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Debug your database with Flame Graphs

docs/kb/flamegraph.rst

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===================================== 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 ScyllaDB code path/functions are using the most time. For instance, when you have latency issues.
  • Need to compare time spent in particular ScyllaDB 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

#. Install flamegraph and flamegraph-stackcollapse-perf

From DNF

.. code-block:: shell

  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

.. code-block:: shell

  git clone https://github.com/brendangregg/FlameGraph
  cd FlameGraph

#. Run the following perf commands, using :doc:Map CPU to ScyllaDB Shards </kb/map-cpu> and :doc:Using the perf utility with ScyllaDB </kb/use-perf> for reference.

.. code-block:: shell

  sudo perf record --call-graph dwarf -C <CPU on which you are onrecording>
  sudo perf script | stackcollapse-perf.pl | flamegraph.pl > some_name.svg

#. 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.

.. image:: flamegraph.svg

Tips

  • On the CPU you are recording, try to load ScyllaDB 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.