[Intel-gfx] [PATCH] drm/i915: Trim the object sg table

[Chris Wilson]

On Wed, Nov 09, 2016 at 02:30:02PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> At the moment we allocate enough sg table entries assuming we
> will not be able to do any coallescing. But since in practice
> we most often can, and more so very effectively, this ends up
> wasting a lot of memory.
> 
> A simple and effective way of trimming the over-allocated
> entries is to copy the table over to a new one allocated to the
> exact size.
> 
> Experiment on my freshly logged and idle desktop (KDE) showed
Experiments
> that by doing this we can save approximately 1 MiB of RAM, or
> when running a typical benchmark like gl_manhattan I have
> even seen a 6 MiB saving.

More complicated techniques such as only copying the last used page and
freeing the rest are left to the reader.

> v2:
>  * Update commit message.
>  * Use temporary sg_table on stack. (Chris Wilson)
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index d2ad73d0b5b9..411aae535abe 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2232,6 +2232,28 @@ static unsigned int swiotlb_max_size(void)
>  #endif
>  }
>  
> +static void i915_sg_trim(struct sg_table *orig_st)
> +{
> +	struct sg_table new_st;
> +	struct scatterlist *sg, *new_sg;
> +	unsigned int i;
> +
> +	if (orig_st->nents == orig_st->orig_nents)
> +		return;
> +
> +	if (sg_alloc_table(&new_st, orig_st->nents, GFP_KERNEL))
> +		return;
> +
> +	new_sg = new_st.sgl;
> +	for_each_sg(orig_st->sgl, sg, orig_st->nents, i) {
> +		sg_set_page(new_sg, sg_page(sg), sg->length, 0);
> +		new_sg = sg_next(new_sg);

Worth a
	/* called before being DMA mapped, no need to copy sg->dma_* */
?

> +	}
> +
> +	sg_free_table(orig_st);
> +	memcpy(orig_st, &new_st, sizeof(*orig_st));

I would have used *orig_st = new;

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
           ^ I remembered it this time!
Took a couple of attempts to spell my name right though.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre