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=-7.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 5ACE7C433ED for ; Thu, 8 Apr 2021 07:53:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 212DD61164 for ; Thu, 8 Apr 2021 07:53:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230329AbhDHHx4 (ORCPT ); Thu, 8 Apr 2021 03:53:56 -0400 Received: from server.lespinasse.org ([63.205.204.226]:42323 "EHLO server.lespinasse.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229505AbhDHHxz (ORCPT ); Thu, 8 Apr 2021 03:53:55 -0400 DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-11-ed; t=1617868424; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=Wculb6jHNxHFT1NPFrH00eMZhPzMfIJ/lXQBt4FvWDY=; b=cY8nm20t8S6Lk/q03f68dYSwJLTlY/tLBudnSuwx5ICtI/So/PXq5zz9VfPvs67OXtfI9 CCWeyrYACiZ2W0xAg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-11-rsa; t=1617868424; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=Wculb6jHNxHFT1NPFrH00eMZhPzMfIJ/lXQBt4FvWDY=; b=agZWXwP+zplT0eY6G7B62LayXa+AEyPxWaG++zb8gm5AeDgBPvl923CbpQPVEhR/p/p1P SEGZ/DGm8o+++EWUvEbDpAH+jWTrCv8Lko7x9NyJYtMAGM6Jt/YI2LzUpG9T5MMTwMRohqa HqA3NHDv0imxpmmewspsowtlXzb4LgXBahwb0jBgnciBTtcqyNPWt0Ely7B2VinQYKSNHZV fuxDf7hCG7+tllz/xN27Zcqxs+JLHPKTkAr1gmms7SqkQxXLIW5OxPjgzPtioFRXjdexO5c nbk+zPZx6q7oxZibyAHLb2NpW0eyNWKCT3oAjTw9AWD9naR5dsF8MVcLp1eQ== Received: by server.lespinasse.org (Postfix, from userid 1000) id 48066160253; Thu, 8 Apr 2021 00:53:44 -0700 (PDT) Date: Thu, 8 Apr 2021 00:53:44 -0700 From: Michel Lespinasse To: Matthew Wilcox Cc: Michel Lespinasse , Linux-MM , Laurent Dufour , Peter Zijlstra , Michal Hocko , Rik van Riel , Paul McKenney , Andrew Morton , Suren Baghdasaryan , Joel Fernandes , Rom Lemarchand , Linux-Kernel Subject: Re: [RFC PATCH 34/37] mm: rcu safe vma freeing only for multithreaded user space Message-ID: <20210408075344.GA27824@lespinasse.org> References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-35-michel@lespinasse.org> <20210407025006.GB2531743@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210407025006.GB2531743@casper.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 07, 2021 at 03:50:06AM +0100, Matthew Wilcox wrote: > On Tue, Apr 06, 2021 at 06:44:59PM -0700, Michel Lespinasse wrote: > > Performance tuning: as single threaded userspace does not use > > speculative page faults, it does not require rcu safe vma freeing. > > Turn this off to avoid the related (small) extra overheads. > > > > For multi threaded userspace, we often see a performance benefit from > > the rcu safe vma freeing - even in tests that do not have any frequent > > concurrent page faults ! This is because rcu safe vma freeing prevents > > recently released vmas from being immediately reused in a new thread. > > Why does that provide a performance benefit? Recently released > VMAs are cache-hot, and NUMA-local. I'd expect the RCU delay to be > performance-negative. I only have the observation and no full explanation for it. Just try it on wis-mmap and wis-malloc threaded cases. Of course this all washes away when dealing with more realistic macro benchmarks.