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Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node
From:   Yang Shi <yang.shi@linux.alibaba.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     mgorman@techsingularity.net, riel@surriel.com, hannes@cmpxchg.org,
        akpm@linux-foundation.org, dave.hansen@intel.com,
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        fengguang.wu@intel.com, fan.du@intel.com, ying.huang@intel.com,
        ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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Date:   Wed, 17 Apr 2019 13:43:44 -0700
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>>
>>>> I would also not touch the numa balancing logic at this stage and 
>>>> rather
>>>> see how the current implementation behaves.
>>> I agree we would prefer start from something simpler and see how it 
>>> works.
>>>
>>> The "twice access" optimization is aimed to reduce the PMEM 
>>> bandwidth burden
>>> since the bandwidth of PMEM is scarce resource. I did compare "twice 
>>> access"
>>> to "no twice access", it does save a lot bandwidth for some once-off 
>>> access
>>> pattern. For example, when running stress test with mmtest's
>>> usemem-stress-numa-compact. The kernel would promote ~600,000 pages 
>>> with
>>> "twice access" in 4 hours, but it would promote ~80,000,000 pages 
>>> without
>>> "twice access".
>> I pressume this is a result of a synthetic workload, right? Or do you
>> have any numbers for a real life usecase?
>
> The test just uses usemem.

I tried to run some more real life like usecases, the below shows the 
result by running mmtest's db-sysbench-mariadb-oltp-rw-medium test, 
which is a typical database workload, with and w/o "twice access" 
optimization.

                              w/                  w/o
promotion          32771           312250

We can see the kernel did 10x promotion w/o "twice access" optimization.

I also tried kernel-devel and redis tests in mmtest, but they can't 
generate enough memory pressure, so I had to run usemem test to generate 
memory pressure. However, this brought in huge noise, particularly for 
the w/o "twice access" case. But, the mysql test should be able to 
demonstrate the improvement achieved by this optimization.

And, I'm wondering whether this optimization is also suitable to general 
NUMA balancing or not.