Linux’s new Hierarchical Constant Bandwidth Server (HCBS) replaces RT_GROUP_SCHED with superior CPU allocation for real-time tasks. Learn how HCBS improves SCHED_FIFO/RR performance, cgroups v2 integration, and its impact on cloud, embedded, and HFT systems.
The Linux kernel is set for a major real-time scheduling upgrade with the introduction of Hierarchical Constant Bandwidth Server (HCBS), a high-performance alternative to the existing RT_GROUP_SCHED implementation.
Designed for mission-critical workloads, HCBS enables precise CPU bandwidth allocation for SCHED_RR/FIFO task groups, ensuring deterministic performance in enterprise and embedded systems.
Why HCBS Matters for Linux Real-Time Scheduling
The current RT_GROUP_SCHED mechanism has limitations in hierarchical task scheduling, particularly for real-time applications requiring strict CPU reservations. HCBS addresses these gaps by:
Replacing RT_GROUP_SCHED with a theoretically sound, hierarchical approach
Enabling dynamic bandwidth reservations via cgroups v2
Optimizing CPU utilization for SCHED_FIFO/RR tasks without excessive kernel modifications
"This patchset introduces Hierarchical RT scheduling, allowing cgroups to reserve CPU bandwidth efficiently. Each cgroup gets dedicated runqueues and deadline servers, activating only when tasks are runnable." — Yuri Andriaccio, HCBS Patch Author
Key Technical Improvements in HCBS
1. Hierarchical CPU Bandwidth Allocation
HCBS introduces per-cgroup runqueues and deadline servers (dl_servers), dynamically allocating resources when tasks are active. This ensures:
Lower latency for real-time workloads
Better isolation between competing task groups
Fine-grained control via cgroup virtual files
2. Enhanced SCHED_FIFO/RR Support
Unlike the legacy system, HCBS:
Reuses existing RT scheduler code for compatibility
Minimizes kernel overhead by activating dl_servers only when needed
Supports dynamic task migration (future updates will expand this)
3. Current Limitations & Future Roadmap
While HCBS is a significant leap forward, some features are still in development:
Capacity-aware bandwidth reservations (for heterogeneous CPUs)
Full task migration support across cores
Per-CPU runtime adjustments
Who Should Care About HCBS?
This update is crucial for:
✔ Cloud & Data Center Operators – Improves real-time VM/container performance
✔ Embedded Linux Developers – Enhances deterministic scheduling for robotics/automotive systems
✔ High-Frequency Trading (HFT) Firms – Reduces CPU scheduling jitter
✔ Kernel Contributors – A major step toward fully hierarchical RT scheduling
How to Test HCBS Today
Developers can review and test the RFC patch series linked in the official Linux kernel mailing list (LKML) discussion. Early benchmarks suggest significant improvements in worst-case latency for multi-tenant RT workloads.
FAQs: HCBS for Linux
Q: Does HCBS work with cgroups v1?
A: No—only cgroups v2 is supported, aligning with modern Linux deployments.
Q: When will HCBS be merged into mainline?
A: No official timeline, but widespread testing could accelerate adoption.
Q: How does HCBS compare to Deadline Scheduling?
A: HCBS focuses on hierarchical RT tasks, while Deadline Scheduling handles time-constrained workloads.
Final Thoughts: A New Era for Linux Real-Time Performance
HCBS represents a major evolution in Linux’s real-time capabilities, offering better predictability, scalability, and control for high-stakes environments. As the patches undergo review, enterprises and developers should prepare for upcoming kernel integrations.
For system architects and performance engineers, this is a development worth watching closely.
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