Samply: Command-line sampling profiler for macOS and Linux
GitHub - mstange/samply: Command-line sampling profiler for macOS and Linux
samply
samply is a command line CPU profiler which uses the Firefox profiler as its UI.
samply works on macOS and Linux. Windows support is planned.
In order to profile the execution of ./my-application, prepend samply record to the command invocation:
samply record ./my-application my-arguments
On Linux, samply uses perf events. You can grant temporary access by running:
echo '1' | sudo tee /proc/sys/kernel/perf_event_paranoid
Installation
You have the following options to install samply:
Install prebuilt binaries via shell script
curl --proto '=https' --tlsv1.2 -LsSf https://github.com/mstange/samply/releases/download/samply-v0.12.0/samply-installer.sh | sh
Install from crates.io with cargo
cargo install --locked samply
Build from source
git clone https://github.com/mstange/samply
cd samply
cargo build --release
./target/release/samply ...
Description
samply record ./my-application my-arguments
This spawns ./my-application my-arguments in a subprocess and records a profile of its execution. When the command finishes, samply opens
profiler.firefox.com in your default browser, loads the recorded profile in it, and starts a local webserver which serves symbol information and source code.
Then you can inspect the profile. And you can upload it.
Here's an example: https://share.firefox.dev/3j3PJoK
This is a profile of dump_syms, running on macOS, recorded as follows:
samply record ./dump_syms ~/mold-opt-libxul.so > /dev/null

You can see which functions were running for how long. You can see flame graphs and timelines. You can double-click functions in the call tree to open the source view, and see which lines of code were sampled how many times.
All data is kept locally (on disk and in RAM) until you choose to upload your profile.
samply is a sampling profiler and collects stack traces, per thread, at some sampling interval (the default 1000Hz, i.e. 1ms). On macOS, both on- and off-cpu samples are collected (so you can see under which stack you were blocking on a lock, for example). On Linux, only on-cpu samples are collected at the moment.
On Linux, samply needs access to performance events system for unprivileged users. For this, you can either:


Grant access until reboot:
echo '1' | sudo tee /proc/sys/kernel/perf_event_paranoid


Grant access more permanently:
sudo sysctl kernel.perf_event_paranoid=1


If using Linux 5.8 or later, you can try setting the CAP_PERFMON capability as effective and permitted for samply, though people have reported mixed results with this approach:
sudo setcap 'cap_perfmon+ep' `which samply`



If you still get a mmap failed error (an EPERM), you might also need to increase the mlock limit, e.g.:
sudo sysctl kernel.perf_event_mlock_kb=2048

Examples
Here's a profile from samply record rustup check: https://share.firefox.dev/3hteKZZ
I'll add some Linux examples when I get a chance.
Turn on debug info for full stacks
If you profile Rust code, make sure to profile a binary which was compiled in release mode and with debug info. This will give you inline stacks and a working source code view.
The best way is the following:

Create a global cargo profile called profiling, see below how.
Compile with cargo build --profile profiling.
Record with samply record ./target/profiling/yourrustprogram.

To create the profiling cargo profile, create a text file at ~/.cargo/config.toml with the following content:
[profile.profiling]
inherits = "release"
debug = true
Similar advice applies to other compiled languages. For C++, you'll want to make sure the -g flag is included in the compiler invocation.
Known issues
On macOS, samply cannot profile system commands, such as the sleep command or system python. This is because system executables are signed in such a way that they block the DYLD_INSERT_LIBRARIES environment variable, which breaks samply's ability to siphon out the mach_port of the process.
But you can profile any binaries that you've compiled yourself, or which are unsigned or locally-signed (such as anything installed by cargo install or by Homebrew).
License
Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.
