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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 84801C433F5 for ; Mon, 25 Apr 2022 19:32:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244953AbiDYTfg (ORCPT ); Mon, 25 Apr 2022 15:35:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57040 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244947AbiDYTfe (ORCPT ); Mon, 25 Apr 2022 15:35:34 -0400 Received: from mail-pj1-x102f.google.com (mail-pj1-x102f.google.com [IPv6:2607:f8b0:4864:20::102f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58F5B111148 for ; Mon, 25 Apr 2022 12:32:28 -0700 (PDT) Received: by mail-pj1-x102f.google.com with SMTP id j8-20020a17090a060800b001cd4fb60dccso322483pjj.2 for ; Mon, 25 Apr 2022 12:32:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=oeezvirplzSzLTVDs1gNBFptZE26wQUimqKa8jXdb5Y=; b=DcMgm0ZRPmqLuBXlvrG+IVaxdGyMozez0lMkhgiAsb1rFFrlAo+EiFEygQczk2veU/ 7euIG+J6TVNbCDHuF4nqQFoK3KT4a9Twwi6a0+Wah2zlGh++9DZhr4YHujmaQohx4b7Y ybdpbndSZU44IcsvIka3R6/pX8/viZpL4iDaxUjLkvInRZPIy5k70JBb+UP3lqySnk4/ a+/1FhiFA6Fsfb6auI8STl+On8SCg2NwjOdFlfWKiKRxSHLeCsiQpzGlNdqHHKeA/isL ab/slbL6gSK0hJmVa29T0+E1vT8wPqQn5ErKEp91yxreT8k3VnuwN4wD5AdgjcZLd8dK Cu7Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=oeezvirplzSzLTVDs1gNBFptZE26wQUimqKa8jXdb5Y=; b=6cY7Cq6w3Hef1J0QIECLIsoNtmfrM2n78OfYe78Dq8yBxvwLnIvmCobHYUC+V08BRr uSrLFVEcO8jnoWyKoRhDhr7msOCGyyQy0aPa6e3mSUpPJ05b+pkyBnMNJPX1HtPp4lgA rujtD4VETdg8tsv8BLnC2X2w6BZSR4fd9AUUti0YRq7i/U0sKmGVquxSCrY3y8PZHP1E N3e9gPp4sJkltLKTOrOOwqN9iX0UbaD8Ed2z25yJPzr+fGJFhV+CMkptk5kDsP0SHZHp Lhs/Z/KeNgkgQwGGUcY8xYHxEfhrKwCLVWFYXXPGFFSJ38tMJXduTmHvSA6ZEidj0EPz EGiA== X-Gm-Message-State: AOAM530YlXNzwdZU0F0v3PzFGrOMVNprAofqR+HHiczFuhOW30JRC1OO 42C5J7Wja6WfRD39CRpZ0dHJAM30KXrG3f8fBqgiwciG2bRNEg== X-Google-Smtp-Source: ABdhPJwBvZDhhnZuuA7aqKCzlJsXI5si0z0hDX8ibOaKh3XVtbZVtAyVMNXVBMw5HvPVjbtnUEUD9Yz4KTFz6sonyfM= X-Received: by 2002:a17:902:8b88:b0:156:2b14:cb6e with SMTP id ay8-20020a1709028b8800b001562b14cb6emr19882747plb.14.1650915147597; Mon, 25 Apr 2022 12:32:27 -0700 (PDT) MIME-Version: 1.0 References: <20220425190040.2475377-1-yosryahmed@google.com> <20220425190040.2475377-2-yosryahmed@google.com> In-Reply-To: From: Yosry Ahmed Date: Mon, 25 Apr 2022 12:31:51 -0700 Message-ID: Subject: Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface To: David Rientjes 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 Mailing List , Linux-MM , linux-kselftest@vger.kernel.org, Michal Hocko Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 25, 2022 at 12:15 PM David Rientjes wrote: > > 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! I think it could be more complex than this. Some functions that get called during reclaim only use the nr_to_reclaim parameter to check if they need one more iteration, but not to limit the actual reclaimed pages per say. For example, nr_to_reclaim is not even passed to shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way to know that they should stop if nr_to_reclaim was already satisfied. I think the general assumption is that each of these calls normally does not reclaim a huge number of pages, so like you said, the kernel should not over-reclaim too much. However, I don't think there are guarantees about this. From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yosry Ahmed Subject: Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface Date: Mon, 25 Apr 2022 12:31:51 -0700 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=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=oeezvirplzSzLTVDs1gNBFptZE26wQUimqKa8jXdb5Y=; b=DcMgm0ZRPmqLuBXlvrG+IVaxdGyMozez0lMkhgiAsb1rFFrlAo+EiFEygQczk2veU/ 7euIG+J6TVNbCDHuF4nqQFoK3KT4a9Twwi6a0+Wah2zlGh++9DZhr4YHujmaQohx4b7Y ybdpbndSZU44IcsvIka3R6/pX8/viZpL4iDaxUjLkvInRZPIy5k70JBb+UP3lqySnk4/ a+/1FhiFA6Fsfb6auI8STl+On8SCg2NwjOdFlfWKiKRxSHLeCsiQpzGlNdqHHKeA/isL ab/slbL6gSK0hJmVa29T0+E1vT8wPqQn5ErKEp91yxreT8k3VnuwN4wD5AdgjcZLd8dK Cu7Q== In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: David Rientjes 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 Mailing List , Linux-MM , linux On Mon, Apr 25, 2022 at 12:15 PM David Rientjes wrote: > > 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! I think it could be more complex than this. Some functions that get called during reclaim only use the nr_to_reclaim parameter to check if they need one more iteration, but not to limit the actual reclaimed pages per say. For example, nr_to_reclaim is not even passed to shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way to know that they should stop if nr_to_reclaim was already satisfied. I think the general assumption is that each of these calls normally does not reclaim a huge number of pages, so like you said, the kernel should not over-reclaim too much. However, I don't think there are guarantees about this.