Ask HN: help needed on VM placement algorithm

https://news.ycombinator.com/item?id=605146

by kuvkir • 17 years ago

8 19 17 years ago

news.ycombinator.com

Hello!

I'm working on a "cloud computing" management system and need help on a VM placement/provision algorithm.

Let us assume we have a cluster of N physical servers. Each server has a single type of resource - slots (Note that in real world servers have more resources CPU/memory/disk/etc...). Let a server have M slots. Servers can run virtual machines, every VM occupies from 1 to M slots. Once the VM (with k slots) starts on a server, the number of free server slots is reduced accordingly. When the VM stops, the occupied slots are freed.

VMs can live-migrate across servers. For example, when there is no server with M free slots (every server runs at least one VM), and we need to start an M-slot VM - there we need to rearrange VMs across a cluster.

The goal is implement a VM placement policy to satisfy the following requirements:

* utilize server resources in a uniform way, e.g. if we have 3 servers and 3 single-slot VMs we should place them on different servers.

* minimize number of VM migrations as it is a costly operation.

It looks like the problem in question has something to do with multiple knapsack problem, so I'm looking for some kind of heuristic algorithm.

Any advise / papers / working solutions ?

Thanks in advance,

Kirill

eru8• 17 years ago

I will look into your problem. Perhaps I can work out an integer linear formulation that you can feed into a standard solver for linear optimization problems.

If you want to read papers on the subject, look for 'scheduling' or 'production planning'. 'Bin packing' and 'cutting stock' might also be related.

Do you need an online solution or do you know all demands in advance? Also what do you do when not enough slots are available for all VMs? That could happen either because the number of slots in demand is higher than the supply --- or a bit more subtle: Say you have n servers, and n+1 VMs each taking M/2+1 slots. The total number of slots suffices, but still your VMs do not fit the machines.

kuvkir7• 17 years ago

Thanks for your reply, eru!

> Do you need an online solution or do you know all demands in advance?

The online solution, as we don't know the type of load in advance. It resembles tetris in some way, meaning that we don't know which piece you'll get next.

> Also what do you do when not enough slots are available for all VMs?

If it's not theoretically possible to host all VMs like in examples you provided, nothing can be done. But it's not an algorithmic problem, in real world new servers should be bought in that case.

rjprins4• 17 years ago

Wait, this is very simple:

Always pick the server with the most slots open.

Only when no server has enough space to accomodate the next machine, but together they have enough space, should you apply a packing algorithm.

eru8• 17 years ago

I guess one solution would be to put the request on hold and serve it as soon as enough slots are open. Or isn't this a problem?

eru8• 17 years ago

If you know something about the probability distribution of VM requests, use that knowledge!

Otherwise: When adding a new VM, just search for the maneuver that minimizes total cost (cost of moving VMs + cost of inequality). You can structure that search as a shortest-path-problem in a graph and solve it with the Dijkstra-Algorithm. See http://en.wikipedia.org/wiki/Levenshtein_distance for comparison.

rjprins4• 17 years ago

Is it first come first serve?

Very interesting, I'll see what I can work out..