Sharing Consumable Resources
CPU Management
(Disclaimer: In this "CPU Management" section, the term "consumable resource" does not include memory. The management of memory as a consumable resource is discussed in its own section below.)
The per-partition OverSubscribe setting applies to the entity
being selected for scheduling:
When the default
select/linearplugin is enabled, the per-partitionOverSubscribesetting controls whether or not the nodes are shared among jobs.When the
select/cons_resorselect/cons_tresplugins are enabled, the per-partitionOverSubscribesetting controls whether or not the configured consumable resources are shared among jobs. When a consumable resource such as a core, socket, or CPU is shared, it means that more than one job can be assigned to it.
The following table describes this new functionality in more detail:
| Selection Setting | Per-partition OverSubscribe Setting |
Resulting Behavior |
|---|---|---|
| SelectType=select/linear | OverSubscribe=NO | Whole nodes are allocated to jobs. No node will run more than one job per partition/queue. |
| OverSubscribe=YES | By default same as OverSubscribe=NO. Nodes allocated to a job may be shared with
other jobs if each job allows sharing via the srun --oversubscribe
option. |
|
| OverSubscribe=FORCE | Each whole node can be allocated to multiple jobs up to the count specified per partition/queue (default 4 jobs per node) | |
| SelectType=select/cons_res or select/cons_tres Plus one of the following: SelectTypeParameters=CR_Core SelectTypeParameters=CR_Core_Memory |
OverSubscribe=NO | Cores are allocated to jobs. No core will run more than one job per partition/queue. |
| OverSubscribe=YES | By default same as OverSubscribe=NO. Cores allocated to a job may be shared with
other jobs if each job allows sharing via the srun --oversubscribe
option. |
|
| OverSubscribe=FORCE | Each core can be allocated to multiple jobs up to the count specified per partition/queue (default 4 jobs per core). | |
| SelectType=select/cons_res or select/cons_tres Plus one of the following: SelectTypeParameters=CR_CPU SelectTypeParameters=CR_CPU_Memory |
OverSubscribe=NO | CPUs are allocated to jobs. No CPU will run more than one job per partition/queue. |
| OverSubscribe=YES | By default same as OverSubscribe=NO. CPUs allocated to a job may be shared with
other jobs if each job allows sharing via the srun --oversubscribe
option. |
|
| OverSubscribe=FORCE | Each CPU can be allocated to multiple jobs up to the count specified per partition/queue (default 4 jobs per CPU). | |
| SelectType=select/cons_res or select/cons_tres Plus one of the following: SelectTypeParameters=CR_Socket SelectTypeParameters=CR_Socket_Memory |
OverSubscribe=NO | Sockets are allocated to jobs. No Socket will run more than one job per partition/queue. |
| OverSubscribe=YES | By default same as OverSubscribe=NO. Sockets allocated to a job may be shared with
other jobs if each job allows sharing via the srun --oversubscribe
option. |
|
| OverSubscribe=FORCE | Each socket can be allocated to multiple jobs up to the count specified per partition/queue (default 4 jobs per socket). |
When OverSubscribe=FORCE is configured, the consumable resources are
scheduled for jobs using a least-loaded algorithm. Thus, idle
CPUs|cores|sockets will be allocated to a job before busy ones, and
CPUs|cores|sockets running one job will be allocated to a job before ones
running two or more jobs. This is the same approach that the
select/linear plugin uses when allocating "shared" nodes.
Note that the granularity of the "least-loaded" algorithm is what
distinguishes the consumable resource and linear plugins
when OverSubscribe=FORCE is configured. With the
select/cons_res or select/cons_tres plugin enabled,
the CPUs of a node are not
overcommitted until all of the rest of the CPUs are overcommitted on the
other nodes. Thus if one job allocates half of the CPUs on a node and then a
second job is submitted that requires more than half of the CPUs, the
consumable resource plugin will attempt to place this new job on other
busy nodes that have more than half of the CPUs available for use. The
select/linear plugin simply counts jobs on nodes, and does not
track the CPU usage on each node.
The sharing functionality in the select/cons_res and
select/cons_tres plugins also support the
new OverSubscribe=FORCE:<num> syntax. If OverSubscribe=FORCE:3
is configured with a consumable resource plugin and CR_Core or
CR_Core_Memory, then the plugin will
run up to 3 jobs on each core of each node in the partition. If
CR_Socket or CR_Socket_Memory is configured, then the
plugin will run up to 3 jobs on each socket
of each node in the partition.
