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From Meng Zhu <m...@mesosphere.com>
Subject Proposing change to the allocatable check in the allocator
Date Mon, 11 Jun 2018 22:47:01 GMT
Hi:

The allocatable
<https://github.com/apache/mesos/blob/1.5.x/src/master/allocator/mesos/hierarchical.cpp#L2471-L2479>
 check in the allocator (shown below) was originally introduced to

help alleviate the situation where a framework receives some resources, but
no

cpu/memory, thus cannot launch a task.


constexpr double MIN_CPUS = 0.01;constexpr Bytes MIN_MEM = Megabytes(32);
bool HierarchicalAllocatorProcess::allocatable(
    const Resources& resources)
{
  Option<double> cpus = resources.cpus();
  Option<Bytes> mem = resources.mem();

  return (cpus.isSome() && cpus.get() >= MIN_CPUS) ||
         (mem.isSome() && mem.get() >= MIN_MEM);
}


Issues

However, there has been a couple of issues surfacing lately surrounding the
check.

   -
   - - MESOS-8935 Quota limit "chopping" can lead to cpu-only and
   memory-only offers.

We introduced fined-grained quota-allocation (MESOS-7099) in Mesos 1.5.
When we

allocate resources to a role, we'll "chop" the available resources of the
agent up to the

quota limit for the role. However, this has the unintended consequence of
creating

cpu-only and memory-only offers, even though there might be other agents
with both

cpu and memory resources available in the cluster.


- MESOS-8626 The 'allocatable' check in the allocator is problematic with
multi-role frameworks.

Consider roleA reserved cpu/memory on an agent and roleB reserved disk on
the same agent.

A framework under both roleA and roleB will not be able to get the reserved
disk due to the

allocatable check. With the introduction of resource providers, the similar
situation will

become more common.

Proposed change

Instead of hardcoding a one-size-fits-all value in Mesos, we are proposing
to add a new master flag

min_allocatable_resources. It specifies one or more scalar resources
quantities that define the

minimum allocatable resources for the allocator. The allocator will only
offer resources that are more

than at least one of the specified resources.  The default behavior *is
backward compatible* i.e.

by default, the flag is set to “cpus:0.01|mem:32”.

Usage

The flag takes in either a simple text of resource(s) delimited by a bar
(|) or a JSON array of JSON

formatted resources. Note, the input should be “pure” scalar quantities
i.e. the specified resource(s)

should only have name, type (set to scalar) and scalar fields set.


Examples:

   - - To eliminate cpu or memory only offer due to the quota chopping,
   - we could set the flag to “cpus:0.01;mem:32”
   -
   - - To enable offering disk only offer, we could set the flag to
   “disk:32”
   -
   - - For both, we could set the flag to “cpus:0.01;mem:32|disk:32”.
   - Then the allocator will only offer resources that at least contain
   “cpus:0.01;mem:32”
   - OR resources that at least contain “disk:32”.


Let me know what you think! Thanks!


-Meng

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