Re: [PATCH] thp: reduce usage of huge zero page's atomic counter

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

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

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