Discussion towards documenting guidance for deciding on a users-per-node size

[consideRatio]

Consider the of active users from ubc-eoas last month:

In this graph there is a peak of 86 users at one point in time, this peak looks like this:

If we can choose the node size to fit any multiple of two amounts of users, what users-per-node size seems to be a good choice in this case?

Extreme 1: all users on one node

• :heavy_plus_sign: only at one point in time would there be startup time, when this node starts
• :heavy_plus_sign: with more users its easier to make effective use of CPU and Memory if limits are higher than requests
• :heavy_minus_sign: the cost incurred for a single user is as large as 100 users

Extreme 2: user dedicated nodes

• :heavy_plus_sign: the cost incurred for a single user is minimized
• :heavy_minus_sign: the startup time is significant for every user
• :heavy_minus_sign: the average cost per user will be high as CPU and Memory use will be very inefficient as CPU is typically idle

Guidande to find the middle ground

Please join me in discussing what middle ground can make sense, so that we can document guidance how to identify it.

Using ubc-eoas usage above as an example, how many users do you think could make sense for them to have per node, and based on what?

[consideRatio]

I suggest we settle on providing guidance based on something simple enough, such as:

• Running server statistics (min, average, median, max) Basic statistics about the running user servers can be easily understood, guessed ahead of time, visually estimated from a dashboard, and even programatically measured.
• User requests for CPU and memory Without this we can't determine how many users could fit on a node of a certain size.

Guidance draft 1

This is my initial guidance suggestion, put in three steps.

1. (lower bound) Choose a node size that at least fits the minimum amount of users
2. (upper bound) Choose a node size that makes peak usage result in having at least around 3 nodes
3. (preference) Prefer a node size from one of the default size set suggested in #3176

Applied to ubc-eoas example above, assuming min users is 0, max users is 100, then we should use a node that fits at most ~33 users. Assuming that requested memory is 1GB, we arrive at a 4 CPU 32 GB node. Here is why:

• 1) the first advice doesn't say much as the minimum amount of users is 0
• 2) with 100 users at most expected, the second advice sais that we should use a node that doesn't fit more than 33 users
• 3) with 1GB per user and preferring one of the three highmem nodes 32GB, 128GB, 512GB, we en up picking the 4CPU 32GB highmem node.