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AJvYcCWy5YeJEG+eGBAxb8e512GQu+jsgWqNsWeu3tpy+ZP5RzavLEPfPAiN1gtJCuLoRfvByUlfdPXqyF6Uoj0VxnWXykQ= X-Gm-Message-State: AOJu0Yw/jHzpsWMQYVjK2yU1ODkoKwk5Wf8DGhCKIRBEPTt6m3lf+4ZO UC/oNoVxdqQekoIRh9eU+Ub7SRl76T8QH+ZcMFV/38sGpmAarmh1wViuaAMFkFKcg9ZS46eM18v BLe8W37H0AjCZ3Bps832v3XzBhoM= X-Google-Smtp-Source: AGHT+IF+5Nveh523y3YiZQa38pUO8yVTBVCEFyzyqKUfHRCpzAxkK6fwK5wXPO9KAvW4JlYNChkmRUfKaB15sthpWOQ= X-Received: by 2002:a05:6122:1d4d:b0:4c9:2540:8520 with SMTP id gd13-20020a0561221d4d00b004c925408520mr18997880vkb.1.1710997960940; Wed, 20 Mar 2024 22:12:40 -0700 (PDT) MIME-Version: 1.0 References: <20240304081348.197341-1-21cnbao@gmail.com> <20240304081348.197341-6-21cnbao@gmail.com> <87wmq3yji6.fsf@yhuang6-desk2.ccr.corp.intel.com> <87sf0rx3d6.fsf@yhuang6-desk2.ccr.corp.intel.com> <87jzm0wblq.fsf@yhuang6-desk2.ccr.corp.intel.com> <9ec62266-26f1-46b6-8bb7-9917d04ed04e@arm.com> <87jzlyvar3.fsf@yhuang6-desk2.ccr.corp.intel.com> <87zfutsl25.fsf@yhuang6-desk2.ccr.corp.intel.com> <87msqts9u1.fsf@yhuang6-desk2.ccr.corp.intel.com> <87a5msrz5e.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: <87a5msrz5e.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Barry Song <21cnbao@gmail.com> Date: Thu, 21 Mar 2024 18:12:29 +1300 Message-ID: Subject: Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole To: "Huang, Ying" Cc: Ryan Roberts , Matthew Wilcox , akpm@linux-foundation.org, linux-mm@kvack.org, chengming.zhou@linux.dev, chrisl@kernel.org, david@redhat.com, hannes@cmpxchg.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, mhocko@suse.com, nphamcs@gmail.com, shy828301@gmail.com, steven.price@arm.com, surenb@google.com, wangkefeng.wang@huawei.com, xiang@kernel.org, yosryahmed@google.com, yuzhao@google.com, Chuanhua Han , Barry Song Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 09FDB20023 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: wih6ors4de3qh5hzzfduw3kzb6bzskrq X-HE-Tag: 1710997966-286233 X-HE-Meta: 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 mUC+qmu3 cup1u394R+vylfQxFNz2SEiRt4+V8N5qczOqOfg3dL4uGbp8ypSMTHicmE7+EmnTE9Zm1LeWbRxqmfJNl8zTEzF0NXZ5iWECcK1yIPBfNfAtERNieTmkxW7h15FRwPI2+rtTFqPpODCvCyUZAPjwg0jU8WjU+MdrGqYQgjmWzpfXWFeemznjpsGJnPGhxcgIk+1p6KE+utqnMx0ebfH/rAs+BwZpFXKlCAPJhJLTJdYfeEZ27pUZakhK2D8qjyljqVNz31R9tE92SqKScsz/bMjJfO+lHq2lHlZPWvragKw9iAvi0bbN2Ufk5n1QC92pvpzBndFhAAMlWGYu/t1dJA2gbuIu/yYcwcmhah5NEfrzTn4NEpaBh5NrYXfrzwtZpahwde/+OPgyUutSVUCEba4pNR4aS+Yu9QqbB8gUU9NJp+vOkibZYII0arquHUa4oQOuqQURpePs8SzA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000294, 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 Thu, Mar 21, 2024 at 5:25=E2=80=AFPM Huang, Ying = wrote: > > Barry Song <21cnbao@gmail.com> writes: > > > On Wed, Mar 20, 2024 at 7:22=E2=80=AFPM Huang, Ying wrote: > >> > >> Barry Song <21cnbao@gmail.com> writes: > >> > >> > On Wed, Mar 20, 2024 at 3:20=E2=80=AFPM Huang, Ying wrote: > >> >> > >> >> Ryan Roberts writes: > >> >> > >> >> > On 19/03/2024 09:20, Huang, Ying wrote: > >> >> >> Ryan Roberts writes: > >> >> >> > >> >> >>>>>> I agree phones are not the only platform. But Rome wasn't bu= ilt in a > >> >> >>>>>> day. I can only get > >> >> >>>>>> started on a hardware which I can easily reach and have enou= gh hardware/test > >> >> >>>>>> resources on it. So we may take the first step which can be = applied on > >> >> >>>>>> a real product > >> >> >>>>>> and improve its performance, and step by step, we broaden it= and make it > >> >> >>>>>> widely useful to various areas in which I can't reach :-) > >> >> >>>>> > >> >> >>>>> We must guarantee the normal swap path runs correctly and