Path: csiph.com!eternal-september.org!feeder.eternal-september.org!reader01.eternal-september.org!.POSTED!not-for-mail From: Anne & Lynn Wheeler Newsgroups: alt.folklore.computers Subject: Re: If Memory Had Been Cheaper Date: Sun, 09 Aug 2020 11:04:02 -1000 Organization: Wheeler&Wheeler Lines: 22 Message-ID: <87zh73h6pp.fsf@localhost> References: <2c8a81a9-9273-49a7-b5e5-632b869f93bco@googlegroups.com> <87r1sgtvqy.fsf@localhost> <2034298831.618684972.130030.peter_flass-yahoo.com@news.eternal-september.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Injection-Info: reader02.eternal-september.org; posting-host="b75fac795ca45d957b53150d14dad279"; logging-data="14411"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/L3G0v03iUPPemRxRx02pxdasRcnw80EY=" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) Cancel-Lock: sha1:kgSui8rt6bbx20bdH14GrD5cJlI= sha1:8QEgJKye0gI4D8eXweMRThNUQ6s= Xref: csiph.com alt.folklore.computers:212621 Peter Flass writes: > Was that you? Doesn’t VM use a “sweep” algorithm? Process all requests that > will keep the arm moving in one direction, then reverse and process all the > requests in the other. Original CP67 was FIFO, at the univ. as undergraduate in the 60s, I changed it to ordered seek ... also did chained requests for paging in one channel program (ordered by rotation, if didn't require seek) ... instead of separate channel program/SIO for every page (which was one of the things retained for vm370, but a lot of other stuff was dropped and/or at least drastically simplified). for 370/125 ... the controller had real-time rotational position, there was delay for rotation and delay for seek arm ... there were some situations where could do a further seek delay might be less than closer arm rotation delay ... it was possible because the whole I/O request queue was exposed to the controller ... and could do real-time reordering ... not only knowing the disk arm position but also the real-time rotational position. -- virtualization experience starting Jan1968, online at home since Mar1970