Hi guys, First of all thank you but, David Marteua and Sebastian Ernst, for the answer. So is impossible to use multiprocessing in Qgis plugin? This is a enormous limitations because Qgis have a lot of data that needs to be evaluated in same cases. Nowadays the cpu have 5,7, 9 cores and we can not use them in a plugin for example is bad. If some one have any idea who we can use multiprocessing in a plugin i will be thankful.
Best regards, João Folgado David Marteau <dmart...@3liz.com> escreveu em sex., 26/03/2021 às 08:57 : > > I am not entirely sure what QGIS' (intended) behavior is in a case like > > this. From the top of my head, having read the relevant portion of its > > source code, I'd expect at least a second application window likely > > followed by non-deterministic behavior (again, depending on the > > use-case, likely resulting in a crash). Can someone elaborate? > > I can confirm that the result is non-deterministic even with headless > PyQgis programs. > > For programs relying heavily on multiprocessing the best strategy is to > use a `fork-server` process > started at the very beginning (before initializing Qgis) managing the > pool of child processes, so that each forked > process starts in the same state. > > David, > > Le 23/03/2021 à 19:22, Sebastian M. Ernst a écrit : > > Hi João, all, > > > > really interesting question. > > > >> however when i run the plugin a new instance of application is open > >> and give me an error. > > I assume that you are using `multiprocessing` [1] from Python's standard > > library. Its default behavior is to use process-based parallelism (as > > the title in the documentation says). I am assuming that you have not > > altered its default behavior (e.g. by configuring it to use threads > > instead). This is why you get at least another "worker" process if you > > e.g. create a process pool. I have successfully used this method with > > QGIS before, but always on Linux and OS X. I am just guessing that you > > are doing this on Windows, right? > > > > Depending on the operating system, `multiprocessing` uses different > > methods to create worker processes. On Linux and OS X, it simply forks > > [2] the main process. This way, you do not see a second application > > window popping up. You "simply" get a second, third, fourth, ... process > > "in the background". On Windows, the `fork` syscall does not exist, > > neither does something similar [3]. `multiprocessing` therefore starts a > > complete new process from scratch. If it was pure Python, it would > > simply start a new Python interpreter process. Because it is QGIS, it > > opens another instance of QGIS. > > > > I am not entirely sure what QGIS' (intended) behavior is in a case like > > this. From the top of my head, having read the relevant portion of its > > source code, I'd expect at least a second application window likely > > followed by non-deterministic behavior (again, depending on the > > use-case, likely resulting in a crash). Can someone elaborate? > > > > Best regards, > > Sebastian > > > > > > 1: https://docs.python.org/3/library/multiprocessing.html > > 2: https://en.wikipedia.org/wiki/Fork_(system_call) > > 3: https://stackoverflow.com/q/985281/1672565 > > > > > > Am 23.03.21 um 13:08 schrieb Joao Folgado: > >> Hi everyone, > >> > >> I had created a plugin for Qgis and I trying to optimize it. So i use > >> in my code the library python that have Queue whit multiprocessing > >> however when i run the plugin a new instance of application is open and > >> give me an error. > >> I had early use this type of multiprocessing before in simple projects > >> in university and it works ok. > >> Can someone help ? I researched in many forums, i see people whit the > >> same question but not with the answer. > >> Thank you very much. > >> > >> Best regards, > >> João Folgado > >> Portugal > >> -- > >> JOÃO FOLGADO > >> > >> _______________________________________________ > >> QGIS-Developer mailing list > >> QGIS-Developer@lists.osgeo.org > >> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > >> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > >> > > _______________________________________________ > > QGIS-Developer mailing list > > QGIS-Developer@lists.osgeo.org > > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > _______________________________________________ > QGIS-Developer mailing list > QGIS-Developer@lists.osgeo.org > List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > -- JOÃO FOLGADO
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