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

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Message-ID: <AANLkTinxVznFtHwM2RM3kNm9-eSXuz9L7TgfaMpgu=uZ@mail.gmail.com>
Date:	Wed, 30 Mar 2011 09:28:07 -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 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).

> And I did play with a new pread/pwrite interface that did as you suggest,
> binding the user pages and performing a blit. But by the time you make the
> interface asynchronous, it becomes much easier to let the client code
> create the mapping and be fully aware of the barriers.
>
> And yes I do concur that vma bookkeeping does impose significant overheads
> and I have been removing as many mappings from our drivers as I can; within
> the limitations of the pwrite interface.
> -Chris
>
> --
> Chris Wilson, Intel Open Source Technology Centre
>

Cheers,
Jerome
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