Sound of Scheduling: Writing Linux Schedulers in Java with eBPF

eBPF is transforming Linux system capabilities, enabling the extension of the kernel with custom process schedulers, firewalls, and more. Thanks to Java's recent native integration and a bit of compiler magic, we can now create these extensions directly in Java — and that's exactly what this talk is all about.

We'll cover the fundamentals of eBPF and scheduling and how to implement a custom scheduler in Java. The session will showcase a scheduler that visualizes scheduling through sound — each process is mapped to a musical note, offering a fun and intuitive way to understand system activity. You’ll even get the chance to control the scheduling yourself using a MIDI keyboard.

Join us to learn how to craft small eBPF programs in Java that you'll really like the sound of.

Transcript

1. Sound of Scheduling timetableworld.com Writing Linux Schedulers in Java Johannes Bechberger (SAP SE)
2. You all know the problem:
3. You all know the problem:
4. You want to produce:
5. But that's not your kitchen
6. It's this
7. Focaccia Cantucini ... Oven
8. Focaccia Cantucini Oven Time
9. Focaccia Cantucini Oven Time Cantucini Focaccia Cantucini
10. Focaccia Cantucini Oven Time Cantucini Focaccia Cantucini
11. Focaccia Cantucini Oven Time Cantucini Focaccia Cantucini
12. Focaccia Cantucini Oven Time Cantucini Focaccia
13. Focaccia Cantucini Oven Time Cantucini Focaccia
14. Scheduling
15. What is scheduling?
16. Process A Process B CPU 1 ... CPU 2 ...
17. Process A Process B CPU Time
18. Process A Process B CPU A Time
19. Process A Process B CPU A Time
20. Process A Process B CPU A Time B
21. Process A Process B CPU A Time B A
22. Process A Process B CPU A Time B A A
23. Process A Process B CPU A Time B A B A
24. Process A Process B CPU A Time B A B A A
25. Hear this sound?
26. It's my scheduler
27. Written in Java
28. Why?
29. The only way of disco- vering the limits of the possible is to venture a little way past them into the impossible. Clarke’s second law https://www.flickr.com/photos/itupictures/16636142906 “
32. How?
33. How to modify the kernel?
34. Traditional ways 1.Change the Kernel 2.Kernel module
35. Traditional ways 1.Change the Kernel 2.Kernel module Interfaces are complicated
36. Traditional ways 1.Change the Kernel 2.Kernel module Not possible with schedulers
38. eBPF is a crazy technology, it’s like putting JavaScript into the Linux kernel Brendan Gregg “ https://www.youtube.com/watch?v=tDacjrSCeq4
39. eBPF is a crazy technology, it’s like putting JavaScript into the Linux kernel Brendan Gregg “ https://www.facesofopensource.com/brendan-gregg/
40. https://ebpf.io/what-is-ebpf/
41. Courtesy of Mohammed Aboullaite eBPF runtime
42. Courtesy of Mohammed Aboullaite eBPF runtime
43. Courtesy of Mohammed Aboullaite
44. How to share data? https://mostlynerdless.de/blog/2024/01/12/hello-ebpf-recording-data-in-basic-ebpf-maps-2/
45. How to share data? https://mostlynerdless.de/blog/2024/01/12/hello-ebpf-recording-data-in-basic-ebpf-maps-2/ Any Problems?
46. How to share data? https://mostlynerdless.de/blog/2024/01/12/hello-ebpf-recording-data-in-basic-ebpf-maps-2/
47. eBPF Maps Courtesy of Mohammed Aboullaite
48. https://ebpf.io
49. https://www.youtube.com/watch?v=X3AWV5lJ6RY
50. user land https://www.youtube.com/watch?v=X3AWV5lJ6RY
51. https://ebpf.io
52. Back to scheduling
54. 1.Ease of experimentation and exploration 2.Customization 3.Rapid scheduler deployments https://lwn.net/Articles/978911/ “
56. Let's create a scheduler
58. What is the performance? Good* * For a typical Java benchmark
59. How does it work?
60. CPU sched-ext User land Kernel land Hardware Java Code C Code Byte Code Loaded attach
61. Task A CPU 1 Local Queue CPU 2 Local Queue Global Queue Scheduler ... ...
62. Schedulers can be simple
63. @BPF(license = "GPL") public abstract class SampleScheduler extends BPFProgram implements Scheduler, Runnable { static final long SHARED_DSQ_ID = 0; @Type static class Stats { long global; long local; } @Override public int init() { return scx_bpf_create_dsq(SHARED_DSQ_ID, -1); } }
64. @BPF(license = "GPL") public abstract class SampleScheduler { @BPFMapDefinition(maxEntries = 100) BPFHashMap<Integer, Stats> statsPerCPU; @BPFMapDefinition(maxEntries = 100000) BPFLRUHashMap<@Unsigned Integer, @Unsigned Long> enqueuesPerProcess; }
65. @BPF(license = "GPL") public abstract class SampleScheduler { @Override public int selectCPU(Ptr<task_struct> p, int prev_cpu, long wake_flags) { boolean is_idle = false; int cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, Ptr.of(is_idle)); if (is_idle) { !/* !!... / } return cpu; } } *
66. @BPF(license = "GPL") public abstract class SampleScheduler { @Override public void enqueue(Ptr<task_struct> p, long enq_flags) { incrementStats(false); recordEnqueue(p); scx_bpf_dispatch(p, SHARED_DSQ_ID, SCX_SLICE_DFL.value(), enq_flags); } }
67. @BPF(license = "GPL") public abstract class SampleScheduler { @Override public void dispatch(int cpu, Ptr<task_struct> prev) { scx_bpf_consume(SHARED_DSQ_ID); } }
68. But they behave differently
69. Demo
70. Lottery Scheduler
71. Task 1 CPU 1 CPU 2 Scheduling Queue treated as a lottery bowl ... ... Draw randomly from queue Ask for new task Return finished task Enqueue task for the first time Task 2 2 1 3 4 7 6 1
72. Demo
73. What else can we do with eBPF
74. Fin. Thanks to Dylan Reimerink sched-ext Slack Johannes Bechberger mostlynerdless.de OpenJDK Developer, SAP