Re: [PATCH i-g-t] tests/gem_mmap_gtt: Use PAGE_SIZE instead of hard coded value

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On 05/04/2015 07:16 AM, Daniel Vetter wrote:
> On Thu, Apr 30, 2015 at 12:22:56PM +0100, Chris Wilson wrote:
>> On Thu, Apr 30, 2015 at 01:28:46PM +0300, Joonas Lahtinen wrote:
>>> On ma, 2015-04-27 at 20:43 +0100, Chris Wilson wrote:
>>>> On Mon, Apr 27, 2015 at 06:35:54PM +0100, Thomas Wood wrote:
>>>>> On 24 April 2015 at 08:38, Joonas Lahtinen
>>>>> <joonas.lahtinen@linux.intel.com> wrote:
>>>>>> Now that there is PAGE_SIZE define, use it.
>>>>>
>>>>> Thanks, I've pushed this patch. I also noticed PAGE_SIZE gets defined
>>>>> in several tests, so at some point it might be worth moving it into
>>>>> the library.
>>>>
>>>> Be wary of these though. PAGE_SIZE should only ever be used wrt to struct
>>>> page and not GPU pages. If you must, please use GTT_PAGE_SIZE instead.
>>>
>>> Do we have a platform/case where these are different? Just asking out of
>>> curiosity :)
>>
>> Yes. We just haven't enabled big pages yet. The thought of getting globs
>> of 64k contiguous physical memory isn't too appealing, but like with
>> hugepages there are likely enough tasks that benefit.
> 
> I thought the verdict thus far was that hw engineers overspecced tlbs and
> 64k pages aren't really worth it except in some corner-case video code
> workloads. Might have changed with the gen8+ pagetables, but I haven't
> seen any new noises about this.

I hadn't heard that; Damien looked at this awhile back but I'm not sure
if he got to the point of getting perf numbers.  Those would be nice...
 there's a lot of added complexity, but if our media processing overhead
goes down by 20% it would probably be worth it!

Jesse

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