Debug your database with Flame Graphs

Flame Graphs are used as a debugging tool to identify latency and the part of the execution path that takes most of the CPU time. Use Flame Graphs when you:

  • Need to understand which Scylla code path/functions are using the most time. For instance, when you have latency issues.

  • Need to compare time spent in particular Scylla code paths/functions on different shards. For instance, when you have latency issues on one CPU but not on the other.

Run a Flame Graph

Procedure

  1. Install flamegraph and flamegraph-stackcollapse-perf

    From DNF

    dnf install flamegraph.noarch flamegraph-stackcollapse-perf.noarch
    

    If the above packages are not available in you distribution’s repositories, you can clone the FlameGraph git repository

    git clone https://github.com/brendangregg/FlameGraph
    cd FlameGraph
    
  2. Run the following perf commands, using Map CPU to Scylla Shards and Using the perf utility with Scylla for reference.

    sudo perf record --call-graph dwarf -C <CPU on which you are onrecording>
    sudo perf script | stackcollapse-perf.pl | flamegraph.pl > some_name.svg
    
  3. The result is an .svg file that is not just a picture but a dynamic diagram where you can search, zoom in, and zoom out. In order to enjoy a Flame Graph properly with all of its features you can open it in a browser such as Chrome or Firefox.

../../_images/flamegraph.svg

Tips

  • On the CPU you are recording, try to load Scylla to consume 100% of the CPU runtime. Otherwise you’ll see a lot of OS functions related to the idle time handling

  • Recording on all shards (e.g. using “perf record” -p parameter) may lead to confusing results recording the same symbol called from different threads (shards). This is not recommended.