From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6115EC6FD1C for ; Thu, 23 Mar 2023 01:50:00 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 762656B0072; Wed, 22 Mar 2023 21:49:59 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 712386B0074; Wed, 22 Mar 2023 21:49:59 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5B4A06B0075; Wed, 22 Mar 2023 21:49:59 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 498176B0072 for ; Wed, 22 Mar 2023 21:49:59 -0400 (EDT) Received: from smtpin05.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 0F7EBA058E for ; Thu, 23 Mar 2023 01:49:59 +0000 (UTC) X-FDA: 80598482118.05.9EB28FE Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by imf25.hostedemail.com (Postfix) with ESMTP id 33EF8A000C for ; Thu, 23 Mar 2023 01:49:53 +0000 (UTC) Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=MkYJsIvm; spf=pass (imf25.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.93 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1679536197; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=xic89HqKIH/H/tRIUN4JEcM2rvHu45DEh80GqaIgIvM=; b=gBr3OkO08SV27GiEQdVS3z9IrDSZ1W9gK4U6D46blzlPEuUdUrm61xJGxUff8iUOi6HNUs uJBLFWWxV5wMFmbK2xuJkW9MXZ0FQBVGaRp4VP0Y4AqyAX3c/V2vuv8FHk5hx+30KaPxQU P1dSk4JkSZ1SMgb4TleE74reBk5S2wo= ARC-Authentication-Results: i=1; imf25.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=MkYJsIvm; spf=pass (imf25.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.93 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1679536197; a=rsa-sha256; cv=none; b=K0qbqgJTtfl616D95GM5sG8oV9zZtn3TYnEHfY1YzWGqT/LhumakOm4muDRv2ocpn4QJTP kA9pwBjQYFkKuqCfWfxA0TAbzhjE2PdVdvHMmkHqovUEFSX139xRLu3PohlZbcK2vMygA8 +XOXcjFj1UcpikN7g5KW5aSNNEvctTo= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1679536195; x=1711072195; h=from:to:cc:subject:references:date:in-reply-to: message-id:mime-version:content-transfer-encoding; bh=n9j64GeHVRJYE5Nazv/wkakx6enjvhFi7d8HGKkxhM8=; b=MkYJsIvmS0W34qcH94c6KzihPFwnqh9F3cJbMPFQNeI5dyk46O15QVkM Pi5mAG1nSgeW6Vtva81EV41FVIkkr3j2uub8yhO3B/7OV6S/t552MV66/ 5Yz3nWqxcqm1qufA4roFoCS8yRToIdpDLm4QGQgsweD26bR//2fKoRWqm 6IWVN9BPNvSGYK0prFG79GcLRL7w3H11oI621IkJDkvI/m0w7F29HV8Gt /f4DaxABPejfrdI0xyOVj+uo9rWkErjj05cSTLFudTQcJEzHTyUgDQdwx lVDvHYhJ7gZniZ2/A1DEO3+cPks0gky/iGnVo9XTsMR1YrAIEDpYM/eOg g==; X-IronPort-AV: E=McAfee;i="6600,9927,10657"; a="336880290" X-IronPort-AV: E=Sophos;i="5.98,283,1673942400"; d="scan'208";a="336880290" Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Mar 2023 18:49:51 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10657"; a="1011590839" X-IronPort-AV: E=Sophos;i="5.98,283,1673942400"; d="scan'208";a="1011590839" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Mar 2023 18:49:46 -0700 From: "Huang, Ying" To: Yosry Ahmed Cc: Chris Li , lsf-pc@lists.linux-foundation.org, Johannes Weiner , Linux-MM , Michal Hocko , Shakeel Butt , David Rientjes , Hugh Dickins , Seth Jennings , Dan Streetman , Vitaly Wool , Yang Shi , Peter Xu , Minchan Kim , Andrew Morton , Aneesh Kumar K V , Michal Hocko , Wei Xu Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap References: <87356e850j.fsf@yhuang6-desk2.ccr.corp.intel.com> <87y1o571aa.fsf@yhuang6-desk2.ccr.corp.intel.com> <87o7ox762m.fsf@yhuang6-desk2.ccr.corp.intel.com> <87bkkt5e4o.fsf@yhuang6-desk2.ccr.corp.intel.com> <87y1ns3zeg.fsf@yhuang6-desk2.ccr.corp.intel.com> Date: Thu, 23 Mar 2023 09:48:45 +0800 In-Reply-To: (Yosry Ahmed's message of "Tue, 21 Mar 2023 22:56:32 -0700") Message-ID: <874jqcteyq.