4 ms·

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

by rjprins 17y 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.

eru 17y ago
If this is so simple, please prove the optimality of your solution. I can imagine that, when you have a perfect fit i.e. a machine that has l slots open and a VM that takes l slots, putting the VM on that machine might prove a good idea. At least when moving VMs is not too cheap compared to the advantages of a homogeneous load.
- rjprins 17y ago
  Okay, you have two conflicting criteria: Spreading load and minimizing VM movements: Algorithm A: Always choose the server with the most slots free spreads the load most equally without moving VM's. (Assuming servers are equal in capacity. Minimizing load-balance is defined as minimizing the difference between highest loaded server and lowest loaded server.) Proof: Given a VM to be placed, v, and a set of servers S of which server s' is has most slots free. Say picking s' to place v does not give the most optimal load-balance. Then there must be server with with more slots free then s', contradiction. Algorithm B: Let S be a series of servers ordered by load (lowest number of free slots first). Put a new VM v on the first server s, on which v fits. This maximizes the initial number of VM's you can place without moving VM's, without prior knowledge. Proof: Because we always try to fit each VM in the first server possible, we maximize the amount of consecutive space on the last server. Thus, without prior knowledge, this maximizes the number of VM's we can place. I take it you like neither of these solutions. You want to compromise between the two solutions. Doing that right (there is not optimal in that sense) would depend on the scale difference between servers slots and space required by VM's. If VM's are small and servers big, you can safely go for load balance. If VM's are big and servers relatively small, you may go for B.
  - eru 17y ago
    If you want a simple solution, your Algorithm A is good for a first try. However if you have more information about the probability distribution of the number of slots in a VM, you could in theory do better than that.
kuvkir 17y ago
That's not that simple. Let us imagine the following configuration. We have 3 servers each of 4 slots, below is the state of each server (the state of a server is a set of VMs it's running) [3], [2, 1] and [2]. Say we need to place a new 3-slot VM to our system. We'll pick the third server as less-loaded and try to accomodate the second server. Together they have enough space, but we can't provision the VM withour using all 3 servers. The solution here is 1) migrate a 1-slot VM from server 2 to server 1 2) migrate a 2-slot VM from server 2 to server 3 3) place the new VM to server 2
- rjprins 17y ago
  Don't get me wrong, a correct packing algorithm for this is far from trivial! What would you say is more important: minimizing the amount of slots to move or the number of VM's?
  - kuvkir 17y ago
    That's not a simple question. But I would guess having one 2-slot migration is better than two 1-slot migrations...

by rjprins 17y ago

eru 17y ago

If this is so simple, please prove the optimality of your solution. I can imagine that, when you have a perfect fit i.e. a machine that has l slots open and a VM that takes l slots, putting the VM on that machine might prove a good idea. At least when moving VMs is not too cheap compared to the advantages of a homogeneous load.

rjprins 17y ago
Okay, you have two conflicting criteria: Spreading load and minimizing VM movements: Algorithm A: Always choose the server with the most slots free spreads the load most equally without moving VM's. (Assuming servers are equal in capacity. Minimizing load-balance is defined as minimizing the difference between highest loaded server and lowest loaded server.) Proof: Given a VM to be placed, v, and a set of servers S of which server s' is has most slots free. Say picking s' to place v does not give the most optimal load-balance. Then there must be server with with more slots free then s', contradiction. Algorithm B: Let S be a series of servers ordered by load (lowest number of free slots first). Put a new VM v on the first server s, on which v fits. This maximizes the initial number of VM's you can place without moving VM's, without prior knowledge. Proof: Because we always try to fit each VM in the first server possible, we maximize the amount of consecutive space on the last server. Thus, without prior knowledge, this maximizes the number of VM's we can place. I take it you like neither of these solutions. You want to compromise between the two solutions. Doing that right (there is not optimal in that sense) would depend on the scale difference between servers slots and space required by VM's. If VM's are small and servers big, you can safely go for load balance. If VM's are big and servers relatively small, you may go for B.
- eru 17y ago
  If you want a simple solution, your Algorithm A is good for a first try. However if you have more information about the probability distribution of the number of slots in a VM, you could in theory do better than that.

kuvkir 17y ago

That's not that simple. Let us imagine the following configuration. We have 3 servers each of 4 slots, below is the state of each server (the state of a server is a set of VMs it's running) [3], [2, 1] and [2]. Say we need to place a new 3-slot VM to our system. We'll pick the third server as less-loaded and try to accomodate the second server. Together they have enough space, but we can't provision the VM withour using all 3 servers. The solution here is 1) migrate a 1-slot VM from server 2 to server 1 2) migrate a 2-slot VM from server 2 to server 3 3) place the new VM to server 2

rjprins 17y ago
Don't get me wrong, a correct packing algorithm for this is far from trivial! What would you say is more important: minimizing the amount of slots to move or the number of VM's?
- kuvkir 17y ago
  That's not a simple question. But I would guess having one 2-slot migration is better than two 1-slot migrations...