[PATCH libdrm 1/2] intel: align reuse buffer's size on page size instead
Chris Wilson
chris at chris-wilson.co.uk
Wed Feb 21 21:43:55 UTC 2018
Quoting James Xiong (2018-02-20 17:48:03)
> From: "Xiong, James" <james.xiong at intel.com>
>
> With gem_reuse enabled, when a buffer size is different than
> the sizes of buckets, it is aligned to the next bucket's size,
> which means about 25% more memory than the requested is allocated
> in the worst senario. For example:
>
> Orignal size Actual
> 32KB+1Byte 40KB
> .
> .
> .
> 8MB+1Byte 10MB
> .
> .
> .
> 96MB+1Byte 112MB
>
> This is very memory expensive and make the reuse feature less
> favorable than it deserves to be.
The reuse feature also misses one important source: reusing temporaries
within a batch.
> This commit aligns the reuse buffer size on page size instead,
> the buffer whose size falls between bucket[n] and bucket[n+1] is
> put in bucket[n] when it's done; And when searching for a cached
> buffer for reuse, it goes through the cached buffers list in the
> bucket until a cached buffer, whose size is larger than or equal
> to the requested size, is found.
So how many times do you have to allocate a new buffer because you
refused to hand back a slightly larger buffer? Have you checked the
impact on !llc? With mmaps? On how wide a workload?
> Signed-off-by: Xiong, James <james.xiong at intel.com>
> ---
> intel/intel_bufmgr_gem.c | 180 +++++++++++++++++++++++++----------------------
> libdrm_lists.h | 6 ++
> 2 files changed, 101 insertions(+), 85 deletions(-)
>
> diff --git a/intel/intel_bufmgr_gem.c b/intel/intel_bufmgr_gem.c
> index 386da30..5b2d0d0 100644
> --- a/intel/intel_bufmgr_gem.c
> +++ b/intel/intel_bufmgr_gem.c
> @@ -402,11 +402,10 @@ drm_intel_gem_bo_bucket_for_size(drm_intel_bufmgr_gem *bufmgr_gem,
> {
> int i;
>
> - for (i = 0; i < bufmgr_gem->num_buckets; i++) {
> - struct drm_intel_gem_bo_bucket *bucket =
> - &bufmgr_gem->cache_bucket[i];
> - if (bucket->size >= size) {
> - return bucket;
> + for (i = 0; i < bufmgr_gem->num_buckets - 1; i++) {
> + if (size >= bufmgr_gem->cache_bucket[i].size &&
> + size < bufmgr_gem->cache_bucket[i+1].size) {
> + return &bufmgr_gem->cache_bucket[i];
So you never return the last bucket?
Given the ordered set of buckets, the test remains correct even when
every bo within the bucket is not the full size (each bo is still at
least bigger than the previous bucket).
> }
> }
>
> @@ -685,25 +684,95 @@ drm_intel_gem_bo_madvise(drm_intel_bo *bo, int madv)
> madv);
> }
>
> -/* drop the oldest entries that have been purged by the kernel */
> +/* drop the entries that are older than the given time */
> static void
> drm_intel_gem_bo_cache_purge_bucket(drm_intel_bufmgr_gem *bufmgr_gem,
> - struct drm_intel_gem_bo_bucket *bucket)
> + struct drm_intel_gem_bo_bucket *bucket,
> + time_t time)
> {
> - while (!DRMLISTEMPTY(&bucket->head)) {
> - drm_intel_bo_gem *bo_gem;
> -
> - bo_gem = DRMLISTENTRY(drm_intel_bo_gem,
> - bucket->head.next, head);
> - if (drm_intel_gem_bo_madvise_internal
> - (bufmgr_gem, bo_gem, I915_MADV_DONTNEED))
> - break;
> -
> - DRMLISTDEL(&bo_gem->head);
> - drm_intel_gem_bo_free(&bo_gem->bo);
> + drm_intel_bo_gem *bo_gem, *temp;
> + DRMLISTFOREACHENTRYSAFE(bo_gem, temp, &bucket->head, head) {
> + if (bo_gem->free_time >= time) {
> + drm_intel_gem_bo_madvise_internal
> + (bufmgr_gem, bo_gem, I915_MADV_DONTNEED);
> + DRMLISTDEL(&bo_gem->head);
> + drm_intel_gem_bo_free(&bo_gem->bo);
> + }
This function is called after the kernel reports that it purged a
buffer, and the intent here is that we find all the buffers that the
kernel purged and evict them. It is not about discarding old buffers,
just throwing out the empty.
