enabling IDE DMA

[Jerry Feldman]

Thanks Glen. 
I also learn new stuff at the Installfests. Most of the volunteers each 
have their own cache of knowledge which they very generously share.
Glenn Burkhardt wrote:

> 	As I was leaving I just happened to overhear you telling
> 	someone that you always recommend as part of a Linux
> 	installation that the installer enable DMA for the IDE drives
> 	by regenerating the kernel.  I was curious to see if there
> 	might not be an easier way, since most users probably don't
> 	want to muck around with the kernel.
> 
> 	I was also curious to see if my systems were using DMA with
> 	the IDE disks, since I've almost always found computers to be
> 	IO bound for my applications.
> 
> 	It looks like kernels can be configured to enable DMA for
> 	hardware that supports it by setting the CONFIG_IDEDMA_AUTO
> 	macro during configuration.  Apparently, most distributions
> 	don't turn this feature on for their stock kernels.  At least,
> 	the three I checked (Mandrake 7.2, RedHat 7.0, SuSE 7.1)
> 	didn't have it turned on.
> 	
> 	I've found two different techniques that can be used to enable
> 	DMA for the disks, with the 2.2 and 2.4 kernels, when the
> 	kernel hasn't been configured for auto enable.
> 
> 	One is to include the boot prompt argument "idex=dma", where
> 
> 	 * "idex=" is recognized for all "x" from "0" to "3", such as "ide1".
> 
> 	This is functionally equivalent to defining CONFIG_IDEDMA_AUTO
> 	during kernel configuration.  The 'hwif->autodma' flag in the
> 	IDE driver gets set, and queries of the drive and controller
> 	chip set determine whether DMA is used with the drive.
> 
> 	A better way is to use hdparm.  It can enable DMA while the
> 	system is running, and so can be added to a start up script,
> 	e.g., rc.local.  Then command would be something like
> 
> 	      hdparm -d1 /dev/hda
> 
> 	Whenever DMA is enabled, it is turned off by the system if
> 	excessive errors occur while accessing the drive.
> 
> 	You can determine if DMA is currently enabled for a particular
> 	drive by executing a command like:
> 
> 		cat /proc/ide/hda/settings
> 
> 	and look for the 'using_dma' line.
> 
> 	The computer I have at home doesn't work in DMA mode at all.
> 	The IDE driver kept turning DMA off.  This computer is not a
> 	newly manufactured system, but has relatively standard
> 	components (Intel Celeron, 440LX chipset, and an Intel IDE
> 	controller).  A query of /proc/pci revealed that the IDE
> 	controller billed itself as "Master Capable", and 'hdparm -i'
> 	indicates that the disk drive prefers UDMA mode 4.  I noticed
> 	that I had a standard 40 conductor IDE cable installed, and
> 	ran out to buy an 80 conductor UDMA cable.  It didn't help.
> 	when I enabled DMA, the disk chattered for a few seconds, and
> 	went silent.  The file /var/log/messages had lines in it like:
> 
> 
> 	kernel: hda: dma_intr: status=0x51 { DriveReady SeekComplete Error } 
> 	kernel: hda: dma_intr: error=0x84 { DriveStatusError BadCRC } 
> 	kernel: hda: DMA disabled 
> 	kernel: ide0: reset: success 
> 
> 
> 	It looked like there were changes to the IDE driver since the
> 	Linux version I was running was released.  There were new
> 	sections in the latest drivers that dealt with CPUs lacking
> 	cache snooping, so I upgraded my computer to use the 2.2.15
> 	kernel.  It still didn't work.  Perhaps there are enough
> 	machines out there that don't work with Linux's IDE driver in
> 	DMA mode that distribution companies leave DMA disabled by
> 	default to avoid dealing with calls from users about funny
> 	messages in system logs.
> 
> 	But newer systems at the office responded quite well.  The
> 	'hdparm' program includes a performance test for the disk:
> 
> 		 hdparm -t -T
> 
> 		 Here are the results:
> 
> 
> 	==============================================================
> 
> 	[root at nimue aoi]# /sbin/hdparm -t -T /dev/hda
> 
> 	/dev/hda:
> 	 Timing buffer-cache reads:   128 MB in  0.69 seconds =185.51 MB/sec
> 	 Timing buffered disk reads:  64 MB in 18.94 seconds =  3.38 MB/sec
> 
> 	[root at nimue aoi]# /sbin/hdparm -d1 /dev/hda
> 
> 	/dev/hda:
> 	 setting using_dma to 1 (on)
> 	 using_dma    =  1 (on)
> 
> 	[root at nimue aoi]# /sbin/hdparm -t -T /dev/hda
> 
> 	/dev/hda:
> 	 Timing buffer-cache reads:   128 MB in  0.95 seconds =134.74 MB/sec
> 	 Timing buffered disk reads:  64 MB in  3.15 seconds = 20.32 MB/sec
> 
> 	==============================================================
> 
> 	[root at raijin glenn]# hdparm -t -T /dev/hda
> 
> 	/dev/hda:
> 	 Timing buffer-cache reads:   128 MB in  4.86 seconds = 26.34 MB/sec
> 	 Timing buffered disk reads:  64 MB in 23.27 seconds =  2.75 MB/sec
> 
> 	[root at raijin glenn]# hdparm -d1 /dev/hda
> 
> 	/dev/hda:
> 	 setting using_dma to 1 (on)
> 	 using_dma    =  1 (on)
> 
> 	[root at raijin glenn]# hdparm -t -T /dev/hda
> 
> 	/dev/hda:
> 	 Timing buffer-cache reads:   128 MB in  3.71 seconds = 34.50 MB/sec
> 	 Timing buffered disk reads:  64 MB in  6.45 seconds =  9.92 MB/sec
> 
> 	==============================================================
> 
> 	But now this enlightenment is going to cost me.  I'll need to
> 	put $300 into my home system, to bring it up to snuff.	
> 
> 
> 
> 

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix user group
http://www.blu.org


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