I've been playing with Mesos cluster for a little bit, and thinking of utilizing Mesos cluster in our production environment. One problem I can't seem to find an answer to: how to properly schedule long running apps that will have varying load?
Marathon has "CPUs" property, where you can set weight for CPU allocation to particular app. (I'm planning on running Docker containers) But from what I've read, it is only a weight, not a reservation, allocation, or limitation that I am setting for the app. It can still use 100% of CPU on the server, if it's the only thing that's running. The problem is that for long running apps, resource demands change over time. Web server, for example, is directly proportional to the traffic. Coupled to Mesos treating this setting as a "reservation," I am choosing between 2 evils: set it too low, and it may start too many processes on the same host and all of them will suffer, with host CPU going past 100%. Set it too high, and CPU will go idle, as reservation is made (or so Mesos think), but there is nothing that's using those resources.
How do you approach this problem? Am I missing something in how Mesos and Marathon handle resources?
I was thinking of an ideal way of doing this:
- Specify weight for CPU for different apps (on the order of, say, 0.1 through 1), so that when going gets tough, higher priority gets more (as is right now)
- Have Mesos slave report "Available LA" with its status (e.g. if 10 minute LA is 2, with 8 CPUs available, report 6 "Available LA")
- Configure Marathon to require "Available LA" resource on the slave to schedule a task (e.g. don't start on particular host if Available LA is < 2)
- When available LA goes to 0 (due to influx of traffic at the same time as some job was started on the same server before the influx) - have Marathon move jobs to another slave, one that has more "Available LA"
Is there a way to achieve any of this?
So far, I gather that I can possible write a custom isolator module that will run on slaves, and report this custom metric to the master. Then I can use it in resource negotiation. Is this true?
I wasn't able to find anything on Marathon rescheduling tasks on different nodes if one becomes overloaded. Any suggestions?