Honestly, it's pointless. The cost of having the empty purged bo around
is insignificant (we reclaim a little bit of mmap virtual space
quicker).
> }
> }
>
> +static drm_intel_bo_gem *
> +drm_intel_gem_bo_cached_for_size(drm_intel_bufmgr_gem *bufmgr_gem,
> + unsigned long size,
> + uint32_t tiling_mode,
> + unsigned long stride,
> + unsigned long alignment,
> + bool for_render)
> +{
> + struct drm_intel_gem_bo_bucket *bucket =
> + drm_intel_gem_bo_bucket_for_size(bufmgr_gem, size);
> +
> + if(bucket != NULL) {
> + drm_intel_bo_gem *bo_gem, *temp_bo_gem;
> +retry:
> + bo_gem = NULL;
> + if (for_render) {
> + /* Allocate new render-target BOs from the tail (MRU)
> + * of the list, as it will likely be hot in the GPU
> + * cache and in the aperture for us.
> + */
> + DRMLISTFOREACHENTRYREVERSE(temp_bo_gem, &bucket->head, head) {
> + if (temp_bo_gem->bo.size >= size) {
> + bo_gem = temp_bo_gem;
> + DRMLISTDEL(&bo_gem->head);
> + bo_gem->bo.align = alignment;
> + break;
> + }
> + }
> + } else {
> + assert(alignment == 0);
This is random.
> + /* For non-render-target BOs (where we're probably
> + * going to map it first thing in order to fill it
> + * with data), check if the last BO in the cache is
> + * unbusy, and only reuse in that case. Otherwise,
> + * allocating a new buffer is probably faster than
> + * waiting for the GPU to finish.
> + */
> + DRMLISTFOREACHENTRY(temp_bo_gem, &bucket->head, head) {
> + if (temp_bo_gem->bo.size >= size &&
> + !drm_intel_gem_bo_busy(&temp_bo_gem->bo)) {
> + bo_gem = temp_bo_gem;
> + DRMLISTDEL(&bo_gem->head);
> + break;
> + }
> + }
> + }
> +
> + if(bo_gem) {
> + if (!drm_intel_gem_bo_madvise_internal(bufmgr_gem, bo_gem,
> + I915_MADV_WILLNEED)) {
> + /* failed to reuse the cached buffer.
> + * the buffers older cannot be reused and can be purged
> + * from the bucket.
> + */
> + drm_intel_gem_bo_cache_purge_bucket(bufmgr_gem, bucket,
> + bo_gem->free_time);
> + drm_intel_gem_bo_free(&bo_gem->bo);
> + return NULL;
> + }
> +
> + if (drm_intel_gem_bo_set_tiling_internal(&bo_gem->bo,
> + tiling_mode,
> + stride)) {
> + drm_intel_gem_bo_free(&bo_gem->bo);
> + goto retry;
> + }
> + }
> + return bo_gem;
> + }
> + return NULL;
> +}
> +
> static drm_intel_bo *
> drm_intel_gem_bo_alloc_internal(drm_intel_bufmgr *bufmgr,
> const char *name,
> @@ -715,81 +784,22 @@ drm_intel_gem_bo_alloc_internal(drm_intel_bufmgr *bufmgr,
> {
> drm_intel_bufmgr_gem *bufmgr_gem = (drm_intel_bufmgr_gem *) bufmgr;
> drm_intel_bo_gem *bo_gem;
> - unsigned int page_size = getpagesize();
> int ret;
> - struct drm_intel_gem_bo_bucket *bucket;
> - bool alloc_from_cache;
> - unsigned long bo_size;
> bool for_render = false;
>
> if (flags & BO_ALLOC_FOR_RENDER)
> for_render = true;
>
> - /* Round the allocated size up to a power of two number of pages. */
> - bucket = drm_intel_gem_bo_bucket_for_size(bufmgr_gem, size);
> -
> - /* If we don't have caching at this size, don't actually round the
> - * allocation up.
> - */
> - if (bucket == NULL) {
> - bo_size = size;
> - if (bo_size < page_size)
> - bo_size = page_size;
> - } else {
> - bo_size = bucket->size;
> - }
> + /* first align the size on page boundary */
> + size = ALIGN(size, getpagesize());
Ime, you want tight (exact) fitting for long lived objects and loose
fitting for temporaries. Do you want to pass in a flag?
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