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.
