On 08/29/2016 02:23 PM, Aaron Lu wrote: > On 08/29/2016 04:49 PM, Anshuman Khandual wrote: >> > On 08/29/2016 12:01 PM, Aaron Lu 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. >>> >> >>> >> Case used for test on Haswell EP: >>> >> usemem -n 72 --readonly -j 0x200000 100G >> > >> > Is this benchmark publicly available ? Does not seem to be this one >> > https://github.com/gnubert/usemem.git, Does it ? > Sorry, forgot to attach its link. > It's this one: > https://git.kernel.org/cgit/linux/kernel/git/wfg/vm-scalability.git > > And the above mentioned usemem is: > https://git.kernel.org/cgit/linux/kernel/git/wfg/vm-scalability.git/tree/usemem.c Hey Aaron, Thanks for pointing out. I did ran similar test on a POWER8 box using 16M steps (huge page size is 16MB on it) instead of 2MB. But the perf profile looked different. The perf command line was like this on a 32 CPU system. perf record ./usemem -n 256 --readonly -j 0x1000000 100G But the relative weight of the above mentioned function came out to be pretty less compared to what you have reported from your experiment which is around 54.03%. 0.07% usemem [kernel.vmlinux] [k] get_huge_zero_page Seems way out of the mark. Can you please confirm your exact perf record command line and how many CPUs you have on the system. - Anshuman -- 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>