On Fri, Jan 03, 2020 at 10:01:28PM +0800, Wei Yang wrote: > On Fri, Jan 03, 2020 at 04:26:50PM +0300, Kirill A. Shutemov wrote: > >On Fri, Jan 03, 2020 at 09:05:54PM +0800, Wei Yang wrote: > >> On Fri, Jan 03, 2020 at 03:18:46PM +0800, Wei Yang wrote: > >> >On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote: > >> >>When page is not NULL, function is called by try_to_unmap_one() with > >> >>TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one() > >> >>with TTU_SPLIT_HUGE_PMD set: > >> >> > >> >> * unmap_page() > >> >> * shrink_page_list() > >> >> > >> >>In both case, the page passed to try_to_unmap_one() is PageHead() of the > >> >>THP. If this page's mapping address in process is not HPAGE_PMD_SIZE > >> >>aligned, this means the THP is not mapped as PMD THP in this process. > >> >>This could happen when we do mremap() a PMD size range to an un-aligned > >> >>address. > >> >> > >> >>Currently, this case is handled by following check in __split_huge_pmd() > >> >>luckily. > >> >> > >> >> page != pmd_page(*pmd) > >> >> > >> >>This patch checks the address to skip some work. > >> > > >> >I am sorry to forget address Kirill's comment in 1st version. > >> > > >> >The first one is the performance difference after this change for a PTE > >> >mappged THP. > >> > > >> >Here is the result:(in cycle) > >> > > >> > Before Patched > >> > > >> > 963 195 > >> > 988 40 > >> > 895 78 > >> > > >> >Average 948 104 > >> > > >> >So the change reduced 90% time for function split_huge_pmd_address(). > > > >Right. > > > >But do we have a scenario, where the performance of > >split_huge_pmd_address() matters? I mean, it it called as part of rmap > >walk, attempt to split huge PMD where we don't have huge PMD should be > >within noise. > > Sorry for my poor English. > > I don't catch the meaning of the last sentence. "within noise" here means > non-huge PMD is an expected scenario and we could tolerate this? Basically, I doubt that this change would bring any measurable perfromance benefits on a real workload. -- Kirill A. Shutemov
