From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Thread: 103376,7272aa7508a3d83f X-Google-Attributes: gid103376,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news4.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!nx01.iad01.newshosting.com!newshosting.com!newsfeed.icl.net!newsfeed.fjserv.net!newsfeed.freenet.de!news.tiscali.de!newsfeed01.sul.t-online.de!t-online.de!news-in.ntli.net!newsrout1-win.ntli.net!ntli.net!newspeer1-win.ntli.net!newsfe2-gui.ntli.net.POSTED!53ab2750!not-for-mail From: "Dr. Adrian Wrigley" Subject: Re: pointer questions User-Agent: Pan/0.14.2 (This is not a psychotic episode. It's a cleansing moment of clarity.) Message-ID: Newsgroups: comp.lang.ada References: <19cfb$4361207d$4995001$19541@ALLTEL.NET> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Date: Fri, 28 Oct 2005 11:57:13 GMT NNTP-Posting-Host: 80.4.127.115 X-Complaints-To: http://www.ntlworld.com/netreport X-Trace: newsfe2-gui.ntli.net 1130500633 80.4.127.115 (Fri, 28 Oct 2005 12:57:13 BST) NNTP-Posting-Date: Fri, 28 Oct 2005 12:57:13 BST Organization: ntl Cablemodem News Service Xref: g2news1.google.com comp.lang.ada:6030 Date: 2005-10-28T11:57:13+00:00 List-Id: On Thu, 27 Oct 2005 19:28:36 +0000, Martin Dowie wrote: > More error related than predictability I hope! Or is there some > dangerous non-determinism associated with access types I haven't come > across yet?! :-) I did meet a case of non-determinism exposed through access types... My stock trading software originally used very large arrays of pricing data (around 1GB of data). This worked fine for many months. One day, I decided to add a level of redirection, using a large array of access values. After that, the program would fail mysteriously every few months. But why? It slowly became apparent that the memory was getting single-bit errors. I remember from the early days of DRAM that alpha particles in packaging could cause soft errors, and researched the current reliability of DRAM. It seems that packaging radiation is now negligable, but cosmic ray strikes are still an issue (depending on altitude). Calculating the rate of soft errors expected, I found that with a gigabyte of non-ECC memory, soft errors might occur about every week. I immediately switched to ECC and the problem vanished. Anyone with a well specced PC should be aware that memory bits will be flipping spontaneously every few days, unless ECC is used. This means that large linked data structures are highly susceptible to failure. Of course non-linked data also gets errors, but these will often disappear, and are slightly less sensitive. It becomes a question of how errors show up. With copious use of access types, failures are often catastophic. Without, they tend to be more insideous. Make your choice! Or get ECC! -- Adrian Quote: "IBM has performed the most comprehensive characterization of soft error rates in DRAM subsystems, collecting data on roughly 800 memory devices over a 10-year period. These studies were performed at various altitudes, including below sea level in underground caves, since the cosmic ray flux increases with altitude. Using this data, a senior engineer for IBM Microelectronics predicted that 256 Mbytes of memory would average one soft error per month due to cosmic rays (EE Times, 6/22/98)."