Re: DMA or not DMA...

[nathan@visi.com (Nathan Mates)]

In article <337C90AF.AA7@library.ucla.edu>,  <pubpc1@library.ucla.edu> wrote:
>nathan@visi.com (Nathan Mates) wrote:
>>>I would.  The loss of DMA is not that big of a deal.  In real world,
>>>random read/write situations, it is not very noticeable (except when
>>>doing sequential operations like verifying a volume).

>>That's adding roughly 30% to a very much real world random access
>>i/o operation.

>Also, booting involves much sequential access (if you installed
>the system on a clean/new partition).

   There is a fair amount of sequential access, but any time you
go load a file, then that init/etc goes and hits a resource fork,
prefs file, etc, your sequential access pattern goes out the window.
There's enough random disk bouncing around to make the even defragged
disks not a linear i/o pattern on booting.

   However, you've got it totally wrong as to what DMA affects. It is
NOT sequential disk access only (that's what the RamFAST excels at,
DMA on or not); DMA speeds up the transfer of each and every block
read simply by telling the processor to take a nap while it shoves
data into memory as fast as the bus allows. DMA off, the processor has
to sit in a tight loop reading out bytes one at a time and copying
them to the destination. Sequential block reads or random are both
equally affected, no matter what the drive speed is-- at the basic
level, each block must be copied from disk to RAM.

Nathan Mates


--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
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