has= no > >> >> >>>>> performance regression when developing SWP_SYNCHRONOUS_IO opt= imization. > >> >> >>>>> So we have to put some effort on the normal path test anyway. > >> >> >>>>> > >> >> >>>>>> so probably we can have a sysfs "enable" entry with default = "n" or > >> >> >>>>>> have a maximum > >> >> >>>>>> swap-in order as Ryan's suggestion [1] at the beginning, > >> >> >>>>>> > >> >> >>>>>> " > >> >> >>>>>> So in the common case, swap-in will pull in the same size of= folio as was > >> >> >>>>>> swapped-out. Is that definitely the right policy for all fol= io sizes? Certainly > >> >> >>>>>> it makes sense for "small" large folios (e.g. up to 64K IMHO= ). But I'm not sure > >> >> >>>>>> it makes sense for 2M THP; As the size increases the chances= of actually needing > >> >> >>>>>> all of the folio reduces so chances are we are wasting IO. T= here are similar > >> >> >>>>>> arguments for CoW, where we currently copy 1 page per fault = - it probably makes > >> >> >>>>>> sense to copy the whole folio up to a certain size. > >> >> >>>>>> " > >> >> >>> > >> >> >>> I thought about this a bit more. No clear conclusions, but hope= d this might help > >> >> >>> the discussion around policy: > >> >> >>> > >> >> >>> The decision about the size of the THP is made at first fault, = with some help > >> >> >>> from user space and in future we might make decisions to split = based on > >> >> >>> munmap/mremap/etc hints. In an ideal world, the fact that we ha= ve had to swap > >> >> >>> the THP out at some point in its lifetime should not impact on = its size. It's > >> >> >>> just being moved around in the system and the reason for our or= iginal decision > >> >> >>> should still hold. > >> >> >>> > >> >> >>> So from that PoV, it would be good to swap-in to the same size = that was > >> >> >>> swapped-out. > >> >> >> > >> >> >> Sorry, I don't agree with this. It's better to swap-in and swap= -out in > >> >> >> smallest size if the page is only accessed seldom to avoid to wa= ste > >> >> >> memory. > >> >> > > >> >> > If we want to optimize only for memory consumption, I'm sure ther= e are many > >> >> > things we would do differently. We need to find a balance between= memory and > >> >> > performance. The benefits of folios are well documented and the k= ernel is > >> >> > heading in the direction of managing memory in variable-sized blo= cks. So I don't > >> >> > think it's as simple as saying we should always swap-in the small= est possible > >> >> > amount of memory. > >> >> > >> >> It's conditional, that is, > >> >> > >> >> "if the page is only accessed seldom" > >> >> > >> >> Then, the page swapped-in will be swapped-out soon and adjacent pag= es in > >> >> the same large folio will not be accessed during this period. > >> >> > >> >> So, I suggest to create an algorithm to decide swap-in order based = on > >> >> swap-readahead information automatically. It can detect the situat= ion > >> >> above via reduced swap readahead window size. And, if the page is > >> >> accessed for quite long time, and the adjacent pages in the same la= rge > >> >> folio are accessed too, swap-readahead window will increase and lar= ge > >> >> swap-in order will be used. > >> > > >> > The original size of do_anonymous_page() should be honored, consider= ing it > >> > embodies a decision influenced by not only sysfs settings and per-vm= a > >> > HUGEPAGE hints but also architectural characteristics, for example > >> > CONT-PTE. > >> > > >> > The model you're proposing may offer memory-saving benefits or reduc= e I/O, > >> > but it entirely disassociates the size of the swap in from the size = prior to the > >> > swap out. > >> > >> Readahead isn't the only factor to determine folio order. For example= , > >> we must respect "never" policy to allocate order-0 folio always. > >> There's no requirements to use swap-out order in swap-in too. Memory > >> allocation has different performance character of storage reading. > > > > Still quite unclear. > > > > If users have only enabled 64KiB (4-ORDER) large folios in sysfs, and t= he > > readahead algorithm requires 16KiB, what should be set as the large fol= io size? > > Setting it to 16KiB doesn't align with users' requirements, while > > setting it to 64KiB > > would be wasteful according to your criteria. > > IIUC, enabling 64KB means you can use 64KB mTHP if appropriate, doesn't > mean that you must use 64KB mTHP. If so, we should use 16KB mTHP in > that situation. A specific large folio size inherently denotes a high-quality resource. For example, a 64KiB folio necessitates only one TLB on ARM64, just as a 2MB large folio accommodates only one TLB. I am skeptical whether a size determined by readahead offers any tangible advantages over simply having small folios. > > >> > Moreover, there's no guarantee that the large folio generated by > >> > the readahead window is contiguous in the swap and can be added to t= he > >> > swap cache, as we are currently dealing with folio->swap instead of > >> > subpage->swap. > >> > >> Yes. We can optimize only when all conditions are satisfied. Just li= ke > >> other optimization. > >> > >> > Incidentally, do_anonymous_page() serves as the initial location for= allocating > >> > large folios. Given that memory conservation is a significant consid= eration in > >> > do_swap_page(), wouldn't it be even more crucial in do_anonymous_pag= e()? > >> > >> Yes. We should consider that too. IIUC, that is why mTHP support is > >> off by default for now. After we find a way to solve the memory usage > >> issue. We may make default "on". > > > > It's challenging to establish a universal solution because various syst= ems > > exhibit diverse hardware characteristics, and VMAs may require differen= t > > alignments. The current sysfs and per-vma hints allow users the opportu= nity > > o customize settings according to their specific requirements. > > IIUC, Linux kernel is trying to provide a reasonable default behavior in > all situations. We are trying to optimize default behavior in the first > place, only introduce customization if we fail to do that. I don't > think that it's a good idea to introduce too much customization if we > haven't tried to optimize the default behavior. I've never been opposed to the readahead case, but I feel it's a second ste= p. My point is to begin with the simplest and most practical approaches that can generate genuine value and contribution. The SWP_SYNCHRONOUS_IO case has been implemented on millions of OPPO's phones and has demonstrated product succe= ss. > > >> > >> > A large folio, by its nature, represents a high-quality resource tha= t has the > >> > potential to leverage hardware characteristics for the benefit of th= e > >> > entire system. > >> > >> But not at the cost of memory wastage. > >> > >> > Conversely, I don't believe that a randomly determined size dictated= by the > >> > readahead window possesses the same advantageous qualities. > >> > >> There's a readahead algorithm which is not pure random. > >> > >> > SWP_SYNCHRONOUS_IO devices are not reliant on readahead whatsoever, > >> > their needs should also be respected. > >> > >> I understand that there are special requirements for SWP_SYNCHRONOUS_I= O > >> devices. I just suggest to work on general code