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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 EFD93C433DF for ; Thu, 21 May 2020 02:51:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CFF1D206F6 for ; Thu, 21 May 2020 02:51:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590029466; bh=pgJQa1w4GgFyUgPZ73SymaRcnYjxRBW/AIT2HJe0HLc=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=vz6XIXp8SjPt1GjOzQ8dRezCsXhgdlbkDKKqdPQU7+2yzmNU1Wb3kN0M0zZl6WRaY qCOxMW0/5cKRvWbwmRJmds9albu5DrdXnkteAwWNBNDxrzWz5TpHG36EOL9JjAEoZn RBnyIufGo6jjIifZHw8eLH9XuMB+8cQr5QgSig/4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727998AbgEUCvG (ORCPT ); Wed, 20 May 2020 22:51:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:57198 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726979AbgEUCvE (ORCPT ); Wed, 20 May 2020 22:51:04 -0400 Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 0FD6420756; Thu, 21 May 2020 02:51:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590029463; bh=pgJQa1w4GgFyUgPZ73SymaRcnYjxRBW/AIT2HJe0HLc=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Zek0SPJjepnZk3PsEnSKqdkox4ZCbwicJqdnqrTtKclqcasDKhsovb68KfaFRaBRI 02Svn1GgX1piNuLo5ArMVEPB+A+GMPkyUJDrdGlGDpi5gxTHghZ15jleLPB+J+WJX3 zwCkeYtP8fyNgWa20Yxz4A1G8niA6f1MSFD1m3QI= Date: Wed, 20 May 2020 19:51:02 -0700 From: Andrew Morton To: Huang Ying Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Daniel Jordan , Michal Hocko , Minchan Kim , Tim Chen , Hugh Dickins Subject: Re: [PATCH -V2] swap: Reduce lock contention on swap cache from swap slots allocation Message-Id: <20200520195102.2343f746e88a2bec5c29ef5b@linux-foundation.org> In-Reply-To: <20200520031502.175659-1-ying.huang@intel.com> References: <20200520031502.175659-1-ying.huang@intel.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 20 May 2020 11:15:02 +0800 Huang Ying wrote: > In some swap scalability test, it is found that there are heavy lock > contention on swap cache even if we have split one swap cache radix > tree per swap device to one swap cache radix tree every 64 MB trunk in > commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks"). > > The reason is as follow. After the swap device becomes fragmented so > that there's no free swap cluster, the swap device will be scanned > linearly to find the free swap slots. swap_info_struct->cluster_next > is the next scanning base that is shared by all CPUs. So nearby free > swap slots will be allocated for different CPUs. The probability for > multiple CPUs to operate on the same 64 MB trunk is high. This causes > the lock contention on the swap cache. > > To solve the issue, in this patch, for SSD swap device, a percpu > version next scanning base (cluster_next_cpu) is added. Every CPU > will use its own per-cpu next scanning base. And after finishing > scanning a 64MB trunk, the per-cpu scanning base will be changed to > the beginning of another randomly selected 64MB trunk. In this way, > the probability for multiple CPUs to operate on the same 64 MB trunk > is reduced greatly. Thus the lock contention is reduced too. For > HDD, because sequential access is more important for IO performance, > the original shared next scanning base is used. > > To test the patch, we have run 16-process pmbench memory benchmark on > a 2-socket server machine with 48 cores. One ram disk is configured What does "ram disk" mean here? Which drivers(s) are in use and backed by what sort of memory? > as the swap device per socket. The pmbench working-set size is much > larger than the available memory so that swapping is triggered. The > memory read/write ratio is 80/20 and the accessing pattern is random. > In the original implementation, the lock contention on the swap cache > is heavy. The perf profiling data of the lock contention code path is > as following, > > _raw_spin_lock_irq.add_to_swap_cache.add_to_swap.shrink_page_list: 7.91 > _raw_spin_lock_irqsave.__remove_mapping.shrink_page_list: 7.11 > _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 2.51 > _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap: 1.66 > _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node: 1.29 > _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages: 1.03 > _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node: 0.93 > > After applying this patch, it becomes, > > _raw_spin_lock.swapcache_free_entries.free_swap_slot.__swap_entry_free: 3.58 > _raw_spin_lock_irq.shrink_inactive_list.shrink_lruvec.shrink_node: 2.3 > _raw_spin_lock_irqsave.swap_cgroup_record.mem_cgroup_uncharge_swap: 2.26 > _raw_spin_lock_irq.shrink_active_list.shrink_lruvec.shrink_node: 1.8 > _raw_spin_lock.free_pcppages_bulk.drain_pages_zone.drain_pages: 1.19 > > The lock contention on the swap cache is almost eliminated. > > And the pmbench score increases 18.5%. The swapin throughput > increases 18.7% from 2.96 GB/s to 3.51 GB/s. While the swapout > throughput increases 18.5% from 2.99 GB/s to 3.54 GB/s. If this was backed by plain old RAM, can we assume that the performance improvement on SSD swap is still good? Does the ram disk actually set SWP_SOLIDSTATE?