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 33EF8A000C X-Stat-Signature: cbb1so7h47s87xzdf7maoefn4bs6b5ee X-Rspam-User: X-HE-Tag: 1679536193-608635 X-HE-Meta: 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 duA4T7XS PqS86NvR2CYHwlsqJCb+xUpD0bWyQhE8OTnqOTBt2+ma1BwPZmP1l50g+3MQYM638cy9As/6JrbVf0ajwbfSMWFL+B9kXK0jVY5A/bv5TG38wHvs/WH2q5rEo8G4qlmfEK3kgs/anqsrDbIape3yGp5YnVtswO7Vnkx4LliJCfy54O333fQRrRiODeOqRRKSqtzomUwXC3r+Wbq968ztIv+6Lvg== 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: Yosry Ahmed writes: > On Sun, Mar 19, 2023 at 7:56=E2=80=AFPM Huang, Ying wrote: >> >> Yosry Ahmed writes: >> >> > On Thu, Mar 16, 2023 at 12:51=E2=80=AFAM Huang, Ying wrote: >> >> >> >> Yosry Ahmed writes: >> >> >> >> > On Sun, Mar 12, 2023 at 7:13=E2=80=AFPM Huang, Ying wrote: >> >> >> >> >> >> Yosry Ahmed writes: >> >> >> >> >> >> >> >> >> > >> >> >> > My current idea is to have one xarray that stores the swap_descs >> >> >> > (which include swap_entry, swapcache, swap_count, etc), and only= for >> >> >> > rotating disks have an additional xarray that maps swap_entry -> >> >> >> > swap_desc for cluster readahead, assuming we can eliminate all o= ther >> >> >> > situations requiring a reverse mapping. >> >> >> > >> >> >> > I am not sure how having separate xarrays help? If we have one x= array, >> >> >> > might as well save the other lookups on put everything in swap_d= esc. >> >> >> > In fact, this should improve the locking today as swapcache / >> >> >> > swap_count operations can be lockless or very lightly contended. >> >> >> >> >> >> The condition of the proposal is "reverse mapping cannot be avoide= d for >> >> >> enough situation". So, if reverse mapping (or cluster readahead) = can be >> >> >> avoided for enough situations, I think your proposal is good. Oth= erwise, >> >> >> I propose to use 2 xarrays. You don't need another reverse mapping >> >> >> xarray, because you just need to read the next several swap_entry = into >> >> >> the swap cache for cluster readahead. swap_desc isn't needed for >> >> >> cluster readahead. >> >> > >> >> > swap_desc would be needed for cluster readahead in my original >> >> > proposal as the swap cache lives in swap_descs. Based on the current >> >> > implementation, we would need a reverse mapping (swap entry -> >> >> > swap_desc) in 3 situations: >> >> > >> >> > 1) __try_to_reclaim_swap(): when trying to find an empty swap slot = and >> >> > failing, we fallback to trying to find swap entries that only have a >> >> > page in the swap cache (no references in page tables or page cache) >> >> > and free them. This would require a reverse mapping. >> >> > >> >> > 2) swapoff: we need to swap in all entries in a swapfile, so we need >> >> > to get all swap_descs associated with that swapfile. >> >> > >> >> > 3) swap cluster readahead. >> >> > >> >> > For (1), I think we can drop the dependency of a reverse mapping if= we >> >> > free swap entries once we swap a page in and add it to the swap cac= he, >> >> > even if the swap count does not drop to 0. >> >> >> >> Now, we will not drop the swap cache even if the swap count becomes 0= if >> >> swap space utility < 50%. Per my understanding, this avoid swap page >> >> writing for read accesses. So I don't think we can change this direc= tly >> >> without necessary discussion firstly. >> > >> > >> > Right. I am not sure I understand why we do this today, is it to save >> > the overhead of allocating a new swap entry if the page is swapped out >> > again soon? I am not sure I understand this statement "this avoid swap >> > page >> > writing for read accesses". >> > >> >> >> >> >> >> > For (2), instead of scanning page tables and shmem page cache to fi= nd >> >> > swapped out pages for the swapfile, we can scan all swap_descs >> >> > instead, we should be more efficient. This is one of the proposal's >> >> > potential advantages. >> >> >> >> Good. >> >> >> >> > (3) is the one that would still need a reverse mapping with the >> >> > current proposal. Today we use swap cluster readahead for anon pages >> >> > if we have a spinning disk or vma readahead is disabled. For shmem,= we >> >> > always use cluster readahead. If we can limit cluster readahead to >> >> > only rotating disks, then the reverse mapping