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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 B437AC433DB for ; Fri, 26 Feb 2021 14:34:49 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 21F0264F04 for ; Fri, 26 Feb 2021 14:34:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 21F0264F04 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 98FC76B0072; Fri, 26 Feb 2021 09:34:48 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 940616B0073; Fri, 26 Feb 2021 09:34:48 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 855FA6B0074; Fri, 26 Feb 2021 09:34:48 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0217.hostedemail.com [216.40.44.217]) by kanga.kvack.org (Postfix) with ESMTP id 6F5516B0072 for ; Fri, 26 Feb 2021 09:34:48 -0500 (EST) Received: from smtpin13.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 2F3518249980 for ; Fri, 26 Feb 2021 14:34:48 +0000 (UTC) X-FDA: 77860665456.13.5FAEA0A Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) by imf09.hostedemail.com (Postfix) with ESMTP id 774B560024A1 for ; Fri, 26 Feb 2021 14:34:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614350087; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=lLxfh5W64Qm8bILSP59FC31GWqzCy9y4iw1b2YdlEyg=; b=g4xZStJf6l3yFIqyn5xvj38h4a8ODZbYZ6+nTgwtpumiZ91lBe5j+eB8dO9zVU8U5rBhG4 hTdqjU+RvKIsCFVt+hE1j+CBUEsfQcUX+MF48Bx4gxt+GH+wJHlq8675AaQV0HzAiStKJD VBMq13cLQoMequ4g5cryE9z92KBTn8c= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-278-aAu7JTolOyaHRWA2B0UJ-A-1; Fri, 26 Feb 2021 09:34:43 -0500 X-MC-Unique: aAu7JTolOyaHRWA2B0UJ-A-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B7A046D4E3; Fri, 26 Feb 2021 14:34:41 +0000 (UTC) Received: from carbon (unknown [10.36.110.51]) by smtp.corp.redhat.com (Postfix) with ESMTP id E9DCB18AAB; Fri, 26 Feb 2021 14:34:36 +0000 (UTC) Date: Fri, 26 Feb 2021 15:34:35 +0100 From: Jesper Dangaard Brouer To: Mel Gorman Cc: linux-mm@kvack.org, chuck.lever@oracle.com, netdev@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, brouer@redhat.com Subject: Re: [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster Message-ID: <20210226153435.6708d171@carbon> In-Reply-To: <20210225153815.GN3697@techsingularity.net> References: <161419296941.2718959.12575257358107256094.stgit@firesoul> <161419301128.2718959.4838557038019199822.stgit@firesoul> <20210225112849.GM3697@techsingularity.net> <20210225161633.53e5f910@carbon> <20210225153815.GN3697@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 774B560024A1 X-Stat-Signature: hq9f6cq18j5ow9ob6rsxxdqqion41nza Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf09; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=63.128.21.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614350083-259562 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 25 Feb 2021 15:38:15 +0000 Mel Gorman wrote: > On Thu, Feb 25, 2021 at 04:16:33PM +0100, Jesper Dangaard Brouer wrote: > > > 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. > > > > Yes, the increase in size is big. For the struct free_area 40 bytes for > > my case and 56 bytes for your case. The real problem is that this is > > multiplied by 11 (MAX_ORDER) and multiplied by number of zone structs > > (is it 5?). Thus, 56*11*5 = 3080 bytes. > > > > Thus, I'm not sure it is worth it! As I'm only saving 2-cycles, for > > something that depends on the compiler generating specific code. And > > the compiler can easily change, and "fix" this on-its-own in a later > > release, and then we are just wasting memory. > > > > I did notice this imul happens 45 times in mm/page_alloc.o, with this > > offset 0x58, but still this is likely not on hot-path. > > > > Yeah, I'm not convinced it's worth it. The benefit of 2 cycles is small and > it's config-dependant. While some configurations will benefit, others do > not but the increased consumption is universal. I think there are better > ways to save 2 cycles in the page allocator and this seems like a costly > micro-optimisation. > > > > > > > > > > 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. > > > > I think it is the size variation. > > > > Yes. > > > > 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? > > > > The purpose of purpose of __pad_to_align_free_list is because struct > > list_head is 16 bytes, thus I wanted to align free_list to 16, given we > > already have wasted the space. > > > > Ok, that's fair enough but it's also somewhat of a micro-optimisation as > whether it helps or not depends on the architecture. > > I don't think I'll pick this up, certainly in the context of the bulk > allocator but it's worth keeping in mind. It's an interesting corner case > at least. I fully agree. Lets drop this patch. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer