Thnk you all for the reply. Maybe I could set up some metrics and count the keys per subtasks/slot by myself. However in the example of the playground there are 6 keys and they get distributed in the 2 slots as 4 and 2: is this a bug (since Piotr said that key groups can have sizes +/- 1 and in this case is 2)?
Il Mer 23 Ott 2019, 18:54 Till Rohrmann <[email protected]> ha scritto: > Currently, we don't work on trying to ensure that the number of key groups > is as evenly spread as possible. As a workaround I would suggest to > increase the number of key groups or to change the key function. > > Cheers, > Till > > On Wed, Oct 23, 2019 at 1:42 PM Piotr Nowojski <[email protected]> > wrote: > >> Hi, >> >> This is a known issue of Flink. For example key groups can have sizes +/- >> 1 and they are currently randomly distributed across the cluster, so some >> machines will get more keys to handle then the others. If the number of >> keys is relatively small, like 3 keys per key group, the load difference >> can be quite large (some machines may get almost only key groups with size >> 2 while others will get mostly with size of 3, making 50% load difference). >> >> Unfortunately I don’t know about any concrete plans to address it. Maybe >> Till will know something more (I CC’ed him). >> >> Also I don’t think it’s exposed via a metric anywhere. >> >> Piotrek >> >> On 22 Oct 2019, at 10:00, Flavio Pompermaier <[email protected]> >> wrote: >> >> Hi to all, >> I was looking into the Flink example of the Flink training trying to >> understand why in the ClickEventCount[1] one task manager was reading >> twice the speed of the other. >> >> I had to debug a lot of internal code of Flink to understand that it >> depends on the adopted hash function (used by Flink to assign keys to >> taskmanagers) that was assigning 4 keys to a TM and 2 to the other. Is >> there a smarter way to monitor this thing (e.g. a metric like >> taskManager_numKeys)? >> >> I also discovered that one cannot force how to partition keys per >> taskmanager (i.e. use keyBy after a customPartition). Is there any >> development effort in this direction? >> >> Best, >> Flavio >> >> [1] >> https://github.com/apache/flink-playgrounds/blob/master/docker/ops-playground-image/java/flink-playground-clickcountjob/src/main/java/org/apache/flink/playgrounds/ops/clickcount/ClickEventCount.java >> >> >>
