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

[Aaron Lu]

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

Regards,
Aaron