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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Hugh Dickins , Nirmoy Das , Dragos Tatulea , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" References: <20260901190123.3511535-1-noren@nvidia.com> <20260902162323.GO3004@cmpxchg.org> <20260902183752.GP3004@cmpxchg.org> <20260917134949.GB1344@cmpxchg.org> Content-Language: en-US From: Nimrod Oren In-Reply-To: <20260917134949.GB1344@cmpxchg.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR4P281CA0106.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:bb::10) To MW4PR12MB6802.namprd12.prod.outlook.com (2603:10b6:303:20f::19) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: MW4PR12MB6802:EE_|PH0PR12MB7095:EE_ X-MS-Office365-Filtering-Correlation-Id: 2b53e0fe-a3f5-471d-4983-08df171986ad X-MS-Exchange-SenderADCheck: 1 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 17/09/2026 16:49, Johannes Weiner wrote: > On Thu, Sep 17, 2026 at 01:19:12PM +0300, Nimrod Oren wrote: >> On 02/09/2026 21:37, Johannes Weiner wrote: >>> On Wed, Sep 02, 2026 at 06:00:55PM +0100, Lorenzo Stoakes (ARM) wrote: >>>> On Wed, Sep 02, 2026 at 12:23:23PM -0400, Johannes Weiner wrote: >>>>> I'm not against carefully evaluating and testing out today's need for >>>>> set_recommended_min_free_kbytes() in real world examples. But this is >>>>> not that. >>>>> >>>>> Nacked-by: Johannes Weiner >>>> >>>> Isn't every possible change to address this kind of issue subject to >>>> exactly the same kind of constraint? >>>> >>>> I'd like to know what not rolling that dice looks like :) or what >>>> constitutes 'careful evaluation'. >>> >>> Usama gave some great examples in his other email. I'm not really >>> arguing to keep things out of tradition. But I think it's fair to say >>> let's at least test the common 4k/2M THP setups under memory pressure >>> before and after the change. >> >> Hi, >> >> I tested this on an x86-64 (4K/2M) virtual machine with one NUMA node >> and 16 GiB online memory, using mmtests config-workload-thpchallenge-fio >> with THPCHALLENGE_MADV_HUGEPAGE=yes. >> min_free_kbytes was 16 MiB patched and 66 MiB unpatched. >> >> I ran each kernel 30 times, rebooting before each run. The results did >> not show a regression in THP fault success rate or latency: >> >> Average THP fault success (Percentage Faults Huge) increased from >> 13.07% unpatched to 13.34% patched, and average fault latency >> (Fault Latencies) decreased by 6.5%. >> >> In contrast, compaction metrics were higher on average with the patch: >> >> Compaction stalls: 1,635 -> 1,713 (+4.7%) >> Compaction failures: 1,350 -> 1,416 (+4.9%) >> Compaction migrate scanned: 4,812,254 -> 5,587,587 (+16.1%) > > A 2% increase in THP success bought with a 16.1% increase in > compaction work looks like a sizable efficiency regression. > > A scan efficiency drop is in line with expectations of what happens > when non-frag placement reserves are taken from the allocator. A > comparison of trace_mm_page_alloc_extfrag rates could be instructive. > > Why the 2% success boost isn't quite clear to me. Allocation latency > improving suggests the extra work is primarily picked up by background > compaction. Reduced reserves could be making proactive compaction more > aggressive. But the improvement is unlikely to hold once you run out > of idle CPUs and the additional compaction work actually eats into the > workload. It could be useful to look closer at who is doing the extra > work and based on what triggers. Thanks. I ran another 30 pairs with the same setup and workload, this time adding identical CPU load to both conditions. CPU utilization during the measured workload averaged approximately 99.7% in both the baseline and the patched runs. The aggregate mpstat %sys value averaged 2.25% in the baseline runs and 2.22% in the patched runs. THP results: baseline avg patched avg change Percentage Faults Huge 9.92% 9.12% -0.80 pp Fault Latencies 4923 us 4856 us -1.36% The average difference in Percentage Faults Huge changed from +0.26 percentage points in the earlier runs to -0.80 points under CPU load. It's worth clarifying, though, that both differences were small relative to the observed run-to-run variability: the baseline/patched standard deviations were 4.52/4.59 percentage points earlier and 2.64/2.95 points under CPU load. The same applies to Fault Latencies, whose baseline/patched SDs were 432/368 us earlier and 181/132 us under CPU load. Compaction and reclaim results: baseline avg patched avg change Compaction migrate scanned 3,983,598 4,563,788 +14.6% Kcompactd migrate scanned 1,242,728 1,944,022 +56.4% Compaction free scanned 17,689,565 20,461,694 +15.7% Kcompactd free scanned 2,608,710 4,426,810 +69.7% Compaction stalls 1,476 1,514 +2.6% Compaction failures 1,265 1,319 +4.3% Direct pages scanned 1,339,044 1,344,907 +0.4% Kswapd pages scanned 2,011,323 2,141,355 +6.5% End-to-end workload results: baseline avg patched avg change Duration Elapsed 98.82 s 98.33 s -0.50% Aggregate fio read bandwidth 0.2142 GiB/s 0.2152 GiB/s +0.48% Aggregate fio read bandwidth is the sum of the per-job read bandwidth values in the fio output.