package uring-trace

uring-trace

eBPF-based tracing tool for visualizing io-uring workloads. This tracer leverages eBPF probes via ocaml libbpf bindings (ocaml_libbpf)) to extract events from the kernel. Traces are generated in fuchsia format to be displayed on Perfetto.

The Mental Model (taken from this blog)

Example trace

Motivation

Debugging code using io-uring is challenging since everything runs behind closed doors inside the kernel. From a users perspective, uring offers 1 system call to send & recieve IO. However, under the hood uring is a sophisticated runtime that makes decisions on how to dispatch your IO.

The current best way to gain some observability into io-uring is to use perf events counters to sample your program. Whilst this works, it can be hard to get a mental picture of how your program flows since perf reports the counts and individual histories of the tracepoints it managed to collect. Our tool on the other hand, traces requests as they go through the kernel and provide an idiomatic way to understand how your IO-requests are handled by io-uring. Under the hood, the tracer uses eBPF technology to hook into kernel tracepoints. Being based on eBPF, this makes it easy to extend and hook into other arbritrary points in the kernel to support future enhancements to tracing.

Visualization features:

• Path of IO requests from submission to completion
• Syscall time slices
• Kernel spawned IO-worker's tracks
• Multiple rings support
• Sampling option to handle high-throughput workloads

Path of IO request from submission to completion

The io-uring runtime makes several decisions on how your request should be processed asynchronously. In particular, there are 3 pathways that your IO request can take:

1. Inline completion (fast path): If the IO request can be carried out immediately and does not need to wait, (i.e. the network interface has pending data), your request will be directly processed by the thread that made the submission and the result will be placed onto the completion queue.
2. Polling (slow path but cheap): For operations that are unable to start immediately but have non-blocking support, the request will be registered to a poll set. When they are ready to be consumed, they are processed by the next thread that enters the ring.
3. Async worker pool (slow path): For operations that are unable to start immediately and do not have non-blockin support, (e.g. regular files, block devices) your request gets punted to a pool of kernel io-workers that will pick up them up and put their results on the completion queue.

This feature visualizes which path a request has taken in the kernel by drawing arrows connecting tracepoints for each request. The lifetime of a request starts from a the io_uring_submit tracepoint and ends with io_uring_complete (NOTE: It's not show when the user process has consumed the result from the ring). By clicking any of the tracepoints in perfetto, the UI will draw arrows to show the path of a request.

Syscall time slices

io-uring is a performance win because users can reduce the number of syscalls by batching them, thereby reducing overhead of context switch from user to kernel modes. One way to see if your program is really benefitting from this is to see how many requests per system call are being called together.

The strace -c summary is useful for getting some quick numbers of syscalls. This method has some caveats though. strace adds quite significant overhead to your running program since it pauses the program to inspect the program state whenever you hit a syscall. By doing so, this can end up altering what the actual interaction with uring looks like. uring-trace solves this by using eBPF, making tracing less invasive. Syscall slices enable users to quickly see how many requests are submitted & completed within that slice. The rough rule of thumb being that the more calls you have in the syscall, the more effective your batching is.

IO-worker tracks

It's not obvious how many workers are involved in processing blocking requests. There might also be workers that are blocked for a long time. This tool shows each spawned io-worker as a separate track and which request it processed.

Multiple uring instance support

Programs may intentionally use multiple rings. This tool can handle these cases since it matches requests to it's ring context

Filtering

More and more programs are using uring. There may be other programs on the system making uring syscalls. uring-trace only registers rings that it has seen the setup command for. This means that other processes using uring that have been running before the program you are tracing will have their uring calls filtered and drop. Thus, your perfetto output won't be garbled with unrelated processes.

Current support

• Path of IO request from submission to completion

  • Tracepoint visualisation support set

    • tracepoint:io_uring:io_uring_complete
    • tracepoint:io_uring:io_uring_cqe_overflow
    • tracepoint:io_uring:io_uring_cqring_wait
    • tracepoint:io_uring:io_uring_create
    • tracepoint:io_uring:io_uring_defer
    • tracepoint:io_uring:io_uring_fail_link
    • tracepoint:io_uring:io_uring_file_get
    • tracepoint:io_uring:io_uring_link
    • tracepoint:io_uring:io_uring_local_work_run
    • tracepoint:io_uring:io_uring_poll_arm
    • tracepoint:io_uring:io_uring_queue_async_work
    • tracepoint:io_uring:io_uring_register
    • tracepoint:io_uring:io_uring_req_failed
    • tracepoint:io_uring:io_uring_short_write
    • tracepoint:io_uring:io_uring_submit_req (previously, tracepoint:io_uring:io_uring_submit_sqe on older kernels)
    • tracepoint:io_uring:io_uring_task_add
    • tracepoint:io_uring:io_uring_task_work_run
  • Trace flow when event flags set IO-uring SQE link to see user enforced ordering of events.
  • Show when the user picks up the completion so that we can see the ring filling/freeing up

• Syscall track

  • io_uring_setup
  • io_uring_register
  • io_uring_enter

• IO-worker tracks

  • Show number of io-workers
  • Connected with flows

Usage

To trace a program, you will first need to run the uring_trace binary in a separate process. The uring_trace will detect when a new uring instance is spawned and record events from there. We do not offer the option to spawn your process you want to trace directly from the uring_trace binary. This is simply because uring_trace requires root priviledges to run and it would be bad to elevate your process run level.

opam install uring-trace
# replace $ with sudo
$ uring-trace

# In a separate terminal
<execute your program>

You can also use uring-trace --help to see the man page for other options.

Once you've finished tracing or would like to stop tracing, hit Ctrl-C and you should find a trace.fxt file in the same directory as uring-trace/src which you can load into perfetto to explore what io-uring was doing under the hood.

Undesirable Behaviours

This tool reads events through a shared ring buffer with the kernel. As such there is a possibility that events are overwritten before they are read and processed when tracing busy workloads. This can result in trace visualizations with missing events that look strange. To workaround this, the tracing tool has a sampling parameter that can be tuned to trace only a percentage of the requests coming in.