From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Rientjes Subject: Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface Date: Mon, 25 Apr 2022 12:15:48 -0700 (PDT) Message-ID: References: <20220425190040.2475377-1-yosryahmed@google.com> <20220425190040.2475377-2-yosryahmed@google.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:in-reply-to:message-id:references :mime-version; bh=ecsUxmyFwe8I77x+9cA1HTTii6mLSDL9ggSg8/7/5Lo=; b=lSv3D+xXudt8kuycOWKphR/+PTAKkjLYPnzKliiGicQEjNUPwftnMMBs2cG9pNFHPV Rx5m56PnLtd6rUJslmzMmG0HpEdKW4PPVfBjF3VEMO4+y8q8bkRuqgB6GFRLiMtUC//+ dIID5FA51QMkhLndJJnDP4lfVmsSD3tZw1djRdoRfaC+JPv8o1fwE6rkxfTqwELj3BHr mXgDscQMSgJLeW+ALlwLXot4+PkLBbSajgNXMds+bAuLTm3k9Vn2cCn7eblMwAcCVs5g 1aPwVBuFa8VfQ32WIX08L2Ouzo2vqpTVIC81Gj14Rz1jpg4RvV3la7P2rAGcA5/ySaJV hj/w== In-Reply-To: <20220425190040.2475377-2-yosryahmed@google.com> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Yosry Ahmed Cc: Johannes Weiner , Michal Hocko , Shakeel Butt , Andrew Morton , Roman Gushchin , Tejun Heo , Zefan Li , Jonathan Corbet , Shuah Khan , Yu Zhao , Dave Hansen , Wei Xu , Greg Thelen , Chen Wandun , Vaibhav Jain , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Tim Chen , cgroups@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Michal On Mon, 25 Apr 2022, Yosry Ahmed wrote: > From: Shakeel Butt > > Introduce a memcg interface to trigger memory reclaim on a memory cgroup. > > Use case: Proactive Reclaim > --------------------------- > > A userspace proactive reclaimer can continuously probe the memcg to > reclaim a small amount of memory. This gives more accurate and > up-to-date workingset estimation as the LRUs are continuously > sorted and can potentially provide more deterministic memory > overcommit behavior. The memory overcommit controller can provide > more proactive response to the changing behavior of the running > applications instead of being reactive. > > A userspace reclaimer's purpose in this case is not a complete replacement > for kswapd or direct reclaim, it is to proactively identify memory savings > opportunities and reclaim some amount of cold pages set by the policy > to free up the memory for more demanding jobs or scheduling new jobs. > > A user space proactive reclaimer is used in Google data centers. > Additionally, Meta's TMO paper recently referenced a very similar > interface used for user space proactive reclaim: > https://dl.acm.org/doi/pdf/10.1145/3503222.3507731 > > Benefits of a user space reclaimer: > ----------------------------------- > > 1) More flexible on who should be charged for the cpu of the memory > reclaim. For proactive reclaim, it makes more sense to be centralized. > > 2) More flexible on dedicating the resources (like cpu). The memory > overcommit controller can balance the cost between the cpu usage and > the memory reclaimed. > > 3) Provides a way to the applications to keep their LRUs sorted, so, > under memory pressure better reclaim candidates are selected. This also > gives more accurate and uptodate notion of working set for an > application. > > Why memory.high is not enough? > ------------------------------ > > - memory.high can be used to trigger reclaim in a memcg and can > potentially be used for proactive reclaim. > However there is a big downside in using memory.high. It can potentially > introduce high reclaim stalls in the target application as the > allocations from the processes or the threads of the application can hit > the temporary memory.high limit. > > - Userspace proactive reclaimers usually use feedback loops to decide > how much memory to proactively reclaim from a workload. The metrics > used for this are usually either refaults or PSI, and these metrics > will become messy if the application gets throttled by hitting the > high limit. > > - memory.high is a stateful interface, if the userspace proactive > reclaimer crashes for any reason while triggering reclaim it can leave > the application in a bad state. > > - If a workload is rapidly expanding, setting memory.high to proactively > reclaim memory can result in actually reclaiming more memory than > intended. > > The benefits of such interface and shortcomings of existing interface > were further discussed in this RFC thread: > https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/ > > Interface: > ---------- > > Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to > trigger reclaim in the target memory cgroup. > > The interface is introduced as a nested-keyed file to allow for future > optional arguments to be easily added to configure the behavior of > reclaim. > > Possible Extensions: > -------------------- > > - This interface can be extended with an additional parameter or flags > to allow specifying one or more types of memory to reclaim from (e.g. > file, anon, ..). > > - The interface can also be extended with a node mask to reclaim from > specific nodes. This has use cases for reclaim-based demotion in memory > tiering systens. > > - A similar per-node interface can also be added to support proactive > reclaim and reclaim-based demotion in systems without memcg. > > - Add a timeout parameter to make it easier for user space to call the > interface without worrying about being blocked for an undefined amount > of time. > > For now, let's keep things simple by adding the basic functionality. > > [yosryahmed@google.com: worked on versions v2 onwards, refreshed to > current master, updated commit message based on recent > discussions and use cases] > > Signed-off-by: Shakeel Butt > Co-developed-by: Yosry Ahmed > Signed-off-by: Yosry Ahmed > Acked-by: Johannes Weiner > Acked-by: Michal Hocko > Acked-by: Wei Xu > Acked-by: Roman Gushchin Acked-by: David Rientjes "can over or under reclaim from the target cgroup" begs the question of how much more memory the kernel can decide to reclaim :) I think it's assumed that it's minimal and that matches the current implementation that rounds up to SWAP_CLUSTER_MAX, though, so looks good. Thanks Yosry!