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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8E719CA9EA0 for ; Fri, 18 Oct 2019 17:03:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6D3002064A for ; Fri, 18 Oct 2019 17:03:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2634419AbfJRRD5 (ORCPT ); Fri, 18 Oct 2019 13:03:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49650 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2634399AbfJRRD4 (ORCPT ); Fri, 18 Oct 2019 13:03:56 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 63ED13083362; Fri, 18 Oct 2019 17:03:56 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-59.bos.redhat.com [10.18.17.59]) by smtp.corp.redhat.com (Postfix) with ESMTP id 590365C21A; Fri, 18 Oct 2019 17:03:55 +0000 (UTC) Subject: Re: [PATCH 00/16] The new slab memory controller To: Roman Gushchin , linux-mm@kvack.org Cc: Michal Hocko , Johannes Weiner , linux-kernel@vger.kernel.org, kernel-team@fb.com, Shakeel Butt , Vladimir Davydov , Christoph Lameter References: <20191018002820.307763-1-guro@fb.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Fri, 18 Oct 2019 13:03:54 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20191018002820.307763-1-guro@fb.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Fri, 18 Oct 2019 17:03:56 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/17/19 8:28 PM, Roman Gushchin wrote: > The existing slab memory controller is based on the idea of replicating > slab allocator internals for each memory cgroup. This approach promises > a low memory overhead (one pointer per page), and isn't adding too much > code on hot allocation and release paths. But is has a very serious flaw:                                                ^it^ > it leads to a low slab utilization. > > Using a drgn* script I've got an estimation of slab utilization on > a number of machines running different production workloads. In most > cases it was between 45% and 65%, and the best number I've seen was > around 85%. Turning kmem accounting off brings it to high 90s. Also > it brings back 30-50% of slab memory. It means that the real price > of the existing slab memory controller is way bigger than a pointer > per page. > > The real reason why the existing design leads to a low slab utilization > is simple: slab pages are used exclusively by one memory cgroup. > If there are only few allocations of certain size made by a cgroup, > or if some active objects (e.g. dentries) are left after the cgroup is > deleted, or the cgroup contains a single-threaded application which is > barely allocating any kernel objects, but does it every time on a new CPU: > in all these cases the resulting slab utilization is very low. > If kmem accounting is off, the kernel is able to use free space > on slab pages for other allocations. In the case of slub memory allocator, it is not just unused space within a slab. It is also the use of per-cpu slabs that can hold up a lot of memory, especially if the tasks jump around to different cpus. The problem is compounded if a lot of memcgs are being used. Memory utilization can improve quite significantly if per-cpu slabs are disabled. Of course, it comes with a performance cost. Cheers, Longman