Resource Limits

Nova supports configuring limits on individual resources including CPU, memory, disk and network. These limits can be used to enforce basic Quality-of-Service (QoS) policies on such resources.

Note

Hypervisor-enforced resource limits are distinct from API-enforced user and project quotas. For information on the latter, refer to Manage quotas.

Warning

This feature is poorly tested and poorly maintained. It may no longer work as expected. Where possible, consider using the QoS policies provided by other services, such as Cinder and Neutron.

Configuring resource limits

Resource quota enforcement support is specific to the virt driver in use on compute hosts.

libvirt

The libvirt driver supports CPU, disk and VIF limits. Unfortunately all of these work quite differently, as discussed below.

CPU limits

Libvirt enforces CPU limits in terms of shares and quotas, configured via quota:cpu_shares and quota:cpu_period / quota:cpu_quota, respectively. Both are implemented using the cgroups cpu controller. Note that allowed values for shares are platform dependent.

CPU shares are a proportional weighted share of total CPU resources relative to other instances. It does not limit CPU usage if CPUs are not busy. There is no unit and the value is purely relative to other instances, so an instance configured with value of 2048 will get twice as much CPU time as a VM configured with the value 1024. For example, to configure a CPU share of 1024 for a flavor:

$ openstack flavor set $FLAVOR --property quota:cpu_shares=1024

The CPU quotas require both a period and quota. The CPU period specifies the enforcement interval in microseconds, while the CPU quota specifies the maximum allowed bandwidth in microseconds that the each vCPU of the instance can consume. The CPU period must be in the range 1000 (1mS) to 1,000,000 (1s) or 0 (disabled). The CPU quota must be in the range 1000 (1mS) to 2^64 or 0 (disabled). Where the CPU quota exceeds the CPU period, this means the guest vCPU process is able to consume multiple pCPUs worth of bandwidth. For example, to limit each guest vCPU to 1 pCPU worth of runtime per period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=1000

To limit each guest vCPU to 2 pCPUs worth of runtime per period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=2000

Finally, to limit each guest vCPU to 0.5 pCPUs worth of runtime per period:

$ openstack flavor set $FLAVOR \
    --property quota:cpu_period=1000 \
    --property quota:cpu_quota=500

Note

Smaller periods will ensure a consistent latency response at the expense of burst capacity.

CPU shares and CPU quotas can work hand-in-hand. For example, if two instances were configured with quota:cpu_shares=1024 and quota:cpu_period=100000 (100mS) for both, then configuring both with a quota:cpu_quota=75000 (75mS) will result in them sharing a host CPU equally, with both getting exactly 50mS of CPU time. If instead only one instance gets quota:cpu_quota=75000 (75mS) while the other gets quota:cpu_quota=25000 (25mS), then the first will get 3/4 of the time per period.

Memory Limits

The libvirt driver does not support memory limits.

Disk I/O Limits

Libvirt enforces disk limits through maximum disk read, write and total bytes per second, using the quota:disk_read_bytes_sec, quota:disk_write_bytes_sec and quota:disk_total_bytes_sec extra specs, respectively. It can also enforce disk limits through maximum disk read, write and total I/O operations per second, using the quota:disk_read_iops_sec, quota:disk_write_iops_sec and quota:disk_total_iops_sec extra specs, respectively. For example, to set a maximum disk write of 10 MB/sec for a flavor:

$ openstack flavor set $FLAVOR \
    --property quota:disk_write_bytes_sec=10485760

Network bandwidth limits

Warning

These limits are enforced via libvirt and will only work where the network is connect to the instance using a tap interface. It will not work for things like SR-IOV VFs. Neutron’s QoS policies should be preferred wherever possible.

Libvirt enforces network bandwidth limits through inbound and outbound average, using the quota:vif_inbound_average and quota:vif_outbound_average extra specs, respectively. In addition, optional peak values, which specifies the maximum rate at which a bridge can send data (kB/s), and burst values, which specifies the amount of bytes that can be burst at peak speed (kilobytes), can be specified for both inbound and outbound traffic, using the quota:vif_inbound_peak / quota:vif_outbound_peak and quota:vif_inbound_burst / quota:vif_outbound_burst extra specs, respectively.

For example, to configure outbound traffic to an average of 262 Mbit/s (32768 kB/s), a peak of 524 Mbit/s, and burst of 65536 kilobytes:

$ openstack flavor set $FLAVOR \
    --property quota:vif_outbound_average=32768 \
    --property quota:vif_outbound_peak=65536 \
    --property quota:vif_outbound_burst=65536

Note

The speed limit values in above example are specified in kilobytes/second, while the burst value is in kilobytes.

VMWare

In contrast to libvirt, the VMWare virt driver enforces resource limits using consistent terminology, specifically through relative allocation levels, hard upper limits and minimum reservations configured via, for example, the quota:cpu_shares_level / quota:cpu_shares_share, quota:cpu_limit, and quota:cpu_reservation extra specs, respectively.

Allocation levels can be specified using one of high, normal, low, or custom. When custom is specified, the number of shares must be specified using e.g. quota:cpu_shares_share. There is no unit and the values are relative to other instances on the host. The upper limits and reservations, by comparison, are measure in resource-specific units, such as MHz for CPUs and will ensure that the instance never used more than or gets less than the specified amount of the resource.

CPU limits

CPU limits are configured via the quota:cpu_shares_level / quota:cpu_shares_share, quota:cpu_limit, and quota:cpu_reservation extra specs.

For example, to configure a CPU allocation level of custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:cpu_shares_level=custom \
    --quota:cpu_shares_share=1024

To configure a minimum CPU allocation of 1024 MHz and a maximum of 2048 MHz:

$ openstack flavor set $FLAVOR \
    --quota:cpu_reservation=1024 \
    --quota:cpu_limit=2048

Memory limits

Memory limits are configured via the quota:memory_shares_level / quota:memory_shares_share, quota:memory_limit, and quota:memory_reservation extra specs.

For example, to configure a memory allocation level of custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:memory_shares_level=custom \
    --quota:memory_shares_share=1024

To configure a minimum memory allocation of 1024 MB and a maximum of 2048 MB:

$ openstack flavor set $FLAVOR \
    --quota:memory_reservation=1024 \
    --quota:memory_limit=2048

Disk I/O limits

Disk I/O limits are configured via the quota:disk_io_shares_level / quota:disk_io_shares_share, quota:disk_io_limit, and quota:disk_io_reservation extra specs.

For example, to configure a disk I/O allocation level of custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:disk_io_shares_level=custom \
    --quota:disk_io_shares_share=1024

To configure a minimum disk I/O allocation of 1024 MB and a maximum of 2048 MB:

$ openstack flavor set $FLAVOR \
    --quota:disk_io_reservation=1024 \
    --quota:disk_io_limit=2048

Network bandwidth limits

Network bandwidth limits are configured via the quota:vif_shares_level / quota:vif_shares_share, quota:vif_limit, and quota:vif_reservation extra specs.

For example, to configure a network bandwidth allocation level of custom with 1024 shares:

$ openstack flavor set $FLAVOR \
    --quota:vif_shares_level=custom \
    --quota:vif_shares_share=1024

To configure a minimum bandwidth allocation of 1024 Mbits/sec and a maximum of 2048 Mbits/sec:

$ openstack flavor set $FLAVOR \
    --quota:vif_reservation=1024 \
    --quota:vif_limit=2048