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#55 in Profiling

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Rust support for the coz Causal Profiler



First, follow the instructions in coz to install the coz command.

Next, coz is a profiler that, for the best results, typically requires source-level modifications of your code. To do this first add this to your Cargo.toml

coz = "0.1"

Then you'll want to either add throughput or latency tracepoints. More information on this can be found upstream. If you have something you'd wish would execute more often, you can add:

fn main() {
    loop {
        // ...

        // For example you wish this `loop` executed more iterations
        coz::progress!(); // equivalent of `COZ_PROGRESS`

Note that coz::progress!("name") is the equivalent of COZ_PROGRESS_NAMED as well.

If you'd like to profile the latency of an operation you can instead use:

// Boy I wish this function executed more quickly...
fn foo() {

    // ...


After you've instrumented your code, you need to also ensure that you're compiling with DWARF debug information. To do this you'll want to configure Cargo.toml again:

debug = 1

Next up you'll build your application with cargo build --release, and then finally you can run it with coz run --- ./target/release/$your_binary.


Known caveats so far to generate a report that collects information are:

  • Rust programs by default segfault when run with coz with an issue related to plasma-umass/coz#110. Rust programs set up a sigaltstack to run segfault handlers to print "you ran out of stack", but this alternate stack is too small to run the SIGPROF handler that coz installs. To handle this this crate provides a coz::thread_init() function which will increase the sigaltstack size that Rust installs by default to something large enough to run coz. If you see segfaults, or corrupt reports, you may wish to manually call coz::thread_init() instead of waiting for this crate to automatically call it for you.

  • Debug information looks to be critical to get a report from coz. Make sure that your program is compiled with at least line-table information (debug = 1) to ensure you get the best experience using coz.

  • Currently coz only works on Linux, and while this crate should compile on all platforms it only actually does something on Linux.


You can find an example toy program at examples/toy.rs in this repository, and we can execute it with coz:

$ cargo build --release
$ coz run --- ./target/release/examples/toy
[profiler.cpp:75] Starting profiler thread

That should generate profile.coz in the current directory, which if you plot that should look something like this:

plot of toy.rs

Note that I'm still learning myself the best sorts of applications to run on as well as the best sorts of questions to ask coz and where to put latency/throughput points. If you've got ideas or good examples, please feel free to add them here!