From owner-freebsd-hardware  Fri May 23 11:38:30 1997
Return-Path: <owner-hardware>
Received: (from root@localhost)
          by hub.freebsd.org (8.8.5/8.8.5) id LAA07184
          for hardware-outgoing; Fri, 23 May 1997 11:38:30 -0700 (PDT)
Received: from lariat.lariat.org (ppp0.lariat.org@[129.72.251.2])
          by hub.freebsd.org (8.8.5/8.8.5) with ESMTP id LAA07164;
          Fri, 23 May 1997 11:38:19 -0700 (PDT)
Received: from solo.lariat.org (ad25-189.compuserve.com [199.174.128.189])
	by lariat.lariat.org (8.8.5/8.8.5) with SMTP id MAA17111;
	Fri, 23 May 1997 12:37:06 -0600 (MDT)
Message-Id: <3.0.1.32.19970522181207.0072d920@lariat.org>
X-Sender: brett@lariat.org
X-Mailer: Windows Eudora Pro Version 3.0.1 (32)
Date: Thu, 22 May 1997 18:12:07 -0600
To: Jim Shankland <jas@flyingfox.com>, gpalmer@FreeBSD.ORG
From: Brett Glass <brett@lariat.org>
Subject: Re: isa bus and boca multiport boards
Cc: HARDWARE@FreeBSD.ORG
In-Reply-To: <199705221647.JAA03333@biggusdiskus.flyingfox.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Sender: owner-hardware@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

At 09:47 AM 5/22/97 -0700, Jim Shankland wrote:
 
>This "tuned asm" thing is perilously close to an urban myth, anyway.
>To the extent there's any "lift" to be gotten from asm at all, it's
>from tiny little pieces in performance-critical inner loops, 

Exactly. The guts of the sio driver ARE some performance-critical inner loops.

>especially
>if there's a hardware-specific instruction that the compiler won't
>generate.

I'd have to look at the generated code to see how well I/O is done. But
I'd HOPE that the macros do the right thing.

>It's one thing to say, "I've just spent 3 weeks of backbreaking labor
>tuning the sio driver, and made it X% faster; then I experimented and
>found that, by replacing the following N lines of C code with assembler,
>I made it Y% faster still"; and another to wave one's hands about
>a fast sio driver written in assembler.  Given the costs in portability,
>maintainability, and effort required for assembler, arguments for its
>use must be supported by strong, empirical evidence of benefit.

My experience on other OS platforms dictates that there IS a large measure
of efficiency to be gained -- on the order of 25% time savings. The
improvements are partly from ASM and partly from good algorithms.

--Brett