From owner-freebsd-hackers  Fri Apr 18 13:00:19 1997
Return-Path: <owner-hackers>
Received: (from root@localhost)
          by freefall.freebsd.org (8.8.5/8.8.5) id NAA29469
          for hackers-outgoing; Fri, 18 Apr 1997 13:00:19 -0700 (PDT)
Received: from phaeton.artisoft.com (phaeton.Artisoft.COM [198.17.250.50])
          by freefall.freebsd.org (8.8.5/8.8.5) with SMTP id NAA29456
          for <freebsd-hackers@freebsd.org>; Fri, 18 Apr 1997 13:00:14 -0700 (PDT)
Received: (from terry@localhost) by phaeton.artisoft.com (8.6.11/8.6.9) id MAA02638; Fri, 18 Apr 1997 12:57:51 -0700
From: Terry Lambert <terry@lambert.org>
Message-Id: <199704181957.MAA02638@phaeton.artisoft.com>
Subject: Re: video capture driver interface to file system?
To: luigi@labinfo.iet.unipi.it (Luigi Rizzo)
Date: Fri, 18 Apr 1997 12:57:51 -0700 (MST)
Cc: dufault@hda.com, hasty@rah.star-gate.com, terry@lambert.org,
        freebsd-hackers@freebsd.org
In-Reply-To: <199704181021.MAA08439@labinfo.iet.unipi.it> from "Luigi Rizzo" at Apr 18, 97 12:21:16 pm
X-Mailer: ELM [version 2.4 PL24]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: owner-hackers@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk

> > Pre-allocate the file; pre-allocate the blocks of a file; pre-set
> > up some control info for a transfer, mmap a set of blocks for the
> > video card driver to dump data into; have the driver chain to the
> > next block and interrupt you on each transfer, dump the video frames
> > to the pre-allocated store.
> 
> and probably, in addition to preallocation, make sure that your write()
> calls are aligned to the FS/device blocksize otherwise (probably) each
> write will be preceeded by a hidden read to take care of fragmentation ?

Actually, the block mapping overhead is higher if you aren't on a page
multiple boundry and not a page size; that appears to be the fastest
path in the VM code.  Otherwise you end up manipulating multiple mappings
for each operation.

Follow /sys/kern/vfs_bio.c, specifically down through vfs_busy_pages()
into the bowels of the VM system for details.

I think in some very specific-use circumstances, the best bet would be
a 4k FS block size.


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.