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

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Date:	Wed, 30 Mar 2011 11:07:08 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
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, Mar 30, 2011 at 10:07 AM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> 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
>

What kind of usage didn't played well with synchronous upload/download ? X, GL ?

Cheers,
Jerome
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