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=unavailable autolearn_force=no version=3.4.4 Path: eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.unit0.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Niklas Holsti Newsgroups: comp.lang.ada Subject: Re: Not to incite a language war but apparently the Corona lockdown was based on 13 year old undocumented C-Code Date: Wed, 13 May 2020 12:54:34 +0300 Organization: Tidorum Ltd Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Trace: individual.net VJ75QIGLQ5QxDpIaey7aYQ/3xdlWRfo6PclDpi7QNfEOXZuQti Cancel-Lock: sha1:smFVeQsc9I1zhEZkQb+ZX0DG/8w= User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 In-Reply-To: Content-Language: en-US Xref: reader01.eternal-september.org comp.lang.ada:58673 Date: 2020-05-13T12:54:34+03:00 List-Id: On 2020-05-13 0:27, Jeffrey R. Carter wrote: > On 5/12/20 9:16 PM, Niklas Holsti wrote: >>> >>> It seems clear that the code that was executed to create the >>> predictions was not deterministic on a single core. >> >> The first of the two cases of single-core "non-determinism" described >> and bewailed in the "review" is that the time evolution of the >> predicted epidemic is different depending on the value of a program >> option -- whether the "network" is created anew, or reused -- that >> should not affect the result. No actual non-determinism is claimed in >> the review, just that the epidemic reaches large infection rates >> sooner for one option value than for the other. It seems that the code >> was using the PRN generator slightly differently in the two cases, >> resulting in different PRN sequences. > > The single-core non-determinism that I gathered from the referenced > interpretation of the review, which I clearly have not studied as > carefully as you, was that the program gave different results on the > same computer for identical inputs: same initial conditions and same RNG > seed(s). Perhaps I misunderstood, but that was what I got from it, and > the only part that seemed troubling. This is different from the kind of > non-determinism of simulations that are intended to be run many times > with different RNG seeds. Possibly that would be caused by accessing > uninitialized variables. The "second analysis" at https://lockdownsceptics.org/second-analysis-of-fergusons-model/ refers to some GitHub change-log entries that do correct uninitialized-variable errors, so there may have been some real non-determinism. The second analysis then tries to evaluate the effect of the various errors and their corrections by running the newest version and the "initial import" version with the same data, and claims "radically altered" results. In fact, the results compared, which are the number of infected but recovered people on day 128 of the simulation, are some 57 million for the initial code and 42 million for the current code. But note that the reviewer compares only one run of each program, so I would say that this change can well be purely statistical and result from small random (PRN-induced) differences in the speed at which the infections take off in the two runs. Even if the two runs use the same initial RNG seed, the changes probably make the sequence of RNG calls, and the uses to which the RNG results are put, different in the two program versions, which means that they effectively use different PRN sequences. I don't intend to spend any more time on this storm in a teapot. -- Niklas Holsti niklas holsti tidorum fi . @ .