GEM-related desktop sluggishness due to linear-time arch_get_unmapped_area_topdown()

Jerome Glisse j.glisse at
Wed Mar 30 06:28:07 PDT 2011

On Wed, Mar 30, 2011 at 3:32 AM, Chris Wilson <chris at> wrote:
> On Wed, 30 Mar 2011 07:57:49 +1000, Dave Airlie <airlied at> wrote:
>> On Wed, Mar 30, 2011 at 7:04 AM, Jerome Glisse <j.glisse at> 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


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