Thanks for taking the time to look into this! Thanks, Yang and Andrew! Best, Lance On Fri, Feb 23, 2024 at 5:11 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, 22 Feb 2024 12:23:21 -0800 Yang Shi <shy828301@xxxxxxxxx> wrote: > > > > 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. > > > > Thanks for following up on this, can you please capture all the > > information in the commit log? > > I added it. > > --- a/txt/mm-khugepaged-bypassing-unnecessary-scans-with-mmf_disable_thp-check.txt > +++ b/txt/mm-khugepaged-bypassing-unnecessary-scans-with-mmf_disable_thp-check.txt > @@ -9,6 +9,24 @@ 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. > > +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% > + > +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. > + > This commit introduces a check before each scanning process to test the > MMF_DISABLE_THP flag for the given mm; if the flag is set, the scanning > process is bypassed, thereby improving the efficiency of khugepaged. >
