon_winsys *ws, struct amdgpu_ib *ib,
                               struct amdgpu_cs_ib_info *info, unsigned ib_type)
 {
-   /* Small IBs are better than big IBs, because the GPU goes idle quicker
-    * and there is less waiting for buffers and fences. Proof:
-    *   http://www.phoronix.com/scan.php?page=article&item=mesa-111-si&num=1
-    */
+   struct amdgpu_winsys *aws = (struct amdgpu_winsys *)ws;
    unsigned buffer_size, ib_size;
 
    switch (ib_type) {
@@ -216,8 +213,18 @@ static bool amdgpu_get_new_ib(struct radeon_winsys *ws, struct amdgpu_ib *ib,
       ib_size = 128 * 1024 * 4;
       break;
    case IB_MAIN:
+      /* Small IBs are often better than big IBs, because the GPU goes idle
+       * quicker and there is less waiting for buffers and fences. Proof:
+       *   http://www.phoronix.com/scan.php?page=article&item=mesa-111-si&num=1
+       * Tune IB size depending on average buffer waiting time, which is an
+       * indicator for the amount of synchronization going on. Some
+       * applications don't cause notable synchronization, so we can use
+       * large IB size for slightly improved throughput.
+       */
       buffer_size = 128 * 1024 * 4;
-      ib_size = 20 * 1024 * 4;
+      ib_size = 32 * 1024 * 4;
+      if (aws->buffer_wait_time_avg > IB_SIZE_WAIT_THRESHOLD_NS)
+         ib_size = 10 * 1024 * 4;
    }
 
    ib->base.cdw = 0;
diff --git a/src/gallium/winsys/amdgpu/drm/amdgpu_cs.h b/src/gallium/winsys/amdgpu/drm/amdgpu_cs.h
index 4ed830b..98e58a2 100644
--- a/src/gallium/winsys/amdgpu/drm/amdgpu_cs.h
+++ b/src/gallium/winsys/amdgpu/drm/amdgpu_cs.h
@@ -35,6 +35,8 @@
 #include "amdgpu_bo.h"
 #include "util/u_memory.h"
 
+#define IB_SIZE_WAIT_THRESHOLD_NS   10000
+
 struct amdgpu_ctx {
    struct amdgpu_winsys *ws;
    amdgpu_context_handle ctx;
diff --git a/src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h b/src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h
index 91b9be4..3bd63b6 100644
--- a/src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h
+++ b/src/gallium/winsys/amdgpu/drm/amdgpu_winsys.h
@@ -54,6 +54,7 @@ struct amdgpu_winsys {
    uint64_t allocated_vram;
    uint64_t allocated_gtt;
    uint64_t buffer_wait_time; /* time spent in buffer_wait in ns */
+   uint64_t buffer_wait_time_avg;
    uint64_t num_cs_flushes;
    unsigned gart_page_size;
 
@@ -76,6 +77,14 @@ amdgpu_winsys(struct radeon_winsys *base)
    return (struct amdgpu_winsys*)base;
 }
 
+static inline
+void amdgpu_winsys_update_buffer_wait_avg(struct amdgpu_winsys *ws,
+                                          uint64_t wait)
+{
+   /* Exponential smoothing with alpha = 0.25 */
+   ws->buffer_wait_time_avg = (3 * ws->buffer_wait_time_avg + wait) / 4;
+}
+
 void amdgpu_surface_init_functions(struct amdgpu_winsys *ws);
 ADDR_HANDLE amdgpu_addr_create(struct amdgpu_winsys *ws);
 
-- 
1.9.1