Re: [linux-next:master] [mm] 7b32f64bc5: pts.svt-av1.Preset13.Bosphorus4K.frames_per_second 45.8% regression

[Pedro Falcato <pfalcato@suse.de>]

On Thu, Jun 18, 2026 at 04:03:43PM +0000, Suren Baghdasaryan wrote:
> On Thu, Jun 18, 2026 at 2:30 PM Lorenzo Stoakes <ljs@kernel.org> wrote:
> >
> > On Thu, Jun 18, 2026 at 11:30:47AM +0200, Jan Kara wrote:
> > > On Thu 18-06-26 16:00:42, kernel test robot wrote:
> > > > Hello,
> > > >
> > > > kernel test robot noticed a 45.8% regression of pts.svt-av1.Preset13.Bosphorus4K.frames_per_second on:
> > >
> > > This one looks serious enough and real. It would be good to figure out what
> > > happens in this benchmark that it benefits from the readahead across VMA
> > > boundaries so much...
> >
> > I think a revert first no? This seems pretty huge for something that isn't key
> > to the kernel, then a new attempt can be tried with this issue addressed
> > perhaps?
> 
> A quick search yields: "The
> pts.svt-av1.Preset13.Bosphorus4K.frames_per_second is a benchmarking
> metric from the Phoronix Test Suite that measures how many frames per
> second a CPU can encode using the open-source SVT-AV1 video encoder."
> 
> If this is a video encoding benchmark I would expect it to explicitly
> prefetch the data from the disk before measuring the encoding speed.
> If limiting readahead caused this regression, I suspect the benchmark
> doesn't explicitly prefetch the data...

Well, commonly video data doesn't actually fit in memory :)

A quick look at the code (I think it's https://gitlab.com/AOMediaCodec/SVT-AV1/-/blob/master/Source/App/app_process_cmd.c#L821)
suggests it is progressively mapping the file data for a given frame
(or frames?). So the old behavior would result in page faults for a given
frame starting readahead for the next few frames. This looks reasonable.

FWIW I suspected this was a really weird case regarding mprotect or
something, and I'm happy it isn't; but at least I had a suggestion for that -
for this, maybe dropping the change (for now?) is the best course of action.

-- 
Pedro