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

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Message-Id: <20160829155021.2a85910c3d6b16a7f75ffccd@linux-foundation.org>
Date:   Mon, 29 Aug 2016 15:50:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        "'Kirill A. Shutemov'" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jerome Marchand <jmarchan@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ebru Akagunduz <ebru.akagunduz@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thp: reduce usage of huge zero page's atomic counter

On Mon, 29 Aug 2016 14:31:20 +0800 Aaron Lu <aaron.lu@...el.com> 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?