By default, the VM is allocated the full amount of RAM configured via RAM_SIZE for its entire lifetime.
If you want the container to dynamically reclaim unused guest RAM based on host memory pressure, you can enable memory ballooning. It is also used to prevent the guest from exceeding the container's memory limit, even when the limit is changed at runtime:
environment:
BALLOONING: "Y"The following optional variables allow you to tune the ballooning behaviour:
| Variable | Default | Description |
|---|---|---|
BALLOONING |
N | Set to Y to enable dynamic memory ballooning |
BALLOONING_MIN_MEM |
33% |
Minimum balloon target, as a percentage of guest max memory (e.g. 33%) or absolute size (e.g. 2G) |
BALLOONING_RAM_THRESHOLD |
80.0 |
Target host RAM usage percentage; the PI controller aims to keep host usage at or below this value |
BALLOONING_RAM_THRESHOLD_HARD |
90.0 |
Host RAM usage percentage above which the balloon target may drop below guest RAM usage, inducing guest memory pressure |
BALLOONING_PSI_PRESSURE |
10.0 |
Host PSI avg10 stall percentage at which the PSI ceiling begins to lower the balloon target |
BALLOONING_PSI_PRESSURE_MAX |
50.0 |
Host PSI avg10 stall percentage at which the PSI ceiling reaches the configured minimum balloon target |
BALLOONING_HYSTERESIS |
128M |
Minimum balloon target change required before a resize is applied, as a percentage (e.g. 2%) or absolute size (e.g. 256M) |
BALLOONING_KP |
0.5 |
PI controller proportional gain; higher values react faster but may oscillate |
BALLOONING_KI |
0.05 |
PI controller integral gain; higher values correct steady-state error faster but risk overshoot |
BALLOONING_INTERVAL |
5 |
Polling interval in seconds |
Note
Memory ballooning uses Linux PSI (/proc/pressure/memory) for progressive pressure detection. Between BALLOONING_PSI_PRESSURE and BALLOONING_PSI_PRESSURE_MAX the PSI ceiling linearly lowers the maximum balloon target from guest max memory down to the configured minimum. If PSI is unavailable (kernel lacks CONFIG_PSI), both thresholds are silently skipped and ballooning continues using host memory usage alone.
Warning
If the container memory limit is reduced at runtime below the guest VM's current memory usage, the container may be killed by the OOM killer if the ballooning driver cannot reclaim memory from the guest fast enough.