Le 24/12/2020 à 19:55, Simon Hobson a écrit : > Didier Kryn <k...@in2p3.fr> wrote: > >> Therefore I suspect the authors managed to launch several threads in order >> to save 0.01s of the boot time. Or to loose more because thread scheduling >> might well consume more than what parallelism saves. > In the general case, parallelism only saves wall clock time IFF you have a > number of processes that have to wait on outside events while not > (significantly) using resources on the machine - or if they are exceedingly > computationally intensive that running tasks across multiple cores gives a > saving (not common during startup). So if you have things like bringing up > interfaces - waiting for WiFi to connect and DHCP to get an address, that > sort of thing. But even then there's probably little to be saved since you > usually have most of the system waiting for the network to be up before it > can proceed. > But otherwise, especially with a spinning disk, parallelism will slow things > down because you force the disk to go off here there and everywhere getting > data for different processes. Not applicable during startup, but there are > memory considerations* too if the jobs are large. With SSD this is much less > of a problem. > > > * As an aside, at a previous job many years ago, they got a network of Apollo > workstations in for running engineering software. The whole thing was > primarily driven by the naval architects for doing complex fluid dynamics and > structural modelling - and at the time Apollo had the higher spec number > cruncher. For context, this was when a 286 with a couple of megs of RAM was > considered high end - Apollo were using (from memory) Motorola 68000 range > processors and I think most of the workstations had 68020. They had to stop > people running their own jobs on the big machine simply because if asked to > run more than one then it would slow to a crawl when it started swapping. But > users were unable to grasp the concept of "wait your f'in turn" (some would > even cancel other running jobs to get theirs to run faster) - so restrictions > were imposed and only the admins could run jobs on it, everyone else had to > put their requests in a queue. > > Simon
I remember these Apollos. They were shining and ran some brand of Unix if I remember well. We had a few in my lab but I never got a chance to touch one. -- Didier _______________________________________________ Dng mailing list Dng@lists.dyne.org https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng
