Date: Sun, 8 Oct 1995 12:32:11 -0400 From: dennis@etinc.com (dennis) To: hackers@freebsd.org Subject: Re: VLB Disk Controllers Message-ID: <199510081632.MAA26478@etinc.com>
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>>Yup, but your whole machine is affected by a PIO device. > >That's mainly because IDE PIO is slow, not because it's PIO. The >overhead for SCSI DMA is not insignificant. For the BT455C, which >has a DMA speed of up to 40MB/sec (I doubt that my bus can keep up >with this), the overhead for reading from a Grand Prix at 5MB/sec >seems to be about 27%. This is hard to measure accurately. I >measured it by running a hog process in the background: on an idle >system it took 7.58 sec (real) and while transferring it took 10.42 >sec. 7.58/10.42 = 73%. Realize that much of this is highly dependent on the MB. Every make of MB is different...literally. We've measured lots of them (with a scope) and its almost impossible to compare any two these days. Dennis
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