From owner-freebsd-hackers Thu Jun 4 12:02:09 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id MAA10203 for freebsd-hackers-outgoing; Thu, 4 Jun 1998 12:02:09 -0700 (PDT) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from dyson.iquest.net (dyson.iquest.net [198.70.144.127]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id MAA10098; Thu, 4 Jun 1998 12:01:42 -0700 (PDT) (envelope-from toor@dyson.iquest.net) Received: (from root@localhost) by dyson.iquest.net (8.8.8/8.8.8) id NAA02065; Thu, 4 Jun 1998 13:59:21 -0500 (EST) (envelope-from toor) Message-Id: <199806041859.NAA02065@dyson.iquest.net> Subject: Re: Recompiling sources with "-O2 -m486 -pipe" In-Reply-To: <19980604134839.09175@follo.net> from Eivind Eklund at "Jun 4, 98 01:48:40 pm" To: eivind@yes.no (Eivind Eklund) Date: Thu, 4 Jun 1998 13:59:21 -0500 (EST) Cc: mike@smith.net.au, dmm125@bellatlantic.net, dyson@FreeBSD.ORG, hackers@FreeBSD.ORG From: "John S. Dyson" Reply-To: dyson@FreeBSD.ORG X-Mailer: ELM [version 2.4ME+ PL38 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG Eivind Eklund said: > > The _only_ thing I've found that affect PPro timing much is > alignement, and alignment of inner loops can make a 50% speed > difference. It caused me a lot of hassle until I found out about it > ("Why the **** does changing that instruction, which is 20 > instructions before the main loop, speed up things by 30%?") > PPro is also sensitive to dirty cache lines. It is best to check a data item before storing to it (for large transfers, like bzero.) Careful scheduling can make some difference also, but the effort is most often not worth the returns. My local versions of swtch.s are carefully scheduled, and they do make a difference, but not much. The PPro can reorder instructions very aggressively, and the order of long:1:1 isn't necessary, since the processor can reorder those very effectively. The P5 is less forgiving. I have spent many hours trying to make a P6 go faster, and it is really hard, and not generally worthwhile. -- John | Never try to teach a pig to sing, dyson@freebsd.org | it just makes you look stupid, jdyson@nc.com | and it irritates the pig. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message