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=-5.3 required=3.0 tests=BAYES_00, 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 B48FCC433DB for ; Thu, 25 Feb 2021 11:31:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6AFEC64EBD for ; Thu, 25 Feb 2021 11:31:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235383AbhBYLaw (ORCPT ); Thu, 25 Feb 2021 06:30:52 -0500 Received: from outbound-smtp29.blacknight.com ([81.17.249.32]:56052 "EHLO outbound-smtp29.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235350AbhBYL3t (ORCPT ); Thu, 25 Feb 2021 06:29:49 -0500 Received: from mail.blacknight.com (pemlinmail06.blacknight.ie [81.17.255.152]) by outbound-smtp29.blacknight.com (Postfix) with ESMTPS id 4D4A0BEC80 for ; Thu, 25 Feb 2021 11:28:51 +0000 (GMT) Received: (qmail 7064 invoked from network); 25 Feb 2021 11:28:51 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 25 Feb 2021 11:28:50 -0000 Date: Thu, 25 Feb 2021 11:28:49 +0000 From: Mel Gorman To: Jesper Dangaard Brouer Cc: linux-mm@kvack.org, chuck.lever@oracle.com, netdev@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster Message-ID: <20210225112849.GM3697@techsingularity.net> References: <161419296941.2718959.12575257358107256094.stgit@firesoul> <161419301128.2718959.4838557038019199822.stgit@firesoul> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <161419301128.2718959.4838557038019199822.stgit@firesoul> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org As a side-node, I didn't pick up the other patches as there is review feedback and I didn't have strong opinions either way. Patch 3 is curious though, it probably should be split out and sent separetly but still; On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote: > Avoid multiplication (imul) operations when accessing: > zone->free_area[order].nr_free > > This was really tricky to find. I was puzzled why perf reported that > rmqueue_bulk was using 44% of the time in an imul operation: > > ??? del_page_from_free_list(): > 44,54 ??? e2: imul $0x58,%rax,%rax > > This operation was generated (by compiler) because the struct free_area have > size 88 bytes or 0x58 hex. The compiler cannot find a shift operation to use > and instead choose to use a more expensive imul, to find the offset into the > array free_area[]. > > The patch align struct free_area to a cache-line, which cause the > compiler avoid the imul operation. The imul operation is very fast on > modern Intel CPUs. To help fast-path that decrement 'nr_free' move the > member 'nr_free' to be first element, which saves one 'add' operation. > > Looking up instruction latency this exchange a 3-cycle imul with a > 1-cycle shl, saving 2-cycles. It does trade some space to do this. > > Used: gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2) > I'm having some trouble parsing this and matching it to the patch itself. First off, on my system (x86-64), the size of struct free area is 72, not 88 bytes. For either size, cache-aligning the structure is a big increase in the struct size. struct free_area { struct list_head free_list[4]; /* 0 64 */ /* --- cacheline 1 boundary (64 bytes) --- */ long unsigned int nr_free; /* 64 8 */ /* size: 72, cachelines: 2, members: 2 */ /* last cacheline: 8 bytes */ }; Are there other patches in the tree? What does pahole say? With gcc-9, I'm also not seeing the imul instruction outputted like you described in rmqueue_pcplist which inlines rmqueue_bulk. At the point where it calls get_page_from_free_area, it's using shl for the page list operation. This might be a compiler glitch but given that free_area is a different size, I'm less certain and wonder if something else is going on. Finally, moving nr_free to the end and cache aligning it will make the started of each free_list cache-aligned because of its location in the struct zone so what purpose does __pad_to_align_free_list serve? -- Mel Gorman SUSE Labs