From owner-freebsd-hackers  Thu Feb  6 12:18:07 1997
Return-Path: <owner-hackers>
Received: (from root@localhost)
          by freefall.freebsd.org (8.8.5/8.8.5) id MAA01936
          for hackers-outgoing; Thu, 6 Feb 1997 12:18:07 -0800 (PST)
Received: from sendero.i-connect.net ([206.190.144.100])
          by freefall.freebsd.org (8.8.5/8.8.5) with ESMTP id MAA01890;
          Thu, 6 Feb 1997 12:17:57 -0800 (PST)
Received: (from shimon@localhost)
          by sendero.i-connect.net (8.8.5/8.8.4)
	  id NAA09330; Thu, 6 Feb 1997 13:17:01 -0800 (PST)
Message-ID: <XFMail.970206131700.Shimon@i-Connect.Net>
X-Mailer: XFMail 1.1-alpha [p0] on FreeBSD
Content-Type: text/plain; charset=iso-8859-8
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
In-Reply-To: <199702060512.AAA10010@dyson.iquest.net>
Date: Thu, 06 Feb 1997 09:55:40 -0800 (PST)
Organization: iConnect Corp.
From: Simon Shapiro <Shimon@i-Connect.Net>
To: "John S. Dyson" <toor@dyson.iquest.net>
Subject: Re: Contigious (spelling?) allocation in kernel
Cc: freebsd-hackers@FreeBSD.ORG, freebsd-scsi@FreeBSD.ORG
Sender: owner-hackers@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk


Hi John S. Dyson;  On 06-Feb-97 you wrote: 

> Take a look at contigmalloc() or vm_page_alloc_contig()
> as defined in /sys/vm/vm_page.c.  These are almost
> guaranteed NOT to work after the system is fully up.

Thanx.

If you (or someone else) please elaborate on the last sentence,
please...

I can re-structure the driver a bit (make it more risky), to 
avoid the need for large contigious blocks, but the (obvious)
question is:  Does the kernel malloc guarantee that allocations
smaller than (or equal to) a page are in the same page?

Having a page or less, limits the Scatter/Gather operations in
the kernel, for most hardware, to 512 entries (segments).  
Under high fragmentation, this can result in 256KB-2MB floating
limit.  While not a problem for most applications (mine
included), it is still a limit that is not absolutely necessary.

Any opposing opinions (before I re-code the darn thing :-)?

BACKGROUND:  One of my engineers, who is heavily involved in 
Linux SCSI development is strongly opposed to calling malloc on
demand in a device driver.  He quotes heavy performance
penalties, and worse;  Failure (under heavy load) to obtain the
memory when needed.

Thanx, Simon