Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE_THP check

[Lance Yang]

Hey,

On an Intel Core i5 CPU, the time taken by
khugepaged to scan the address space of
the process, which has been set with the
MMF_DISABLE_THP flag after being added
to the mm_slots list, is as follows (shorter is better):

VMA Count |   Old   |   New   |  Change
---------------------------------------
    50     |   23us  |    9us  |  -60.9%
   100     |   32us  |    9us  |  -71.9%
   200     |   44us  |    9us  |  -79.5%
   400     |   75us  |    9us  |  -88.0%
   800     |   98us  |    9us  |  -90.8%

IIUC, once the count of VMAs for the process
exceeds page_to_scan, khugepaged needs to
wait for scan_sleep_millisecs ms before scanning
the next process. IMO, unnecessary scans could
actually be skipped with a very inexpensive
mm->flags check in this case.

Best,
Lance

On Wed, Jan 31, 2024 at 5:30 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
>
> Updating the change log.
[...]

> On Mon, Jan 29, 2024 at 1:46 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >
> > khugepaged scans the entire address space in the
> > background for each given mm, looking for
> > opportunities to merge sequences of basic pages
> > into huge pages. However, when an mm is inserted
> > to the mm_slots list, and the MMF_DISABLE_THP flag
> > is set later, this scanning process becomes
> > unnecessary for that mm and can be skipped to avoid
> > redundant operations, especially in scenarios with
> > a large address space.
[...]
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>