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SA3PR12MB8047.namprd12.prod.outlook.com (2603:10b6:806:31b::8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8606.32; Mon, 7 Apr 2025 11:03:32 +0000 Received: from SJ0PR12MB6966.namprd12.prod.outlook.com ([fe80::6688:276b:7e47:17d5]) by SJ0PR12MB6966.namprd12.prod.outlook.com ([fe80::6688:276b:7e47:17d5%5]) with mapi id 15.20.8583.045; Mon, 7 Apr 2025 11:03:31 +0000 Message-ID: <432f63d5-cf96-4041-97b0-4f218876b9f4@amd.com> Date: Mon, 7 Apr 2025 16:33:23 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: pcp: scale batch to reduce number of high order pcp flushes on deallocation To: "Huang, Ying" , Nikhil Dhama Cc: akpm@linux-foundation.org, bharata@amd.com, huang.ying.caritas@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mgorman@techsingularity.net, raghavendra.kodsarathimmappa@amd.com, oe-lkp@lists.linux.dev, lkp@intel.com References: <875xjmuiup.fsf@DESKTOP-5N7EMDA> 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000005, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/7/2025 1:08 PM, Huang, Ying wrote: > Nikhil Dhama writes: > >> On 4/3/2025 7:06 AM, Huang, Ying wrote: >> >>> Nikhil Dhama writes: >>> >>>> On 3/30/2025 12:22 PM, Huang, Ying wrote: >>>> >>>>> Hi, Nikhil, >>>>> >>>>> Nikhil Dhama writes: >>>>> >>>>>> In old pcp design, pcp->free_factor gets incremented in nr_pcp_free() >>>>>> which is invoked by free_pcppages_bulk(). So, it used to increase >>>>>> free_factor by 1 only when we try to reduce the size of pcp list or >>>>>> flush for high order. >>>>>> and free_high used to trigger only for order > 0 and order < >>>>>> costly_order and free_factor > 0. >>>>>> >>>>>> and free_factor used to scale down by a factor of 2 on every successful >>>>>> allocation. >>>>>> >>>>>> for iperf3 I noticed that with older design in kernel v6.6, pcp list was >>>>>> drained mostly when pcp->count > high (more often when count goes above >>>>>> 530). and most of the time free_factor was 0, triggering very few >>>>>> high order flushes. >>>>>> >>>>>> Whereas in the current design, free_factor is changed to free_count to keep >>>>>> track of the number of pages freed contiguously, >>>>>> and with this design for iperf3, pcp list is getting flushed more >>>>>> frequently because free_high heuristics is triggered more often now. >>>>>> >>>>>> In current design, free_count is incremented on every deallocation, >>>>>> irrespective of whether pcp list was reduced or not. And logic to >>>>>> trigger free_high is if free_count goes above batch (which is 63) and >>>>>> there are two contiguous page free without any allocation. >>>>>> (and with cache slice optimisation). >>>>>> >>>>>> With this design, I observed that high order pcp list is drained as soon >>>>>> as both count and free_count goes about 63. >>>>>> >>>>>> and due to this more aggressive high order flushing, applications >>>>>> doing contiguous high order allocation will require to go to global list >>>>>> more frequently. >>>>>> >>>>>> On a 2-node AMD machine with 384 vCPUs on each node, >>>>>> connected via Mellonox connectX-7, I am seeing a ~30% performance >>>>>> reduction if we scale number of iperf3 client/server pairs from 32 to 64. >>>>>> >>>>>> So, though this new design reduced the time to detect high order flushes, >>>>>> but for application which are allocating high order pages more >>>>>> frequently it may be flushing the high order list pre-maturely. >>>>>> This motivates towards tuning on how late or early we should flush >>>>>> high order lists. >>>>>> >>>>>> for free_high heuristics. I tried to scale batch and tune it, >>>>>> which will delay the free_high flushes. >>>>>> >>>>>> >>>>>> score # free_high >>>>>> ----------- ----- ----------- >>>>>> v6.6 (base) 100 4 >>>>>> v6.12 (batch*1) 69 170 >>>>>> batch*2 69 150 >>>>>> batch*4 74 101 >>>>>> batch*5 100 53 >>>>>> batch*6 100 36 >>>>>> batch*8 100 3 >>>>>> >>>>>> scaling batch for free_high heuristics with a factor of 5 or above restores >>>>>> the performance, as it is reducing the number of high order flushes. >>>>>> >>>>>> On 2-node AMD server with 384 vCPUs each,score for other benchmarks with >>>>>> patch v2 along with iperf3 are as follows: >>>>> Em..., IIUC, this may disable the free_high optimizations. free_high >>>>> optimization is introduced by Mel Gorman in commit f26b3fa04611 >>>>> ("mm/page_alloc: limit number of high-order pages on PCP during bulk >>>>> free"). So, this may trigger regression for the workloads in the >>>>> commit. Can you try it too? >>>>> >>>> Hi, I ran netperf-tcp as in commit f26b3fa04611 ("mm/page_alloc: limit >>>> number of high-order pages on PCP during bulk free"), >>>> >>>> On a 2-node AMD server with 384 vCPUs, results I observed are as follows: >>>> >>>> 6.12 6.12 >>>> vanilla freehigh-heuristicsopt >>>> Hmean 64 732.14 ( 0.00%) 736.90 ( 0.65%) >>>> Hmean 128 1417.46 ( 0.00%) 1421.54 ( 0.29%) >>>> Hmean 256 2679.67 ( 0.00%) 2689.68 ( 0.37%) >>>> Hmean 1024 8328.52 ( 0.00%) 8413.94 ( 1.03%) >>>> Hmean 2048 12716.98 ( 0.00%) 12838.94 ( 0.96%) >>>> Hmean 3312 15787.79 ( 0.00%) 15822.40 ( 0.22%) >>>> Hmean 4096 17311.91 ( 0.00%) 17328.74 ( 0.10%) >>>> Hmean 8192 20310.73 ( 0.00%) 20447.12 ( 0.67%) >>>> >>>> It is not regressing for netperf-tcp. >>> Thanks a lot for your data! >>> >>> Think about this again. Compared with the pcp->free_factor solution, >>> the pcp->free_count solution will trigger free_high heuristics more >>> early, this causes performance regression in your workloads. So, it's >>> reasonable to raise the bar to trigger free_high. And, it's also >>> reasonable to use a stricter threshold, as you have done in this patch. >>> However, "5 * batch" appears too magic and adapt to one type of machine. >>> >>> Let's step back to do some analysis. In the original pcp->free_factor >>> solution, free_high is triggered for contiguous freeing with size >>> ranging from "batch" to "pcp->high + batch". So, the average value is >>> about "batch + pcp->high / 2". While in the pcp->free_count solution, >>> free_high will be triggered for contiguous freeing with size "batch". >>> So, to restore the original behavior, it seems that we can use the >>> threshold "batch + pcp->high_min / 2". Do you think that this is >>> reasonable? If so, can you give it a try? >> Hi, >> >> I have tried your suggestion as setting threshold to "batch + pcp->high_min / 2", >> scores for different benchmarks on the same machine >> (2-Node AMD server with 384 vCPUs each) are as follows: >> >> iperf3 lmbench3 netperf kbuild >> (AF_UNIX) (SCTP_STREAM_MANY) >> ------- --------- ----------------- ------ >> v6.6 vanilla (base) 100 100 100 100 >> v6.12 vanilla 69 113 98.5 98.8 >> v6.12 avg_threshold 100 110.3 100.2 99.3 >> >> and for netperf-tcp, it is as follows: >> >> 6.12 6.12 >> vanilla avg_free_high_threshold >> Hmean 64 732.14 ( 0.00%) 730.45 ( -0.23%) >> Hmean 128 1417.46 ( 0.00%) 1419.44 ( 0.14%) >> Hmean 256 2679.67 ( 0.00%) 2676.45 ( -0.12%) >> Hmean 1024 8328.52 ( 0.00%) 8339.34 ( 0.13%) >> Hmean 2048 12716.98 ( 0.00%) 12743.68 ( 0.21%) >> Hmean 3312 15787.79 ( 0.00%) 15887.25 ( 0.63%) >> Hmean 4096 17311.91 ( 0.00%) 17332.68 ( 0.12%) >> Hmean 8192 20310.73 ( 0.00%) 20465.09 ( 0.76%) > Thanks a lot for test and results! > > It looks good to me. Can you submit a formal patch? Thank you Huang Ying,  Yes,  I have submitted a formal patch with this. Patch v3: https://lore.kernel.org/linux-mm/20250407105219.55351-1-nikhil.dhama@amd.com/ --- Thanks, Nikhil Dhama