On Mon, Aug 29, 2016 at 07:17:58PM +0530, Anshuman Khandual wrote: > 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. Haswell EP has 72 CPUs. Since the huge page size is 16MB on your system, maybe you can try: perf record ./usemem -n 32 --readonly -j 0x1000000 800G Regards, 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>