Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755282AbcKKKan (ORCPT ); Fri, 11 Nov 2016 05:30:43 -0500 Received: from mail-pf0-f170.google.com ([209.85.192.170]:36160 "EHLO mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750714AbcKKKam (ORCPT ); Fri, 11 Nov 2016 05:30:42 -0500 Date: Fri, 11 Nov 2016 02:30:39 -0800 (PST) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Joonsoo Kim cc: Andrew Morton , Greg Thelen , Aruna Ramakrishna , Christoph Lameter , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [patch] mm, slab: faster active and free stats In-Reply-To: <20161111055326.GA16336@js1304-P5Q-DELUXE> Message-ID: References: <20161108151727.b64035da825c69bced88b46d@linux-foundation.org> <20161111055326.GA16336@js1304-P5Q-DELUXE> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1543 Lines: 39 On Fri, 11 Nov 2016, Joonsoo Kim wrote: > Hello, David. > > Maintaining acitve/free_slab counters looks so complex. And, I think > that we don't need to maintain these counters for faster slabinfo. > Key point is to remove iterating n->slabs_partial list. > > We can calculate active slab/object by following equation as you did in > this patch. > > active_slab(n) = n->num_slab - the number of free_slab > active_object(n) = n->num_slab * cachep->num - n->free_objects > > To get the number of free_slab, we need to iterate n->slabs_free list > but I guess it would be small enough. > > If you don't like to iterate n->slabs_free list in slabinfo, just > maintaining the number of slabs_free would be enough. > Hi Joonsoo, It's a good point, although I don't think the patch has overly complex logic to keep track of slab state. We don't prefer to do any iteration in get_slabinfo() since users can read /proc/slabinfo constantly; it's better to just settle the stats when slab state changes instead of repeating an expensive operation over and over if someone is running slabtop(1) or /proc/slabinfo is scraped regularly for stats. That said, I imagine there are more clever ways to arrive at the same answer, and you bring up a good point about maintaining a n->num_slabs and n->free_slabs rather than n->active_slabs and n->free_slabs. I don't feel strongly about either approach, but I think some improvement, such as what this patch provides, is needed to prevent how expensive simply reading /proc/slabinfo can be.