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Resource Limits

Cgroups v2 provides a unified interface for managing resource limits and scheduling policies for processes and containers. This section demonstrates how to enforce CPU shares, memory limits, and throttling using cgroups v2 controllers. These techniques are critical for isolating workloads, preventing resource starvation, and ensuring predictable performance in multi-tenant environments.

CPU Limiting

Absolute CPU Limits

To enforce an absolute CPU limit (e.g., 50% of a single CPU), use the cpu.max property. This sets a hard cap on CPU time consumption, measured in microseconds per CPU.

# Create a cgroup and set CPU limit
sudo cgcreate -g cpu:/mycpu_limit
echo 500000 | sudo tee /sys/fs/cgroup/cpu/mycpu_limit/cpu.max
  • 500000 microseconds = 0.5 seconds per CPU cycle.
  • This limits the group to 50% of a single CPU core.

Relative CPU Shares

For relative CPU allocation (e.g., 2x the default share), use cpu.weight (0–100000). This is useful for prioritizing workloads without absolute caps.

# Set relative CPU weight (2x default)
echo 200000 | sudo tee /sys/fs/cgroup/cpu/mycpu_limit/cpu.weight

Note: Absolute limits (cpu.max) take precedence over relative weights.

Memory Limiting

Memory Usage Limits

Set a hard limit on memory consumption using memory.max. This prevents processes from exceeding the specified size.

# Create a cgroup and set memory limit
sudo cgcreate -g memory:/mymem_limit
echo 1073741824 | sudo tee /sys/fs/cgroup/memory/mymem_limit/memory.max
  • 1073741824 bytes = 1 GiB.
  • Exceeding this limit triggers the OOM killer, which may terminate processes in the group.

OOM Control

Adjust how the OOM killer handles memory limits using memory.oom_control:

# Prevent OOM killer from killing processes in this group
echo 0 | sudo tee /sys/fs/cgroup/memory/mymem_limit/memory.oom_control
  • 0 disables OOM killer intervention (high risk for system instability).
  • 1 enables OOM killer (default behavior).

Throttling

CPU Throttling

Limit CPU usage rate using cpu.throttling. This is useful for preventing processes from monopolizing CPU cycles.

# Set CPU throttling rate (e.g., 1000 cycles per second)
echo 1000 | sudo tee /sys/fs/cgroup/cpu/mycpu_limit/cpu.throttling
  • This enforces a maximum of 1000 cycles per second, effectively capping CPU usage.

Memory Throttling

Memory throttling is less common but can be achieved via memory.throttling (requires kernel support). It limits memory bandwidth usage, useful for I/O-bound workloads.

# Set memory throttling rate (e.g., 1000 bytes per second)
echo 1000 | sudo tee /sys/fs/cgroup/memory/mymem_limit/memory.throttling

Key takeaways

  • Use cpu.max for absolute CPU limits and cpu.weight for relative shares.
  • Set memory.max to enforce hard memory limits and use memory.oom_control to manage OOM behavior.
  • Throttling via cpu.throttling and memory.throttling ensures controlled resource consumption.
  • Always verify cgroup paths and kernel support for cgroups v2 before applying limits.