From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753691Ab3KDO4m (ORCPT ); Mon, 4 Nov 2013 09:56:42 -0500 Received: from mail-qc0-f173.google.com ([209.85.216.173]:50100 "EHLO mail-qc0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753371Ab3KDO4l (ORCPT ); Mon, 4 Nov 2013 09:56:41 -0500 MIME-Version: 1.0 In-Reply-To: <20131104073640.GF13030@gmail.com> References: <1383337039.2653.18.camel@buesod1.americas.hpqcorp.net> <1383537862.2373.14.camel@buesod1.americas.hpqcorp.net> <20131104073640.GF13030@gmail.com> Date: Mon, 4 Nov 2013 06:56:38 -0800 Message-ID: Subject: Re: [PATCH] mm: cache largest vma From: Michel Lespinasse To: Ingo Molnar Cc: Davidlohr Bueso , Linus Torvalds , Andrew Morton , Hugh Dickins , Mel Gorman , Rik van Riel , Guan Xuetao , "Chandramouleeswaran, Aswin" , Linux Kernel Mailing List , linux-mm Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 3, 2013 at 11:36 PM, Ingo Molnar wrote: > So I think it all really depends on the hit/miss cost difference. It makes > little sense to add a more complex scheme if it washes out most of the > benefits! > > Also note the historic context: the _original_ mmap_cache, that I > implemented 16 years ago, was a front-line cache to a linear list walk > over all vmas (!). > > Today we have the vma rbtree, which is self-balancing and a lot faster > than your typical linear list walk search ;-) > > So I'd _really_ suggest to first examine the assumptions behind the cache, > it being named 'cache' and it having a hit rate does in itself not > guarantee that it gives us any worthwile cost savings when put in front of > an rbtree ... Agree. We have made the general case a lot faster, and caches in front of it may not pull their weight anymore - the fact that we are wondering how to even measure that, to me, means that we probably shouldn't even bother. That's what I did when I implemented the augmented rbtree to search for allocatable spaces between vmas: I removed the cache for the last used gap, and nobody has complained about it since. Absent some contrary data, I would actually prefer we remove the mmap_cache as well. And if a multiple-entry cache is necessary, I would also prefer it to be LRU type rather than something ad-hoc (if there is a benefit to caching the largest VMA, then LRU would capture that as well...) -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.