Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932406AbXLQTEW (ORCPT ); Mon, 17 Dec 2007 14:04:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757395AbXLQTEM (ORCPT ); Mon, 17 Dec 2007 14:04:12 -0500 Received: from mx1.redhat.com ([66.187.233.31]:56441 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757287AbXLQTEK (ORCPT ); Mon, 17 Dec 2007 14:04:10 -0500 Date: Mon, 17 Dec 2007 17:03:39 -0200 From: Arnaldo Carvalho de Melo To: Arjan van de Ven Cc: Arnaldo Carvalho de Melo , Andrew Morton , linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] Reduce overhead of CONFIG_TIMER_STATS Message-ID: <20071217190339.GE25780@ghostprotocols.net> Mail-Followup-To: Arnaldo Carvalho de Melo , Arjan van de Ven , Andrew Morton , linux-kernel@vger.kernel.org, Ingo Molnar References: <20071216212944.GA5128@ghostprotocols.net> <20071217090337.0b1af816@laptopd505.fenrus.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20071217090337.0b1af816@laptopd505.fenrus.org> X-Url: http://oops.ghostprotocols.net:81/blog User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2732 Lines: 61 Em Mon, Dec 17, 2007 at 09:03:37AM -0800, Arjan van de Ven escreveu: > On Sun, 16 Dec 2007 19:29:44 -0200 > Arnaldo Carvalho de Melo wrote: > > > Hi, > > While looking at the pahole output for struct timer_list on > > recent kernels I noticed that there is a 4 bytes padding on struct > > timer_list that gets propagated to many structs on 64 bits > > architectures: > > > > [acme@doppio linux-2.6]$ pahole -C timer_list /tmp/tcp.o.before > > struct timer_list { > > struct list_head entry; /* 0 16 */ > > long unsigned int expires; /* 16 8 */ > > void (*function)(long unsigned int); /* > > 24 8 */ long unsigned int data; /* 32 8 */ > > struct tvec_t_base_s *base; /* 40 8 */ > > void * start_site; /* 48 8 */ > > char start_comm[16]; /* 56 16 */ > > /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */ > > int start_pid; /* 72 4 */ > > > > /* size: 80, cachelines: 2 */ > > /* padding: 4 */ > > /* last cacheline: 16 bytes */ > > }; > > [acme@doppio linux-2.6]$ > > > > So the attached patch reduces the 4 bytes hole overhead of > > CONFIG_TIMER_STATS on 64bit architectures by shrinking the field for > > the process name by 4 bytes. > > > > Statistically this doesn't affects that many process names as > > most are less than 12 bytes. As CONFIG_TIMER_STATS is enabled at least > > on fedora kernels I think that we can, with this patch, still reap the > > benefits of powertopping. > > I'm still worried that this means that PowerTOP will end up displaying shortened names.. > it's already sometimes tricky to know who's guilty at 16... at 12 I fear it just gets worse. > Isn't there some other reorder that you can do to still get rid of the hole? Nope. As shown above there is no other hole that we can conbine with the 4 bytes padding on struct timer_list. What we could do, perhaps, would be to struct start_comm to be a pointer and store the strings in a list, refcounted, so instead of 16 or 12 bytes we would use 8, but would incur in some CPU overhead definetely bigger than a simple memcpy, but that would reduce even further the memory footprint. But the hole would still be there, as it would be sizeof(void *) + sizeof(start_pid) :-) - Arnaldo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/