On 08/30/2016 06:50 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... > > 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? Sorry for the confusion, the above line is extracted from perf report. It shows the percent of CPU cycles executed in a specific function. The above two perf lines are used to show get_huge_zero_page doesn't consume that much CPU cycles after applying the patch. > > Do we have any numbers for something which is more real-wordly? Unfortunately, no real world numbers. We think the global atomic counter could be an issue for performance so I'm trying to solve the problem. Thanks, Aaron -- 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>