Nodes in Multiple Partitions
Slurm has supported configuring nodes in more than one partition since version
0.7.0. The following table describes how nodes configured in two partitions with
different OverSubscribe settings will be allocated to jobs. Note that
"shared" jobs are jobs that are submitted to partitions configured with
OverSubscribe=FORCE or with OverSubscribe=YES and the job requested
sharing with the srun --oversubscribe option. Conversely, "non-shared"
jobs are jobs that are submitted to partitions configured with
OverSubscribe=NO or OverSubscribe=YES and the job did not
request shareable resources.
| First job "shareable" | First job not "shareable" | |
|---|---|---|
| Second job "shareable" | Both jobs can run on the same nodes and may share resources | Jobs do not run on the same nodes |
| Second job not "shareable" | Jobs do not run on the same nodes | Jobs can run on the same nodes but will not share resources |
The next table contains several scenarios with the select/cons_res
or select/cons_tres plugins enabled to further
clarify how a node is used when it is configured in more than one partition and
the partitions have different "OverSubscribe" policies.
| Slurm configuration | Resulting Behavior |
|---|---|
Two OverSubscribe=NO partitions assigned the same set of nodes |
Jobs from either partition will be assigned to all available consumable resources. No consumable resource will be shared. One node could have 2 jobs running on it, and each job could be from a different partition. |
Two partitions assigned the same set of nodes: one partition is
OverSubscribe=FORCE, and the other is OverSubscribe=NO |
A node will only run jobs from one partition at a time. If a node is
running jobs from the OverSubscribe=NO partition, then none of its
consumable resources will be shared. If a node is running jobs from the
OverSubscribe=FORCE partition, then its consumable resources can be
shared. |
Two OverSubscribe=FORCE partitions assigned the same set of nodes |
Jobs from either partition will be assigned consumable resources. All consumable resources can be shared. One node could have 2 jobs running on it, and each job could be from a different partition. |
Two partitions assigned the same set of nodes: one partition is
OverSubscribe=FORCE:3, and the other is OverSubscribe=FORCE:5 |
Generally the same behavior as above. However no consumable resource will ever run more than 3 jobs from the first partition, and no consumable resource will ever run more than 5 jobs from the second partition. A consumable resource could have up to 8 jobs running on it at one time. |
Note that the "mixed shared setting" configuration (row #2 above) introduces the
possibility of starvation between jobs in each partition. If a set of
nodes are running jobs from the OverSubscribe=NO partition, then these
nodes will continue to only be available to jobs from that partition, even if
jobs submitted to the OverSubscribe=FORCE partition have a higher
priority. This works in reverse also, and in fact it's easier for jobs from the
OverSubscribe=FORCE partition to hold onto the nodes longer because the
consumable resource "sharing" provides more resource availability for new jobs
to begin running "on top of" the existing jobs. This happens with the
select/linear plugin also, so it's not specific to the
select/cons_res or select/cons_tres plugins.
Memory Management
The management of memory as a consumable resource remains unchanged and can be used to prevent oversubscription of memory, which would result in having memory pages swapped out and severely degraded performance.
| Selection Setting | Resulting Behavior |
|---|---|
| SelectType=select/linear | Memory allocation is not tracked. Jobs are allocated to nodes without considering if there is enough free memory. Swapping could occur! |
| SelectType=select/linear plus SelectTypeParameters=CR_Memory |
Memory allocation is tracked. Nodes that do not have enough available memory to meet the jobs memory requirement will not be allocated to the job. |
| SelectType=select/cons_res or select/cons_tres Plus one of the following: SelectTypeParameters=CR_Core SelectTypeParameters=CR_CPU SelectTypeParameters=CR_Socket |
Memory allocation is not tracked. Jobs are allocated to consumable resources without considering if there is enough free memory. Swapping could occur! |
| SelectType=select/cons_res or select/cons_tres Plus one of the following: SelectTypeParameters=CR_Memory SelectTypeParameters=CR_Core_Memory SelectTypeParameters=CR_CPU_Memory SelectTypeParameters=CR_Socket_Memory |
Memory allocation for all jobs are tracked. Nodes that do not have enough available memory to meet the jobs memory requirement will not be allocated to the job. |
Users can specify their job's memory requirements one of two ways. The
srun --mem=<num> option can be used to specify the jobs
memory requirement on a per allocated node basis. This option is recommended
for use with the select/linear plugin, which allocates
whole nodes to jobs. The
srun --mem-per-cpu=<num> option can be used to specify the
jobs memory requirement on a per allocated CPU basis. This is recommended
for use with the select/cons_res or select/cons_tres
plugins, which can allocate individual CPUs to jobs.
Default and maximum values for memory on a per node or per CPU basis can
be configured by the system administrator using the following
slurm.conf options: DefMemPerCPU,
DefMemPerNode, MaxMemPerCPU and
MaxMemPerNode.
Users can use the --mem or --mem-per-cpu option
at job submission time to override the default value, but they cannot exceed
the maximum value.
Enforcement of a jobs memory allocation is performed by setting the "maximum
data segment size" and the "maximum virtual memory size" system limits to the
appropriate values before launching the tasks. Enforcement is also managed by
the accounting plugin, which periodically gathers data about running jobs. Set
JobAcctGather and JobAcctFrequency to
values suitable for your system.
Last modified 9 June 2021