Date: Mon, 1 Mar 1999 19:59:32 -0800 (PST) From: Tom <tom@uniserve.com> To: David Kelly <dkelly@hiwaay.net> Cc: Andrew McNaughton <andrew@squiz.co.nz>, freebsd-stable@FreeBSD.ORG Subject: Re: rc5des slows tape thruput Message-ID: <Pine.BSF.4.02A.9903011955220.21869-100000@shell.uniserve.ca> In-Reply-To: <199903020244.UAA02047@nospam.hiwaay.net>
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On Mon, 1 Mar 1999, David Kelly wrote: > Andrew McNaughton writes: > > > > Does the tape process get niced also then? > > No. > > > Is the nice setting reasonable? > > Yes. > > > If not then why is rc5des allowed to slow it so much? > > The picture that I'm getting is a single block is written to the tape > drive at a time. When the tape drive is ready for another block > rc5des's current time slice is not aborted but allowed to complete > before dd gets a time slice to queue another block to the tape. > > The drive at home has 1M of internal RAM, the DDS-3 drives at work have > 2M buffers. It would appear FreeBSD doesn't make use of these buffers. I don't see how that could be. SCSI tapes are SCSI tapes. Any buffering done by the device should be invisible. There might be a SCSI mode page flag to turn it on or off though. > > I don't imagine that dd to tape would need that much CPU. I'd expect > > it to bottleneck on IO. Is it something to do with needing very small > > chunks of CPU t ime and running into lots of small waits for the > > kernel's CPU time allocation? > > A couple of percent. Using dd to read /dev/zero and write /dev/null > yields 208M/sec w/o rc5des running, 203M/sec with, writing 100000 > blocks of 10k (roughly 1G) so the test runs long enough to be valid. Those figures are pretty weird. 208MB/s or ? That is a pretty fast tape drive. /dev/zero isn't a good test source due to drive compression and encoding stuff. > -- > David Kelly N4HHE, dkelly@nospam.hiwaay.net > ===================================================================== > The human mind ordinarily operates at only ten percent of its > capacity -- the rest is overhead for the operating system. > > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-stable" in the body of the message > > Tom Systems Support Uniserve To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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