On Tue, 30 Aug 2016 10:44:21 +0530 Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx> wrote: > On 08/30/2016 04:20 AM, Andrew Morton wrote: > > On Mon, 29 Aug 2016 14:31:20 +0800 Aaron Lu <aaron.lu@xxxxxxxxx> wrote: > > > >> > > >> > The global zero page is used to satisfy an anonymous read fault. If > >> > THP(Transparent HugePage) is enabled then the global huge zero page is used. > >> > The global huge zero page uses an atomic counter for reference counting > >> > and is allocated/freed dynamically according to its counter value. > >> > > >> > CPU time spent on that counter will greatly increase if there are > >> > a lot of processes doing anonymous read faults. This patch proposes a > >> > way to reduce the access to the global counter so that the CPU load > >> > can be reduced accordingly. > >> > > >> > To do this, a new flag of the mm_struct is introduced: MMF_USED_HUGE_ZERO_PAGE. > >> > With this flag, the process only need to touch the global counter in > >> > two cases: > >> > 1 The first time it uses the global huge zero page; > >> > 2 The time when mm_user of its mm_struct reaches zero. > >> > > >> > Note that right now, the huge zero page is eligible to be freed as soon > >> > as its last use goes away. With this patch, the page will not be > >> > eligible to be freed until the exit of the last process from which it > >> > was ever used. > >> > > >> > And with the use of mm_user, the kthread is not eligible to use huge > >> > zero page either. Since no kthread is using huge zero page today, there > >> > is no difference after applying this patch. But if that is not desired, > >> > I can change it to when mm_count reaches zero. > > > I suppose we could simply never free the zero huge page - if some > > process has used it in the past, others will probably use it in the > > future. One wonders how useful this optimization is... > > Yeah, what prevents us from doing away with this lock altogether and > keep one zero filled huge page (after a process has used it once) for > ever to be mapped across all the read faults ? A 16MB / 2MB huge page > is too much of memory loss on a THP enabled system ? We can also save > on allocation time. Sounds OK to me. But only if it makes a useful performance benefit to something that someone cares about! otoh, that patch is simple enough... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>