.. _sardana-explanation-performance:

=============================
Performance and scalability
=============================

This chapter collects the main concepts behind Sardana's performance
characteristics. It is intentionally short and only states what can be
grounded in the code base and the project changelog.

Events instead of polling
=========================

Sardana relies on the Tango_ event system rather than on client polling.
The sardana Tango_ devices configure their attributes for change events
and *push* them from the server code whenever the kernel produces a new
value — e.g. position updates while a motor is moving, value updates of
experimental channels, state transitions, or macro output and status from a
Door. Clients (Spock, Taurus_ widgets, the macro :term:`API` acting on
elements) subscribe to these events and react when something actually
changes.

The consequence for scalability is that the cost of monitoring is
proportional to the rate of *change*, not to the number of connected
clients times a polling frequency: many GUIs can watch the same beamline
without multiplying the load on the Pool, and updates arrive with low
latency instead of at the next polling period. During fast operations the
event rate itself can become the bottleneck on the client side; for
example, Spock output of high frequency (100 Hz) scans can be decimated
(``OutputDecimation`` view option, disabled by default).

MacroServer startup
===================

When the MacroServer starts (and whenever macro libraries are reloaded), it
scans all macro libraries on its ``MacroPath``. Up to Sardana 3.6 this
included eager source-code introspection of every discovered macro. Since
Sardana 3.7.0 the code introspection in `~sardana.sardanameta.SardanaCode`
and `~sardana.sardanameta.SardanaLibrary` is *lazy*: source lines are only
extracted (and cached) the first time they are actually requested, e.g. when
a client asks to display a macro's code. With large macro libraries this
significantly improves the MacroServer startup time, since startup cost is
reduced to importing the modules and registering the macro metadata.

Large installations
===================

A few architectural properties matter when an installation grows to many
elements:

* **The sardana server can be split.** A single process may host both the
  Pool and the MacroServer, but they can also be deployed as separate
  servers, and one MacroServer can connect to several Pools (see
  :ref:`sardana-devel-global-overview`). This allows distributing hardware
  over several processes/hosts and isolating slow hardware in its own Pool.
* **Controllers group hardware access.** Operations on multiple elements of
  one controller are dispatched through the grouped controller :term:`API`
  (``PreStartAll``/``StartAll``-style interfaces), so a controller can
  translate, for instance, the start of many axes or channels into a single
  hardware command instead of one communication per element.
* **Measurement group size.** The acquisition of a measurement group is
  orchestrated per controller, and its overhead grows with the number of
  involved controllers and channels. For software synchronized continuous
  scans, large measurement groups increase the risk that channels cannot
  follow the synchronization pace, requiring a larger latency time or data
  interpolation (see :ref:`sardana-users-scan-continuous`).
* **Continuous scans move the load off the software stack.** Where step scan
  performance is dominated by per-point software overhead (motion start/stop,
  acquisition start, readout), hardware synchronized continuous scans reduce
  the software role to preparation and data collection (see
  :ref:`sardana-explanation-synchronization`), including the velocity
  adjustment and path extension mechanics described in
  :ref:`sardana-users-scan-continuous`.

.. todo:: Provide measured figures (element counts, event rates, startup
   times) for representative installations. No verified numbers are
   available in the repository at the time of writing.

.. _Tango: http://www.tango-controls.org/
.. _Taurus: http://packages.python.org/taurus/