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[217.86.160.110]) by smtp.gmail.com with ESMTPSA id p3-20020a05600c1d8300b00401e32b25adsm1193765wms.4.2023.08.31.01.09.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 31 Aug 2023 01:09:20 -0700 (PDT) Message-ID: Date: Thu, 31 Aug 2023 10:09:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v5 3/5] mm: LARGE_ANON_FOLIO for improved performance Content-Language: en-US To: "Yin, Fengwei" , "Huang, Ying" , Ryan Roberts Cc: Andrew Morton , Matthew Wilcox , Yu Zhao , Catalin Marinas , Anshuman Khandual , Yang Shi , Zi Yan , Luis Chamberlain , Itaru Kitayama , "Kirill A. Shutemov" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20230810142942.3169679-1-ryan.roberts@arm.com> <20230810142942.3169679-4-ryan.roberts@arm.com> <87v8dg6lfu.fsf@yhuang6-desk2.ccr.corp.intel.com> <5c9ba378-2920-4892-bdf0-174e47d528b7@arm.com> <87cyz43s63.fsf@yhuang6-desk2.ccr.corp.intel.com> <4e14730b-4e4c-de30-04bb-9f3ec4a93754@redhat.com> <747deb43-68c8-449f-b41a-91864820a699@intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <747deb43-68c8-449f-b41a-91864820a699@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 31.08.23 10:02, Yin, Fengwei wrote: > > > On 8/31/2023 3:57 PM, David Hildenbrand wrote: >> On 31.08.23 03:40, Huang, Ying wrote: >>> Ryan Roberts writes: >>> >>>> On 15/08/2023 22:32, Huang, Ying wrote: >>>>> Hi, Ryan, >>>>> >>>>> Ryan Roberts writes: >>>>> >>>>>> Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be >>>>>> allocated in large folios of a determined order. All pages of the large >>>>>> folio are pte-mapped during the same page fault, significantly reducing >>>>>> the number of page faults. The number of per-page operations (e.g. ref >>>>>> counting, rmap management lru list management) are also significantly >>>>>> reduced since those ops now become per-folio. >>>>>> >>>>>> The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig, >>>>>> which defaults to disabled for now; The long term aim is for this to >>>>>> defaut to enabled, but there are some risks around internal >>>>>> fragmentation that need to be better understood first. >>>>>> >>>>>> Large anonymous folio (LAF) allocation is integrated with the existing >>>>>> (PMD-order) THP and single (S) page allocation according to this policy, >>>>>> where fallback (>) is performed for various reasons, such as the >>>>>> proposed folio order not fitting within the bounds of the VMA, etc: >>>>>> >>>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena >>>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always >>>>>> ----------------|-----------|-------------|---------------|------------- >>>>>> no hint         | S         | LAF>S       | LAF>S         | THP>LAF>S >>>>>> MADV_HUGEPAGE   | S         | LAF>S       | THP>LAF>S     | THP>LAF>S >>>>>> MADV_NOHUGEPAGE | S         | S           | S             | S >>>>> >>>>> IMHO, we should use the following semantics as you have suggested >>>>> before. >>>>> >>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena >>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always >>>>> ----------------|-----------|-------------|---------------|------------- >>>>> no hint         | S         | S           | LAF>S         | THP>LAF>S >>>>> MADV_HUGEPAGE   | S         | S           | THP>LAF>S     | THP>LAF>S >>>>> MADV_NOHUGEPAGE | S         | S           | S             | S >>>>> >>>>> Or even, >>>>> >>>>>                  | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena >>>>>                  | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always >>>>> ----------------|-----------|-------------|---------------|------------- >>>>> no hint         | S         | S           | S             | THP>LAF>S >>>>> MADV_HUGEPAGE   | S         | S           | THP>LAF>S     | THP>LAF>S >>>>> MADV_NOHUGEPAGE | S         | S           | S             | S >>>>> >>>>>  From the implementation point of view, PTE mapped PMD-sized THP has >>>>> almost no difference with LAF (just some small sized THP).  It will be >>>>> confusing to distinguish them from the interface point of view. >>>>> >>>>> So, IMHO, the real difference is the policy.  For example, prefer >>>>> PMD-sized THP, prefer small sized THP, or fully auto.  The sysfs >>>>> interface is used to specify system global policy.  In the long term, it >>>>> can be something like below, >>>>> >>>>> never:      S               # disable all THP >>>>> madvise:                    # never by default, control via madvise() >>>>> always:     THP>LAF>S       # prefer PMD-sized THP in fact >>>>> small:      LAF>S           # prefer small sized THP >>>>> auto:                       # use in-kernel heuristics for THP size >>>>> >>>>> But it may be not ready to add new policies now.  So, before the new >>>>> policies are ready, we can add a debugfs interface to override the >>>>> original policy in /sys/kernel/mm/transparent_hugepage/enabled.  After >>>>> we have tuned enough workloads, collected enough data, we can add new >>>>> policies to the sysfs interface. >>>> >>>> I think we can all imagine many policy options. But we don't really have much >>>> evidence yet for what it best. The policy I'm currently using is intended to >>>> give some flexibility for testing (use LAF without THP by setting sysfs=never, >>>> use THP without LAF by compiling without LAF) without adding any new knobs at >>>> all. Given that, surely we can defer these decisions until we have more data? >>>> >>>> In the absence of data, your proposed solution sounds very sensible to me. But >>>> for the purposes of scaling up perf testing, I don't think its essential given >>>> the current policy will also produce the same options. >>>> >>>> If we were going to add a debugfs knob, I think the higher priority would be a >>>> knob to specify the folio order. (but again, I would rather avoid if possible). >>> >>> I totally understand we need some way to control PMD-sized THP and LAF >>> to tune the workload, and nobody likes debugfs knob. >>> >>> My concern about interface is that we have no way to disable LAF >>> system-wise without rebuilding the kernel.  In the future, should we add >>> a new policy to /sys/kernel/mm/transparent_hugepage/enabled to be >>> stricter than "never"?  "really_never"? >> >> Let's talk about that in a bi-weekly MM session. (I proposed it as a topic for next week). > > The time slot of the meeting is not friendly to our timezone. Like > it's 1 or 2 AM. Yes. I know it's very hard to find a good time slot > for US, EU and Asia. :(. :/ Yeah, even for me in Germany it's usually already around 6-7pm. > > So maybe we still need to discuss it through mail? I don't think we'll be done discussing that in one session. One of the main goals is to get some input from the wider MM community. -- Cheers, David / dhildenb