Path: utzoo!mnetor!uunet!husc6!think!ames!aurora!labrea!decwrl!ucbvax!LBL.GOV!nagy%warner.hepnet
From: nagy%warner.hepnet@LBL.GOV (Frank J. Nagy, VAX Wizard & Guru)
Newsgroups: comp.os.vms
Subject: RE: Buffered vs. direct I/O's
Message-ID: <880119060242.20e00e4f@LBL.Gov>
Date: 19 Jan 88 14:02:42 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 52

First, the difference between buffered and direct I/Os is determined
by the device driver.  Devices which can do direct I/O (disks, array
processors, image processors, tapes, etc.) do their DMA directly
to or from buffers in the user's virtual memory space.  Buffered
devices (terminals, some network operations, etc.) use buffers
allocated in system space in nonpaged pool.  In earlier versions
of VMS the difference reflected that between DMA devices and programmed
I/O devices (usually the DZ11 terminal controllers).  So to summarize
the differences are concerned with data transfers to/from the buffers
in the user process's virtual address space:

	direct I/O devices transfer data directly to/from the user
	buffers by locking those buffers into memory, double-mapping
	them into system address space (to allow non-process context
	reference by the device driver) and initiating DMA operations
	between these buffers and the device.

	buffered I/O devices transfer data between the user buffers and
	system buffers in nonpaged pool.  The device driver then uses
	either DMA (new terminal controllers, network interfaces) or
	programmed I/O to move data between the system buffers and the
	device.
     
> In terms of CPU and other resource usage, I would like to know which of these
> is more expensive and precisely why. 

Buffered I/O can be more expensive in CPU time (extra memory move) but
the amount of data transferred in each operation is usually fairly small
for buffered I/O.  Depending upon the device (such as DZ11s), programmed
I/O between the system buffers and the device can use *lots* of CPU time
(i.e. DZ11 causes lots of interrupt processing).  Buffered I/O uses
system memory resources (nonpaged pool).

Direct I/O requires CPU overhead to setup (map the user buffers into
system space, lock buffers into memory, setup bus adapter maps for the
Unibus or QBus) but then the DMA operation does not require the CPU.
Typically direct I/O is used for DMA devices and large operations
(disks and tapes).  

Note that disk I/O still uses direct I/O even with RMS.  The RMS buffers
the direct I/O is to/from are in the process's virtual address space.
So, RMS buffering does not count as buffered I/O per se.  RMS buffering
allows lots of record operations to be done to/from the RMS buffers and
then direct I/O to/from the disks in large chunks as set by the RMS
buffer size and multibuffer count parameters.


= Frank J. Nagy   "VAX Guru & Wizard"
= Fermilab Research Division EED/Controls
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