linux-kernel - Re: GEM-related desktop sluggishness due to linear-time arch_get_unmapped_area

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Message-Id: <b7da2f$qtmdi3@fmsmga001.fm.intel.com>
Date:	Wed, 30 Mar 2011 15:07:28 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	Dave Airlie <airlied@...il.com>, r6144 <rainy6144@...il.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: GEM-related desktop sluggishness due to linear-time arch_get_unmapped_area_topdown()

On Wed, 30 Mar 2011 09:28:07 -0400, Jerome Glisse <j.glisse@...il.com> wrote:
> On Wed, Mar 30, 2011 at 3:32 AM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> > On Wed, 30 Mar 2011 07:57:49 +1000, Dave Airlie <airlied@...il.com> wrote:
> >> On Wed, Mar 30, 2011 at 7:04 AM, Jerome Glisse <j.glisse@...il.com> wrote:
> >> > What i had in mind was something little bit more advance that pwrite,
> >> > somethings that would take width,height,pitch of userpage and would be
> >> > able to perform proper blit. But yes pwrite in intel is kind of
> >> > limited.
> >>
> >> TTM has support for userpage binding we just don't use it.
> >
> > Yes, and I've been experimenting with the same in GEM to great effect in
> > the DDX. The complication remains in managing the CPU synchronisation,
> > which suggests that it would only be useful for STREAM_DRAW objects (and
> > perhaps the sub-region updates to STATIC_DRAW). (And for readback, if
> > retrieving the data were the actual bottleneck.)
> 
> What do you mean by CPU synchronisation ? In what i had in mind the
> upload/download would block userspace until operation is, this would
> make upload/dowload barrier of course it doesn't play well with
> usecase where you keep uploading/downloading (idea to aleviate that is
> to allow several download/upload in one ioctl call).

Yes, that is the issue: having to control access to the user pages whilst
they are in use by the GPU. A completely synchronous API for performing
a single pwrite with the blitter is too slow, much slower than doing an
uncached write with the CPU and queueing up multiple blits (as we
currently do).

The API I ended up with for the pwrite using the BLT was to specify a 2D
region (addr, width, height, stride, flags etc) and list of clip rects. At
which point I grew disenchanted, and realised that simply creating a bo
for mapping user pages was the far better solution.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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