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[2003:cb:c702:5e00:8e78:71f3:6243:77f0]) by smtp.gmail.com with ESMTPSA id k12-20020adfe8cc000000b002c7b229b1basm10137726wrn.15.2023.04.03.10.04.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Apr 2023 10:04:57 -0700 (PDT) Message-ID: <39deb9d5-9f21-9d3d-0847-54e90491b0b1@redhat.com> Date: Mon, 3 Apr 2023 19:04:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.1 Subject: Re: [PATCH v4 0/3] mm: process/cgroup ksm support Content-Language: en-US To: Stefan Roesch Cc: Johannes Weiner , Andrew Morton , kernel-team@fb.com, linux-mm@kvack.org, riel@surriel.com, mhocko@suse.com, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, Hugh Dickins References: <20230310182851.2579138-1-shr@devkernel.io> <20230328160914.5b6b66e4a5ad39e41fd63710@linux-foundation.org> <37dcd52a-2e32-c01d-b805-45d862721fbc@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On 03.04.23 18:34, Stefan Roesch wrote: >> >> In contrast to e.g.: >> >> 1) THP resulted in many zeropages we end up deduplicating again. The THP >> placement was unfortunate. >> >> 2) Unoptimized memory allocators that leave many identical pages mapped >> after freeing up memory (e.g., zeroed pages, pages all filled with >> poison values) instead of e.g., using MADV_DONTNEED to free up that >> memory. >> >> > > I repeated an experiment with and without KSM. In terms of THP there is > no huge difference between the two. On a 64GB main memory machine I see > between 100 - 400MB in AnonHugePages. > >>> /sys/kernel/mm/ksm/pages_shared is over 10000 when we run this on an >>> Instagram workload. The workload consists of 36 processes plus a few >>> sidecar processes. >> >> Thanks! To which value is /sys/kernel/mm/ksm/max_page_sharing set in that >> environment? >> > > It's set to the standard value of 256. > > In the meantime I have run experiments with different settings for > pages_to_scan. With the default value of 100, we only get a relatively > small benefit of KSM. If I increase the value to for instance to 2000 or > 3000 the savings are substantial. (The workload is memory bound, not > CPU bound). Interesting. > > Here are some stats for setting pages_to_scan to 3000: > > full_scans: 560 > general_profit: 20620539008 > max_page_sharing: 256 > merge_across_nodes: 1 > pages_shared: 125446 > pages_sharing: 5259506 > pages_to_scan: 3000 > pages_unshared: 1897537 > pages_volatile: 12389223 > run: 1 > sleep_millisecs: 20 > stable_node_chains: 176 > stable_node_chains_prune_millisecs: 2000 > stable_node_dups: 2604 > use_zero_pages: 0 > zero_pages_sharing: 0 > > >> What would be interesting is pages_shared after max_page_sharing was set to a >> very high number such that pages_shared does not include duplicates. Then >> pages_shared actually expresses how many different pages we deduplicate. No need >> to run without THP in that case. >> > > Thats on my list for the next set of experiments. Splendid. >> Similarly, enabling "use_zero_pages" could highlight if your workload ends up >> deduplciating a lot of zeropages. But maxing out max_page_sharing would be >> sufficient to understand what's happening. >> >> > > I already run experiments with use_zero_pages, but they didn't make a > difference. I'll repeat the experiment with a higher pages_to_scan > value. Okay, so it's most certainly not the zeropage. Thanks for that information and running the experiments! -- Thanks, David / dhildenb