linux-kernel - Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs

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Date:   Fri, 28 May 2021 11:03:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, "Tang, Feng" <feng.tang@...el.com>
Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active
 CPUs

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