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Apple OSes and IDE DMA Support?

Posted by Cliff on from the they-couldn't-have-overlooked-this-could-they dept.

KFox wishes to get to the core of this particular issue: "I just recently purchased an iBook and I have noticed that even in Jaguar, the system gets choppy from disk I/O. It appears that Mac OS 9 and Mac OS X don't support DMA access for hard drives, even if the drives are initialized in a UDMA mode. Wintel has had support in this area for a long time (since Win95b). Has anyone in the Mac world had any experience with DMA support on hard drives? Is it supported on iBooks? If so, which models?"

6 of 49 comments (clear)

  1. Drive Setup v1.1 by brejc8 · · Score: 4, Informative

    The ChangeLog seems to have added UDMA in version 1.1 and later updated in 1.7 .
    Try searching for UDMA Ultra-ATA ATA33/66/100 because DMA doesnt allways show up.

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  2. Choppy Disk or VM switching out by Anonymous Coward · · Score: 5, Informative

    Are you sure it's the disk causing the problem vs delays due to VM switching in and out. There's a memory utility (can't remember name, Memory Tracker or Memory Monitor?) that will beep each time memory is paged in or out (or you could use top if you can interpret the numbers).

    I've found it can be pretty easy to use up all free RAM if you have a number of programs open or they leak memory. I've found at times I've had over 2GB of switch files.

    This is with 896 MB of RAM, iBooks can be under spec'd in that department.

    1. Re:Choppy Disk or VM switching out by coolgeek · · Score: 3, Informative

      Me too. Esp. if running OS X w/256MB (or less), you need the WindowServer compression hack or else all your available RAM goes to window buffers, which inevitably causes swapping.

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    2. Re:Choppy Disk or VM switching out by frankie · · Score: 3, Informative

      Note that window buffer compression is enabled by default in Jaguar. I still wouldn't run OS X with less than 256MB though, and 512+ is much faster.

  3. try this by austad · · Score: 4, Informative

    # sysctl -a | grep dma

    Look for a setting that's related to DMA and set it from 0 to 1 with "sysctl -w". You should be able to place the setting in /etc/sysctl.conf to make it enable it on every boot also.

    Unfortunately, I don't have an OSX box to test it on right now, so I can't give you the exact settings. OSX is based on FreeBSD, and FreeBSD enables DMA by default, however, it's possible that apple found a need to disable it by default. Some drive/chipset combos have problems with DMA, and if they were unsure of which brands of drives or chipsets they would be using, disabling it by default would be the safe way to go.

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  4. Not a DMA issue by h0tblack · · Score: 3, Informative

    Mac's have had DMA support for a long time in both hardware and software, see this technote and the Developer Notes on hardware here for details. This includes the original clamshell iBooks. The iBooks may not have the fastest drives or interfaces but I don't think this is the problem. It's much more likely a Virtual Memory issue with lots of caching going on. check the contents of /private/var/vm/ for the number of 80Mb swapfiles you have, also try issuing vm_stat from the CLI to see whats going on. Two suggestions, either get more RAM or setup a 'clean' swap-drive partition.