Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!genrad!teddy!panda!talcott!harvard!seismo!brl-tgr!tgr!speck@cit-vax From: speck@cit-vax (Don Speck) Newsgroups: net.unix-wizards Subject: Re: How to make 4.2bsd ``/etc/dumps faster Message-ID: <7098@brl-tgr.ARPA> Date: Tue, 8-Jan-85 12:07:30 EST Article-I.D.: brl-tgr.7098 Posted: Tue Jan 8 12:07:30 1985 Date-Received: Fri, 11-Jan-85 05:26:49 EST Sender: news@brl-tgr.ARPA Organization: Ballistic Research Lab Lines: 29 After reading Chris Torek's trial of my dump mods I waited until I could try it on a TU80 myself before saying anything. Sorry if I seemed too silent... I timed the TU77 and found that a tight write() loop only makes it go at 110 ips, not 125. I was mistaking the smooth motion of the reels for top speed - the vacuum column chatter should have told me that it's coming to a stop between every tape write. Trying this on a TU80 on a 750, the TU80 does stream, but keeps going in and out of 100 ips mode every few seconds. (This is in single-user mode). The 750 can just *barely* finish the current write() and start another one before the interrecord gap flys by, even though the TU80 is stretching that interrecord gap as far as it can (1.2 inches). Dump has to do more processing between writes than the tight-loop write() program. When running dump, the TU80 tries 100 ips a few times but discovers that the time between tape blocks is over the limit. Thereafter the TU80 just sticks to 25 ips streaming. So the problem is that overhead on write() is much higher than I'd dreamed. Unless someone knows offhand where the time goes, I'll have to profile the kernel and find out where. (Probably physio()). Btw, how do I get an alternate profiling clock? To those who've asked for the dump mods, please be patient. I don't want to give out any more copies of it until it works *well*. Don Speck Caltech CS (818) 356-6886