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

[Andrew Morton]

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?

Do we have any numbers for something which is more real-wordly?