can only be maintained >> >> > for swapfiles on rotating disks. Otherwise, we will need to maintai= n a >> >> > reverse mapping for all swapfiles. >> >> >> >> For shmem, I think that it should be good to readahead based on shmem >> >> file offset instead of swap device offset. >> >> >> >> It's possible that some pages in the readahead window are from HDD wh= ile >> >> some other pages aren't. So it's a little hard to enable cluster read >> >> for HDD only. Anyway, it's not common to use HDD for swap now. >> >> >> >> >> >> >> >> > If the point is to store the swap_desc directly inside the xarra= y to >> >> >> > save 8 bytes, I am concerned that having multiple xarrays for >> >> >> > swapcache, swap_count, etc will use more than that. >> >> >> >> >> >> The idea is to save the memory used by reverse mapping xarray. >> >> > >> >> > I see. >> >> > >> >> >> >> >> >> >> >> >> >> >> >> >> > Keep in mind that the current overhead is 1 byte O(max swa= p pages) not >> >> >> >> >> > O(swapped). Also, 1 byte is assuming we do not use the swap >> >> >> >> >> > continuation pages. If we do, it may end up being more. We= also >> >> >> >> >> > allocate continuation in full 4k pages, so even if one swa= p_map >> >> >> >> >> > element in a page requires continuation, we will allocate = an entire >> >> >> >> >> > page. What I am trying to say is that to get an actual com= parison you >> >> >> >> >> > need to also factor in the swap utilization and the rate o= f usage of >> >> >> >> >> > swap continuation. I don't know how to come up with a form= ula for this >> >> >> >> >> > tbh. >> >> >> >> >> > >> >> >> >> >> > Also, like Johannes said, the worst case overhead (32 byte= s if you >> >> >> >> >> > count the reverse mapping) is 0.8% of swapped memory, aka = 8M for every >> >> >> >> >> > 1G swapped. It doesn't sound *very* bad. I understand that= it is pure >> >> >> >> >> > overhead for people not using zswap, but it is not very aw= ful. >> >> >> >> >> > >> >> >> >> >> >> >> >> >> >> >> >> It seems what you really need is one bit of information t= o indicate >> >> >> >> >> >> this page is backed by zswap. Then you can have a seperat= e pointer >> >> >> >> >> >> for the zswap entry. >> >> >> >> >> > >> >> >> >> >> > If you use one bit in swp_entry_t (or one of the available= swap types) >> >> >> >> >> > to indicate whether the page is backed with a swapfile or = zswap it >> >> >> >> >> > doesn't really work. We lose the indirection layer. How do= we move the >> >> >> >> >> > page from zswap to swapfile? We need to go update the page= tables and >> >> >> >> >> > the shmem page cache, similar to swapoff. >> >> >> >> >> > >> >> >> >> >> > Instead, if we store a key else in swp_entry_t and use thi= s to lookup >> >> >> >> >> > the swp_entry_t or zswap_entry pointer then that's essenti= ally what >> >> >> >> >> > the swap_desc does. It just goes the extra mile of unifyin= g the >> >> >> >> >> > swapcache as well and storing it directly in the swap_desc= instead of >> >> >> >> >> > storing it in another lookup structure. >> >> >> >> >> >> >> >> >> >> If we choose to make sizeof(struct swap_desc) =3D=3D 8, that= is, store only >> >> >> >> >> swap_entry in swap_desc. The added indirection appears to b= e another >> >> >> >> >> level of page table with 1 entry. Then, we may use the simi= lar method >> >> >> >> >> as supporting system with 2 level and 3 level page tables, l= ike the code >> >> >> >> >> in include/asm-generic/pgtable-nopmd.h. But I haven't thoug= ht about >> >> >> >> >> this deeply. >> >> >> >> > >> >> >> >> > Can you expand further on this idea? I am not sure I fully un= derstand. >> >> >> >> >> >> >> >> OK. The goal is to avoid the overhead if indirection isn't ena= bled via >> >> >> >> kconfig. >> >> >> >> >> >> >> >> If indirection isn't enabled, store swap_entry in PTE directly. >> >> >> >> Otherwise, store index of swap_desc in PTE. Different function= s (e.g., >> >> >> >> to get/set swap_entry in PTE) are implemented based on kconfig. >> >> >> > >> >> >> > >> >> >> > I thought about this, the problem is that we will have multiple >> >> >> > implementations of multiple things. For example, swap_count with= out >> >> >> > the indirection layer lives in the swap_map (with continuation l= ogic). >> >> >> > With the indirection layer, it lives in the swap_desc (or somewh= ere >> >> >> > else). Same for the swapcache. Even if we keep the swapcache in = an >> >> >> > xarray and not inside swap_desc, it would be indexed by swap_ent= ry if >> >> >> > the indirection is disabled, and by swap_desc (or similar) if the >> >> >> > indirection is enabled. I think maintaining separate implementat= ions >> >> >> > for when the indirection is enabled/disabled would be adding too= much >> >> >> > complexity. >> >> >> > >> >> >> > WDYT? >> >> >> >> >> >> If we go this way, swap cache and swap_count will always be indexe= d by >> >> >> swap_entry. swap_desc just provides a indirection to make it poss= ible >> >> >> to move between swap devices. >> >> >> >> >> >> Why must we index swap cache and swap_count by swap_desc if indire= ction >> >> >> is enabled? Yes, we can save one xarray indexing if we do so, but= I >> >> >> don't think the overhead of one xarray indexing is a showstopper. >> >> >> >> >> >> I think this can be one intermediate step towards your final targe= t. >> >> >> The changes to current implementation can be smaller. >> >> > >> >> > IIUC, the idea is to have two xarrays: >> >> > (a) xarray that stores a pointer to a struct containing swap_count = and >> >> > swap cache. >> >> > (b) xarray that stores the underlying swap entry or zswap entry. >> >> > >> >> > When indirection is disabled: >> >> > page tables & page cache have swap entry directly like today, xarray >> >> > (a) is indexed by swap entry, xarray (b) does not exist. No reverse >> >> > mapping needed. >> >> > >> >> > In this case we have an extra overhead of 12-16 bytes (the struct >> >> > containing swap_count and swap cache) vs. 24 bytes of the swap_desc. >> >> > >> >> > When indirection is enabled: >> >> > page tables & page cache have a swap id (or swap_desc index), xarray >> >> > (a) is indexed by swap id, >> >> >> >> xarray (a) is indexed by swap entry. >> > >> > >> > How so? With the indirection enabled, the page tables & page cache >> > have the swap id (or swap_desc index), which can point to a swap entry >> > or a zswap entry -- which can change when the page is moved between >> > zswap & swapfiles. How is xarray (a) indexed by the swap entry in this >> > case? Shouldn't be indexed by the abstract swap id so that the >> > writeback from zswap is transparent? >> >> In my mind, >> >> - swap core will define a abstract interface to swap implementations >> (zswap, swap device/file, maybe more in the future), like VFS. >> >> - zswap will be a special swap implementation (compressing instead of >> writing to disk). >> >> - swap core will manage the indirection layer and swap cache. >> >> - swap core can move swap pages between swap implementations (e.g., from >> zswap to a swap device, or from one swap device to another swap >> device) with the help of the indirection layer. >> >> In this design, the writeback from zswap becomes moving swapped pages >> from zswap to a swap device. > > > All the above matches my understanding of this proposal. swap_desc is > the proposed indirection layer, and the swap implementations are zswap > & swap devices. For now, we only have 2 static swap implementations > (zswap->swapfile). In the future, we can make this more dynamic as the > need arises. Great to align with you on this. >> >> If my understanding were correct, your suggestion is kind of moving >> zswap logic to the swap core? And zswap will be always at a higher >> layer on top of swap device/file? > > > We do not want to move the zswap logic into the swap core, we want to > make the swap core independent of the swap implementation, and zswap > is just one possible implementation. Good! I found that you put zswap related data structure inside struct swap_desc directly. I think that we should avoid that as much as possible. >> >> >> >> >> >> >> > xarray (b) is indexed by swap id as well >> >> > and contain swap entry or zswap entry. Reverse mapping might be >> >> > needed. >> >> >> >> Reverse mapping isn't needed. >> > >> > >> > It would be needed if xarray (a) is indexed by the swap id. I am not >> > sure I understand how it can be indexed by the swap entry if the >> > indirection is enabled. >> > >> >> >> >> >> >> > In this case we have an extra overhead of 12-16 bytes + 8 bytes for >> >> > xarray (b) entry + memory overhead from 2nd xarray + reverse mapping >> >> > where needed. >> >> > >> >> > There is also the extra cpu overhead for an extra lookup in certain= paths. >> >> > >> >> > Is my analysis correct? If yes, I agree that the original proposal = is >> >> > good if the reverse mapping can be avoided in enough situations, and >> >> > that we should consider such alternatives otherwise. As I mentioned >> >> > above, I think it comes down to whether we can completely restrict >> >> > cluster readahead to rotating disks or not -- in which case we need= to >> >> > decide what to do for shmem and for anon when vma readahead is >> >> > disabled. >> >> >> >> We can even have a minimal indirection implementation. Where, swap >> >> cache and swap_map[] are kept as they ware before, just one xarray is >> >> added. The xarray is indexed by swap id (or swap_desc index) to store >> >> the corresponding swap entry. >> >> >> >> When indirection is disabled, no extra overhead. >> >> >> >> When indirection is enabled, the extra overhead is just 8 bytes per >> >> swapped page. >> >> >> >> The basic migration support can be build on top of this. >> >> >> >> I think that this could be a baseline for indirection support. Then >> >> further optimization can be built on top of it step by step with >> >> supporting data. >> > >> > >> > I am not sure how this works with zswap. Currently swap_map[] >> > implementation is specific for swapfiles, it does not work for zswap >> > unless we implement separate swap counting logic for zswap & >> > swapfiles. Same for the swapcache, it currently supports being indexed >> > by a swap entry, it would need to support being indexed by a swap id, >> > or have a separate swap cache for zswap. Having separate >> > implementation would add complexity, and we would need to perform >> > handoffs of the swap count/cache when a page is moved from zswap to a >> > swapfile. >> >> We can allocate a swap entry for each swapped page in zswap. > > > This is exactly what the current implementation does and what we want > to move away from. The current implementation uses zswap as an > in-memory compressed cache on top of an actual swap device, and each > swapped page in zswap has a swap entry allocated. With this > implementation, zswap cannot be used without a swap device. I totally agree that we should avoid to use an actual swap device under zswap. And, as an swap implementation, zswap can manage the swap entry inside zswap without an underlying actual swap device. For example, when we swap a page to zswap (actually compress), we can allocate a (virtual) swap entry in the zswap. I understand that there's overhead to manage the swap entry in zswap. We can consider how to reduce the overhead. Best Regards, Huang, Ying >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> Depending on how much you are going to reuse the swap cac= he, you might >> >> >> >> >> >> need to have something like a swap_info_struct to keep th= e locks happy. >> >> >> >> >> > >> >> >> >> >> > My current intention is to reimplement the swapcache compl= etely as a >> >> >> >> >> > pointer in struct swap_desc. This would eliminate this nee= d and a lot >> >> >> >> >> > of the locking we do today if I get things right. >> >> >> >> >> > >> >> >> >> >> >> >> >> >> >> >> >> > Another potential concern is readahead. With this desig= n, we have no >> >> >> >> >> >> >> >> >> >> >> >> Readahead is for spinning disk :-) Even a normal swap fil= e with an SSD can >> >> >> >> >> >> use some modernization. >> >> >> >> >> > >> >> >> >> >> > Yeah, I initially thought we would only need the swp_entry= _t -> >> >> >> >> >> > swap_desc reverse mapping for readahead, and that we can o= nly store >> >> >> >> >> > that for spinning disks, but I was wrong. We need for othe= r things as >> >> >> >> >> > well today: swapoff, when trying to find an empty swap slo= t and we >> >> >> >> >> > start trying to free swap slots used only by the swapcache= . However, I >> >> >> >> >> > think both of these cases can be fixed (I can share more d= etails if >> >> >> >> >> > you want). If everything goes well we should only need to = maintain the >> >> >> >> >> > reverse mapping (extra overhead above 24 bytes) for swap f= iles on >> >> >> >> >> > spinning disks for readahead. >> >> >> >> >> > >> >> >> >> >> >> >> >> >> >> >> >> Looking forward to your discussion. >> >> >> >> >> >> >> >> Per my understanding, the indirection is to make it easy to move >> >> >> >> (swapped) pages among swap devices based on hot/cold. This is = similar >> >> >> >> as the target of memory tiering. It appears that we can extend= the >> >> >> >> memory tiering (mm/memory-tiers.c) framework to cover swap devi= ces too? >> >> >> >> Is it possible for zswap to be faster than some slow memory med= ia? >> >> >> > >> >> >> > >> >> >> > Agree with Chris that this may require a much larger overhaul. A= slow >> >> >> > memory tier is still addressable memory, swap/zswap requires a p= age >> >> >> > fault to read the pages. I think (at least for now) there is a >> >> >> > fundamental difference. We want reclaim to eventually treat slow >> >> >> > memory & swap as just different tiers to place cold memory in wi= th >> >> >> > different characteristics, but otherwise I think the swapping >> >> >> > implementation itself is very different. Am I missing something? >> >> >> >> >> >> Is it possible that zswap is faster than a really slow memory >> >> >> addressable device backed by NAND? TBH, I don't have the answer. >> >> > >> >> > I am not sure either. >> >> > >> >> >> >> >> >> Anyway, do you need a way to describe the tiers of the swap device= s? >> >> >> So, you can move the cold pages among the swap devices based on th= at? >> >> > >> >> > For now I think the "tiers" in this proposal are just zswap and nor= mal >> >> > swapfiles. We can later extend it to support more explicit tiering. >> >> >> >> IIUC, in original zswap implementation, there's 1:1 relationship betw= een >> >> zswap and normal swapfile. But now, you make demoting among swap >> >> devices more general. Then we need some general way to specify which >> >> swap devices are fast and which are slow, and the demoting relationsh= ip >> >> among them. It can be memory tiers or something else, but we need on= e. >> > >> > >> > I think for this proposal, there are only 2 hardcoded tiers. Zswap is >> > fast, swapfile is slow. In the future, we can support more dynamic >> > tiering if the need arises. >> >> We can start from a simple implementation. And I think that it's better >> to consider the general design too. Try not to make it impossible now. > > > Right. I am proposing we come up with an abstract generic interface > for swap implementations, and have 2 implementations statically > defined (swapfiles and zswap). If the need arises, we can make swap > implementations more dynamic in the future. > >> >> >> Best Regards, >> Huang, Ying