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[172.125.78.211]) by smtp.gmail.com with ESMTPSA id df18-20020a05690c0f9200b0054643d99e21sm3390137ywb.133.2023.04.04.11.59.33 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Apr 2023 11:59:34 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3731.400.51.1.1\)) Subject: Re: [External] RE(2): FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL From: "Viacheslav A.Dubeyko" In-Reply-To: Date: Tue, 4 Apr 2023 11:59:22 -0700 Cc: Adam Manzanares , Mike Rapoport , Kyungsan Kim , "lsf-pc@lists.linux-foundation.org" , "linux-mm@kvack.org" , "linux-fsdevel@vger.kernel.org" , "linux-cxl@vger.kernel.org" , "dan.j.williams@intel.com" , "seungjun.ha@samsung.com" , "wj28.lee@samsung.com" Content-Transfer-Encoding: quoted-printable Message-Id: References: <20230331114525.400375-1-ks0204.kim@samsung.com> <20230404175754.GA633356@bgt-140510-bm01> To: Gregory Price X-Mailer: Apple Mail (2.3731.400.51.1.1) X-Rspam-User: X-Rspamd-Queue-Id: 3C74618000D X-Rspamd-Server: rspam01 X-Stat-Signature: hjnuhbrw83bwsztwm8cm7b4fg5i5jp1q X-HE-Tag: 1680634776-621099 X-HE-Meta: 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 8d41HFhz 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.017957, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Apr 1, 2023, at 3:51 AM, Gregory Price = wrote: >=20 > On Tue, Apr 04, 2023 at 05:58:05PM +0000, Adam Manzanares wrote: >> On Tue, Apr 04, 2023 at 11:31:08AM +0300, Mike Rapoport wrote: >>>=20 >>> The point of zswap IIUC is to have small and fast swap device and >>> compression is required to better utilize DRAM capacity at expense = of CPU >>> time. >>>=20 >>> Presuming CXL memory will have larger capacity than DRAM, why not = skip the >>> compression and use CXL as a swap device directly? >>=20 >> I like to shy away from saying CXL memory should be used for swap. I = see a=20 >> swap device as storing pages in a manner that is no longer directly = addressable >> by the cpu.=20 >>=20 >> Migrating pages to a CXL device is a reasonable approach and I = believe we >> have the ability to do this in the page reclaim code.=20 >>=20 >=20 > The argument is "why do you need swap if memory itself is elastic", = and > I think there are open questions about how performant using large > amounts of high-latency memory is. >=20 > Think 1us-1.5us+ cross-rack attached memory. >=20 > Does it make sense to use that as CPU-addressible and migrate it on > first use? Isn't that just swap with more steps? What happens if we > just use it as swap, is the performance all that different? >=20 > I think there's a reasonable argument for exploring the idea at the > higher ends of the latency spectrum. And the simplicity of using an > existing system (swap) to implement a form of proto-tiering is rather > attractive in my opinion. >=20 I think the problem with swap that we need to take into account the = additional latency of swap-in/swap-out logic. I assume that this logic is expensive = enough. And if we considering the huge graph, for example, I am afraid the = swap-in/swap-out logic could be expensive. So, the question here is about use-case. Which = use-case could have benefits to employ the swap as a big space of high-latency memory? = I see your point that such swap could be faster than persistent storage. But which = use-case can be happy user of this space of high-latency memory? Thanks, Slava.