Some questions about A/UX

[Dave Martindale]

In article <19528 at apple.Apple.COM> phil at Apple.COM (Phil Ronzone) writes:
>
>Assume you are running a typical one user I/O load of 40 to 80 1K blocks
>a second. When the Apple HD80 presents the data requested, then A/UX
>"yanks" the 1K, 4 bytes at a time, in a very tight loop. There is hardware
>assist to make for very quick "yanking". How quick? 3.657 bytes per
>microsecond. Or, 280 microseconds to "yank" the 1K block.
>
>If you are doing swapping of a 160K "chunk" every second, then it will
>save you 4.9% of your total CPU cycles every second.

>From my perspective, this is an argument that using the CPU to copy
data is fine as long as you are using the System V 1K-block filesystem,
since the filesystem so thoroughly throttles the disk.  But if you
ever switch to a filesystem with more throughput, you'll be in trouble.

For comparison, our old, slow, vax 780 running 4.3BSD always reads 8K
blocks on the filesystems that store images, and it manages to get about
60 blocks per second through the filesystem.  That's about 500 Kb/sec,
an order of magnitude larger than Phil's figures.  And this is on
old Eagle disks, where the average user data rate coming off the head
is about 1.6 Mb/sec.  More recent disks, even small Winchesters, are
considerably faster.

Our old Silicon Graphics workstation with a 70 Mb Vertex disk still
manages about 200 Kb/sec, using SGI's proprietary "extent filesystem".

So, if A/UX ever switches to a filesystem that allows access to some
reasonable fraction of the disk's real bandwidth (say 500 Kb/sec to
1 Mb/sec), like other workstation manufacturers provide,
having a DMA controller will suddenly become essential.

Remember that once the data is in kernel memory, UNIX has to copy it to
user memory, so the 68020 is going to be really busy just handling that.

I hope Apple switches to a better filesystem soon....

	Dave Martindale