[Intel-gfx] [RFC] drm/i915 : Reduce the shmem page allocation time by using blitter engines for clearing pages.

[Chris Wilson]

On Tue, May 06, 2014 at 04:40:58PM +0530, sourab.gupta at intel.com wrote:
> From: Sourab Gupta <sourab.gupta at intel.com>
> 
> This patch is in continuation of and is dependent on earlier patch
> series to 'reduce the time for which device mutex is kept locked'.
> (http://lists.freedesktop.org/archives/intel-gfx/2014-May/044596.html)
> 
> This patch aims to reduce the allocation time of pages from shmem
> by using blitter engines for clearing the pages freshly alloced.
> This is utilized in case of fresh pages allocated in shmem_preallocate
> routines in execbuffer path and page_fault path only.
> 
> Even though the CPU memset routine is optimized, but still sometimes
> the time consumed in clearing the pages of a large buffer comes in
> the order of milliseconds.
> We intend to make this operation asynchronous by using blitter engine,
> so irrespective of the size of buffer to be cleared, the execbuffer
> ioctl processing time will not be affected. Use of blitter engine will
> make the overall execution time of 'execbuffer' ioctl lesser for
> large buffers.
> 
> There may be power implications here on using blitter engines, and
> we have to evaluate this. As a next step, we can selectively enable
> this HW based memset only for large buffers, where the overhead of
> adding commands in a blitter ring(which will otherwise be idle),
> cross ring synchronization, will be negligible compared to using the
> CPU to clear out the buffer.

You leave a lot of holes by which you leak the uncleared pages to
userspace.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre