Some questions about A/UX

[Alexis Rosen]

Sorry I took so long to respond to this...

In article <19816 at apple.Apple.COM> phil at Apple.COM (Phil Ronzone) writes:
"In article <964 at ccnysci.UUCP> alexis at ccnysci.UUCP (Alexis Rosen) writes:
">That said, I still have some questions. The Mac can transfer a 1KByte block
">in 280 usecs. That's fine, but it's not the whole story. If it were, it could
">do about 3.5 MBytes/sec. In fact, it can do less than one tenth of that
">Speed. So what causes that discrepancy? My guess (uneducated, so please
">correct me if I'm wrong) is that it's the overhead for transferring that
">block. Can the Mac transfer 10 or 100 blocks in 2800 or 28000 microseconds? I
">don't think so. So again, what am I missing?
"
"DMA is merely a hardware assist for transferring data from the (typically and
"hopefully) buffered SCSI device, such as a hard disk, into the Mac system
"memory.
"
"To do this, the hard disk is instructed to read/write "N" blocks of
"data starting at a certain address. For example, opening a thousand
"1 block files, each of which is located near the end of a slower hard disk,
"could, in ABSOLUTE worst case, can take as follows:
"     1000 seeks to front of the disk to read the inode
"     and then go back to near the end of the disk to read one 1K block
"     = 60ms * 1000 = 60 secs = about 16KB per second!!!!
"
"Rememberm that hard disk is getting more or less 40 - 100 requests for
"blocks per second under load - and each block has a more or less random
"block address in UNIX (to oversimplify it).
"
"On the other hand, a SCSI device that is actually 16M (16 megabytes!) of
"cache in the SCSI adapter (to ESDI) gave up to 400K/s data transfer rates.
"You could write at even higher rates, but every 1 or 2 seconds it hit
"an internal count of "writes pending" and accepted no more I/O until ALL
"the writes were flushed. So you had 2-3 seconds of immense data transfer,
"no matter how much "seeking" was implied, then 2 - 3 seconds of NO data
"transfer while the device flushed.
"
"So DMA (or "PIO") is but a small part of the equation for disk throughput.
"Consider interleave, disk seek, average bytes per transfer, "randomness"
"and so on.

This is useful information to have, but it really doesn't answer the question
at all. Writing to a buffered SCSI disk can go up to 400K/s. That's great...
but not too great. The Wrens can sustain a throughput of 1MB/s. That's really
great. So why is the Mac's "great" != the Wren's "great"?

In other words, if the lack of DMA isn't slowing down the Mac, what is???
It is clearly not going as fast as it should.

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Alexis Rosen                       alexis at dasys1.UUCP  or  alexis at ccnysci.UUCP
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