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... But the patch is simple enough. > Case used for test on Haswell EP: > usemem -n 72 --readonly -j 0x200000 100G > Which spawns 72 processes and each will mmap 100G anonymous space and > then do read only access to that space sequentially with a step of 2MB. > > perf report for base commit: > 54.03% usemem [kernel.kallsyms] [k] get_huge_zero_page > perf report for this commit: > 0.11% usemem [kernel.kallsyms] [k] mm_get_huge_zero_page Does this mean that overall usemem runtime halved? Do we have any numbers for something which is more real-wordly? -- 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>