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
