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[2003:cb:c707:9800:59ba:1006:9052:fb40]) by smtp.gmail.com with ESMTPSA id 3-20020a05600c230300b003f7ea771b5dsm1487358wmo.1.2023.06.16.00.57.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Jun 2023 00:57:20 -0700 (PDT) Message-ID: <576b7ba6-4dcd-48c9-3917-4e2a25aaa823@redhat.com> Date: Fri, 16 Jun 2023 09:57:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.12.0 Subject: Re: [External] Re: [RFC PATCH 1/3] zram: charge the compressed RAM to the page's memcgroup Content-Language: en-US To: Yosry Ahmed , =?UTF-8?B?6LS65Lit5Z2k?= Cc: Yu Zhao , minchan@kernel.org, senozhatsky@chromium.org, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrea Arcangeli , Fabian Deutsch References: <20230615034830.1361853-1-hezhongkun.hzk@bytedance.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: 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 16.06.23 09:37, Yosry Ahmed wrote: > On Thu, Jun 15, 2023 at 9:41 PM 贺中坤 wrote: >> >>> Thanks Fabian for tagging me. >>> >>> I am not familiar with #1, so I will speak to #2. Zhongkun, There are >>> a few parts that I do not understand -- hopefully you can help me out >>> here: >>> >>> (1) If I understand correctly in this patch we set the active memcg >>> trying to charge any pages allocated in a zspage to the current memcg, >>> yet that zspage will contain multiple compressed object slots, not >>> just the one used by this memcg. Aren't we overcharging the memcg? >>> Basically the first memcg that happens to allocate the zspage will pay >>> for all the objects in this zspage, even after it stops using the >>> zspage completely? >> >> It will not overcharge. As you said below, we are not using >> __GFP_ACCOUNT and charging the compressed slots to the memcgs. >> >>> >>> (2) Patch 3 seems to be charging the compressed slots to the memcgs, >>> yet this patch is trying to charge the entire zspage. Aren't we double >>> charging the zspage? I am guessing this isn't happening because (as >>> Michal pointed out) we are not using __GFP_ACCOUNT here anyway, so >>> this patch may be NOP, and the actual charging is coming from patch 3 >>> only. >> >> YES， the actual charging is coming from patch 3. This patch just >> delivers the BIO page's memcg to the current task which is not the >> consumer. >> >>> >>> (3) Zswap recently implemented per-memcg charging of compressed >>> objects in a much simpler way. If your main interest is #2 (which is >>> what I understand from the commit log), it seems like zswap might be >>> providing this already? Why can't you use zswap? Is it the fact that >>> zswap requires a backing swapfile? >> >> Thanks for your reply and review. Yes, the zswap requires a backing >> swapfile. The I/O path is very complex, sometimes it will throttle the >> whole system if some resources are short , so we hope to use zram. > > Is the only problem with zswap for you the requirement of a backing swapfile? > > If yes, I am in the early stages of developing a solution to make > zswap work without a backing swapfile. This was discussed in LSF/MM > [1]. Would this make zswap usable in for your use case? Out of curiosity, are there any other known pros/cons when using zswap-without-swap instead of zram? I know that zram requires sizing (size of the virtual block device) and consumes metadata, zswap doesn't. -- Cheers, David / dhildenb