before specific > >> optimization. > > > > I disagree with your definition of "special" and "general". According > > to your logic, > > non-SWP_SYNCHRONOUS_IO devices could also be classified as "special". > > SWP_SYNCHRONOUS_IO devices also use general code path. They just use > some special optimization in some special situation (__swap_count(entry) > =3D=3D 1). Optimization in general path benefits everyone. > > > Furthermore, the number of systems running SWP_SYNCHRONOUS_IO is > > significantly greater than those running non-SWP_SYNCHRONOUS_IO, > > contradicting your assertion. > > > > SWP_SYNCHRONOUS_IO devices have a minor chance of being involved > > in readahead. > > Then it loses an opportunity to determine the appropriate folio order. > We can consider how to balance between the overhead and benefit of > readahead. IIUC, compared with original SWP_SYNCHRONOUS_IO swap-in, > mTHP is a kind of readahead too. > > BTW, because we have added more and more swap cache related operations > (swapcache_prepare(), clear_shadow_from_swap_cache(), swapcache_clear(), > etc.) in SWP_SYNCHRONOUS_IO code path, I suspect whether the benefit of > SWP_SYNCHRONOUS_IO is still large enough. We may need to re-evaluate > it. Obviously SWP_SYNCHRONOUS_IO is still quite valuable as Kairui has the data in his commit 13ddaf26be324a ("mm/swap: fix race when skipping swapcache") "Performance overhead is minimal, microbenchmark swapin 10G from 32G zram: Before: 10934698 us After: 11157121 us Cached: 13155355 us (Dropping SWP_SYNCHRONOUS_IO flag) " > > > However, in OPPO's code, which hasn't been sent in the > > LKML yet, we use the exact same size as do_anonymous_page for readahead= . > > Without a clear description of how you want the new readahead > > algorithm to balance memory waste and users' hints from sysfs and > > per-vma flags, it appears to be an ambiguous area to address. > > > > Please provide a clear description of how you would like the new readah= ead > > algorithm to function. I believe this clarity will facilitate others > > in attempting to > > implement it. > > For example, if __swapin_nr_pages() > 4, we can try to allocate an > order-2 mTHP if other conditions are satisfied. There is no evidence suggesting that an order-2 or any other orders determined by readahead are superior to having four small folios. > > >> > >> >> > You also said we should swap *out* in smallest size possible. Hav= e I > >> >> > misunderstood you? I thought the case for swapping-out a whole fo= lio without > >> >> > splitting was well established and non-controversial? > >> >> > >> >> That is conditional too. > >> >> > >> >> >> > >> >> >>> But we only kind-of keep that information around, via the swap > >> >> >>> entry contiguity and alignment. With that scheme it is possible= that multiple > >> >> >>> virtually adjacent but not physically contiguous folios get swa= pped-out to > >> >> >>> adjacent swap slot ranges and then they would be swapped-in to = a single, larger > >> >> >>> folio. This is not ideal, and I think it would be valuable to t= ry to maintain > >> >> >>> the original folio size information with the swap slot. One way= to do this would > >> >> >>> be to store the original order for which the cluster was alloca= ted in the > >> >> >>> cluster. Then we at least know that a given swap slot is either= for a folio of > >> >> >>> that order or an order-0 folio (due to cluster exhaustion/scann= ing). Can we > >> >> >>> steal a bit from swap_map to determine which case it is? Or are= there better > >> >> >>> approaches? > >> >> >> > >> >> >> [snip] > >> > > -- > Best Regards, > Huang, Ying Thanks Barry