From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Paul Eggert Newsgroups: gmane.emacs.devel Subject: Re: Lisp_Marker size on 32bit systems Date: Fri, 7 Sep 2018 20:04:52 -0700 Organization: UCLA Computer Science Department Message-ID: <499c51c7-37c0-0b4f-442c-8f83f1574523@cs.ucla.edu> References: <5a2c709e-aa49-b5b6-3fbe-fb8bd33acb23@cs.ucla.edu> <06d01a4b-9d98-df5b-be8a-aeda449acad7@cs.ucla.edu> <0e358c21-1e67-32f9-d24b-fa039753a2de@cs.ucla.edu> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Trace: blaine.gmane.org 1536375786 11830 195.159.176.226 (8 Sep 2018 03:03:06 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 8 Sep 2018 03:03:06 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 Cc: emacs-devel@gnu.org To: Stefan Monnier Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Sat Sep 08 05:03:02 2018 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fyTWV-0002vL-Ac for ged-emacs-devel@m.gmane.org; Sat, 08 Sep 2018 05:02:59 +0200 Original-Received: from localhost ([::1]:41127 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fyTYb-0004h0-Hw for ged-emacs-devel@m.gmane.org; Fri, 07 Sep 2018 23:05:09 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:60330) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fyTYV-0004gt-4z for emacs-devel@gnu.org; Fri, 07 Sep 2018 23:05:03 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fyTYP-0002ll-0F for emacs-devel@gnu.org; Fri, 07 Sep 2018 23:05:00 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:42566) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fyTYO-0002k3-Q4 for emacs-devel@gnu.org; Fri, 07 Sep 2018 23:04:56 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id A763B160F92; Fri, 7 Sep 2018 20:04:53 -0700 (PDT) Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id 39Zv4ZtCfhAm; Fri, 7 Sep 2018 20:04:53 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id ED006160FA8; Fri, 7 Sep 2018 20:04:52 -0700 (PDT) X-Virus-Scanned: amavisd-new at zimbra.cs.ucla.edu Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id bFtSQJpQciDK; Fri, 7 Sep 2018 20:04:52 -0700 (PDT) Original-Received: from [192.168.1.9] (cpe-23-242-74-103.socal.res.rr.com [23.242.74.103]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id C4F34160F92; Fri, 7 Sep 2018 20:04:52 -0700 (PDT) In-Reply-To: Content-Language: en-US X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x X-Received-From: 131.179.128.68 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:229468 Archived-At: Stefan Monnier wrote: > I think 32 byte objects aren't supposed to be much more likely to be > aligned on 32 byte boundaries than on 32B + 16B. True, I miscalculated. If the cache line size is 64 bytes and objects are allocated on 16-byte boundaries (the case now), the probability that a randomly-placed 24-byte marker (allocated as 32 bytes) will straddle into two cache lines is (2-1)/4, or 25%. Whereas if objects are allocated in 8-byte boundaries as you're suggesting, the probability that the same marker will straddle is (3-1)/8, which is still 25%. So for this particular case the straddling issue should be a wash. > Those perf-stats also show improved I$ performance, which isn't > explained by your suggested explanation. Similarly, they show a reduced > number of instructions. Yes, it could well be that the 32-byte allocation is faster than the 24 partly due to some reason other than d-cache effects. Although there is a smaller percentage of cache misses in the 32-byte version, it could be that this is because the 32-byte version uses simpler code that would be faster even if the cache miss rate were the same.