Document choosing cluster options

[TomAugspurger]

cc @chiaral. I think this is why you weren't getting all your workers. 15Gb / worker happens to not schedule well onto our physical machines (with ~26GB of memory). Something like 11GB will work better.

You might also try 4 cores / worker with ~ 25 GB / worker. I think that would also get better utilization. I plan to verify this once things quiet down on the cluster a bit :)

[chiaral]

that's exactly what I have been wondering about - what is the right combination to squeeze the most out of it. I will try it out and see. I have a combination of a lot of tasks and need of memory, so I will have to play a bit, but the fact of the 26gb mem is helpful!

[chiaral]

@TomAugspurger I tried to scale down worker_memory to 11 and requests more workers - but I don't get them all either. I then tried to scale up the memory (to both 25 and 22) and the workers are not starting at all. @rabernat looked at the dashboard and it seems like the node pools is not scaling up. Can you help?

This is the latest clusters I tried to request that didn't start at all

import dask_gateway
from dask.distributed import Client
gateway = dask_gateway.Gateway()
options = gateway.cluster_options() 
options.worker_memory=25
options.worker_cores = 1
cluster = gateway.new_cluster(options)
cluster.scale(10)
client = Client(cluster)
client
 

and

import dask_gateway
from dask.distributed import Client
gateway = dask_gateway.Gateway()
options = gateway.cluster_options() 
options.worker_memory=22
options.worker_cores = 1
cluster = gateway.new_cluster(options)
cluster.scale(10)
client = Client(cluster)
client
 

[chiaral]

When I request 11gb instead I get something out of it. So it appears like it's better to be on the 11gb max? I am not trying to be difficult :) I am just trying to understand!

[TomAugspurger]

I think the memory per core should (probably) always be a factor of about 6, since that's what the physical machine has (6.25 - 6.5 GB/core)

options.worker_memory = 25
options.worker_cores = 4

or

options.worker_memory = 12
options.worker_cores = 2

That ensures that we hit the CPU and memory limits at about the same time.

[rabernat]

Tom, thanks for helping to debug this.

I think the memory per core should (probably) always be a factor of about 6,

The problem with this suggestion is that it fixes the memory / CPU ratio. Some workloads, like @chiaral's just require more memory per core due to the nature of the data and the computation being performed. How do you recommend handling this?

[TomAugspurger]

Mmm so there are a couple layers here:

1. From the point of view of the kubernetes cluster, there's nothing wrong with some users requesting 3/4 of a nodes memory and 1/4 of its CPU. We might also have other users requesting 1/4 the memory and 3/4 the CPU, and we schedule stuff just fine.
2. So this only becomes an issue of wasted resources when a single user's requests is hitting the cluster limits
3. Ideally Dask would handle the extra cores just fine. We'd notice the worker has high memory and not schedule any more tasks, but we know that's not always the case. Does this particular workload finish with 4 cores & 25 GB of memory / worker? Does it benefit from those extra cores at all?

But the short answer is yes: the application's workload is the first factor to consider. Past that we're just optimizing resources.

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I noticed that our limits didn't have the right ratio. The max memory:CPU ratio should reflect the type of machines we're using: pangeo-data/pangeo-cloud-federation#807