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Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active
 CPUs
To:     Mel Gorman <mgorman@techsingularity.net>,
        Dave Hansen <dave.hansen@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, "Tang, Feng" <feng.tang@intel.com>
References: <20210525080119.5455-1-mgorman@techsingularity.net>
 <7177f59b-dc05-daff-7dc6-5815b539a790@intel.com>
 <20210528085545.GJ30378@techsingularity.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <54ff0363-2f39-71d1-e26c-962c3fddedae@redhat.com>
Date:   Fri, 28 May 2021 11:03:22 +0200
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On 28.05.21 10:55, Mel Gorman wrote:
> On Thu, May 27, 2021 at 12:36:21PM -0700, Dave Hansen wrote:
>> Hi Mel,
>>
>> Feng Tang tossed these on a "Cascade Lake" system with 96 threads and
>> ~512G of persistent memory and 128G of DRAM.  The PMEM is in "volatile
>> use" mode and being managed via the buddy just like the normal RAM.
>>
>> The PMEM zones are big ones:
>>
>>          present  65011712 = 248 G
>>          high       134595 = 525 M
>>
>> The PMEM nodes, of course, don't have any CPUs in them.
>>
>> With your series, the pcp->high value per-cpu is 69584 pages or about
>> 270MB per CPU.  Scaled up by the 96 CPU threads, that's ~26GB of
>> worst-case memory in the pcps per zone, or roughly 10% of the size of
>> the zone.

When I read about having such big amounts of free memory theoretically 
stuck in PCP lists, I guess we really want to start draining the PCP in 
alloc_contig_range(), just as we do with memory hotunplug when offlining.

-- 
Thanks,

David / dhildenb