tgravescs commented on issue #26284: [SPARK-29415][Core]Stage Level Sched: Add 
base ResourceProfile and Request classes
URL: https://github.com/apache/spark/pull/26284#issuecomment-553481259
 
 
   I think that matching resources to existing Executors complicates a lot of 
things. Personally, I think that actually makes it harder for the user to 
reason about it.
   In your scenario if I have executors with 9GB and then have a stage that 
needs 8GB, then yes I may want to release the old executors because each one is 
using an additional 1GB. If I have 1000 executors, that is 1000GB that some 
other application could be using. Now its possible that I don't want to release 
those either if it takes a long time to get new ones.  But the user can always 
just create one ResourceProfile with 9GB and use it for both. The only way that 
doesn't work is if you wanted to change just  task requirements for some other 
resource across them.
   
   I think if we clearly document it saying a new ResourceProfile means new 
executors with the resources specified, then it should be clear.  If we don't 
do that, then I think it is more effort for the user to try to reason about 
what spark would internally be doing.  You would also need an option here that 
says don't reuse executors and get me new ones for when users don't want to 
waste resources by reusing executors.  I think all of that could be added 
later, the main first target for this is the ETL/ML use case  where in most 
cases you would want new executors because you want your ETL to run on 
executors without GPUs and probably have more cores and then when you go to the 
ML side you would have less cores and a GPU on the executors.

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