Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758522AbWLDWpO (ORCPT ); Mon, 4 Dec 2006 17:45:14 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759741AbWLDWpO (ORCPT ); Mon, 4 Dec 2006 17:45:14 -0500 Received: from sp604001mt.neufgp.fr ([84.96.92.60]:54924 "EHLO Smtp.neuf.fr" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1758522AbWLDWpM (ORCPT ); Mon, 4 Dec 2006 17:45:12 -0500 Date: Mon, 04 Dec 2006 23:45:28 +0100 From: Eric Dumazet Subject: Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to_index() In-reply-to: <20061204.135625.48528445.davem@davemloft.net> To: David Miller Cc: akpm@osdl.org, clameter@sgi.com, linux-kernel@vger.kernel.org Message-id: <4574A508.1090805@cosmosbay.com> MIME-version: 1.0 Content-type: text/plain; charset=ISO-8859-1; format=flowed Content-transfer-encoding: 8BIT References: <200612041918.29682.dada1@cosmosbay.com> <20061204114954.165107b6.akpm@osdl.org> <45749465.6030601@cosmosbay.com> <20061204.135625.48528445.davem@davemloft.net> User-Agent: Thunderbird 1.5.0.8 (Windows/20061025) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2222 Lines: 59 David Miller a ?crit : > From: Eric Dumazet > Date: Mon, 04 Dec 2006 22:34:29 +0100 > >> On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle >> for a multiply. > > For UltraSPARC I and II (which is what this 200mhz guy probably is), > it's 4 cycle latency for a multiply (32-bit or 64-bit) and 68 cycles > for a 64-bit divide (32-bit divide is 37 cycles). I must have Ultra-2 (running Solaris :( ) for (ui = 0 ; ui < 100000000 ; ui++) val += reciprocal_divide(ui, reciproc); 100000cb0: 83 31 20 00 srl %g4, 0, %g1 100000cb4: 82 48 40 12 mulx %g1, %l2, %g1 100000cb8: 83 30 70 20 srlx %g1, 0x20, %g1 100000cbc: 88 01 20 01 inc %g4 100000cc0: 80 a1 00 05 cmp %g4, %g5 100000cc4: 08 4f ff fb bleu %icc, 100000cb0 100000cc8: b0 06 00 01 add %i0, %g1, %i0 I confirm that this block uses 20 cycles/iteration, while next one uses 72 cycles/iteration for (ui = 0 ; ui < 100000000 ; ui++) val += ui / value; 100000ca8: 83 31 20 00 srl %g4, 0, %g1 100000cac: 82 68 40 11 udivx %g1, %l1, %g1 100000cb0: 88 01 20 01 inc %g4 100000cb4: 80 a1 00 05 cmp %g4, %g5 100000cb8: 08 4f ff fc bleu %icc, 100000ca8 100000cbc: b0 06 00 01 add %i0, %g1, %i0 > > UltraSPARC-III and IV are worse, 6 cycles for multiply and 40/71 > cycles (32/64-bit) for integer divides. > > Niagara is even worse :-) 11 cycle integer multiply and a 72 cycle > integer divide (regardless of 32-bit or 64-bit). > > (more details in gcc/config/sparc/sparc.c:{ultrasparc,ultrasparc3,niagara}_cost). > > So this change has tons of merit for sparc64 chips at least :-) > > Also, the multiply can parallelize with other operations but it > seems that integer divide stalls the pipe for most of the duration > of the calculation. So this makes the divide even worse. - 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/