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[2003:d8:2f4d:e00:298:59cc:2514:52]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e18537589dsm5619251f8f.54.2025.09.05.04.55.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Sep 2025 04:55:22 -0700 (PDT) Message-ID: <65ce71c1-72a5-415e-9059-027167abf129@redhat.com> Date: Fri, 5 Sep 2025 13:55:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support To: Lorenzo Stoakes , Nico Pache Cc: Usama Arif , Baolin Wang , Dev Jain , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com References: <2a141eef-46e2-46e1-9b0f-066ec537600d@linux.alibaba.com> <286e2cb3-6beb-4d21-b28a-2f99bb2f759b@redhat.com> <17075d6a-a209-4636-ae42-2f8944aea745@gmail.com> <287f3b64-bc34-48d9-9778-c519260c3dba@redhat.com> <61afc355-1877-4530-86b7-e0aa2b6fb827@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: mmhqfkfpdmfh88qi9dzya5jrf4czicpp X-Rspam-User: X-Rspamd-Queue-Id: 5D8F42002A X-Rspamd-Server: rspam01 X-HE-Tag: 1757073328-828998 X-HE-Meta: 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 l65bT9JF 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 05.09.25 13:48, Lorenzo Stoakes wrote: > On Wed, Sep 03, 2025 at 08:54:39PM -0600, Nico Pache wrote: >> On Tue, Sep 2, 2025 at 2:23 PM Usama Arif wrote: >>>>>> So I question the utility of max_ptes_none. If you can't tame page faults, then there is only >>>>>> limited sense in taming khugepaged. I think there is vale in setting max_ptes_none=0 for some >>>>>> corner cases, but I am yet to learn why max_ptes_none=123 would make any sense. >>>>>> >>>>>> >>>>> >>>>> For PMD mapped THPs with THP shrinker, this has changed. You can basically tame pagefaults, as when you encounter >>>>> memory pressure, the shrinker kicks in if the value is less than HPAGE_PMD_NR -1 (i.e. 511 for x86), and >>>>> will break down those hugepages and free up zero-filled memory. >>>> >>>> You are not really taming page faults, though, you are undoing what page faults might have messed up :) >>>> >>>> I have seen in our prod workloads where >>>>> the memory usage and THP usage can spike (usually when the workload starts), but with memory pressure, >>>>> the memory usage is lower compared to with max_ptes_none = 511, while still still keeping the benefits >>>>> of THPs like lower TLB misses. >>>> >>>> Thanks for raising that: I think the current behavior is in place such that you don't bounce back-and-forth between khugepaged collapse and shrinker-split. >>>> >>> >>> Yes, both collapse and shrinker split hinge on max_ptes_none to prevent one of these things thrashing the effect of the other. >> I believe with mTHP support in khugepaged, the max_ptes_none value in >> the shrinker must also leverage the 'order' scaling to properly >> prevent thrashing. > > No please do not extend this 'scalling' stuff somewhere else, it's really horrid. > > We have to find an alternative to that, it's extremely confusing in what is > already extremely confusing THP code. > > As I said before, if we can't have a boolean we need another interface, which > makes most sense to be a ratio or in practice, a percentage sysctl. > > Speaking with David off-list, maybe the answer - if we must have this - is to > add a new percentage interface and have this in lock-step with the existing > max_ptes_none interface. One updates the other, but internally we're just using > the percentage value. Yes, I'll try hacking something up and sending it as an RFC. > >> I've been testing a patch for this that I might include in the V11. >>> >>>> There are likely other ways to achieve that, when we have in mind that the thp shrinker will install zero pages and max_ptes_none includes >>>> zero pages. >>>> >>>>> >>>>> I do agree that the value of max_ptes_none is magical and different workloads can react very differently >>>>> to it. The relationship is definitely not linear. i.e. if I use max_ptes_none = 256, it does not mean >>>>> that the memory regression of using THP=always vs THP=madvise is halved. >>>> >>>> To which value would you set it? Just 510? 0? Sorry, I missed Usama's reply. Thanks Usama! >>>> >>> >>> There are some very large workloads in the meta fleet that I experimented with and found that having >>> a small value works out. I experimented with 0, 51 (10%) and 256 (50%). 51 was found to be an optimal >>> comprimise in terms of application metrics improving, having an acceptable amount of memory regression and >>> improved system level metrics (lower TLB misses, lower page faults). I am sure there was a better value out >>> there for these workloads, but not possible to experiment with every value. > > (->Usama) It's a pity that such workloads exist. But then the percentage solution should work. Good. So if there is no strong case for > 255, that's already valuable for mTHP. -- Cheers David / dhildenb