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

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Message-ID: <3b8deaf7-2e7b-ff22-be72-31b1a7ebb3eb@intel.com>
Date:   Mon, 29 Aug 2016 16:53:09 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Linux Memory Management List <linux-mm@...ck.org>
Cc:     "'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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 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