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Addressed inaccuracies in performance docs 

* Move the discussion of CachedStaticFilesStorage to the section about
  HTTP. It's really about client-side caching. It doesn't fit with the
  caching utilities from django.utils.functional.

* Tone down the warning against Pypy as per Alex' feedback. It's a valid
  choice for advanced users who are comfortable using a non-standard
  stack.

* Generally reworded the 'Using different versions of available software'
  section.

* Some other minor adjustments to the document.
This commit is contained in:
Aymeric Augustin 2013-09-21 10:07:10 +02:00 committed by evildmp
parent a991b01167
commit 11187386f4
1 changed files with 68 additions and 34 deletions
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102
docs/topics/performance.txt
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@ -150,8 +150,8 @@ so there can be huge benefit in saving the value to a quickly accessible cache,
ready for the next time it's required.
It's a sufficiently significant and powerful technique that Django includes a
comprehensive caching framework, as well as numerous other opportunities to
make use of caching.
comprehensive caching framework, as well as other smaller pieces of caching
functionality.
:doc:`The caching framework </topics/cache>`
--------------------------------------------
@ -168,14 +168,8 @@ Implementing caching should not be regarded as an alternative to improving code
that's performing poorly because it has been written badly. It's one of the
final steps towards producing well-performing code, not a shortcut.
Other opportunities for caching
-------------------------------
Beyond the caching framework, Django offers other smaller pieces of caching
functionality.
:class:`~django.utils.functional.cached_property`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-------------------------------------------------
It's common to have to call a class instances's method more than once. If
that function is expensive, then doing so can be wasteful.
@ -186,14 +180,8 @@ the saved value rather than re-computing it. Note that this only works on
methods that take ``self`` as their only argument and that it changes the
method to a property.
:class:`~django.contrib.staticfiles.storage.CachedStaticFilesStorage`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
:class:`~django.contrib.staticfiles.storage.CachedStaticFilesStorage` appends a
content-dependent tag to the filenames of :doc:`static files
</ref/contrib/staticfiles>` to make it safe for browsers to cache them
long-term without missing future changes - when a file changes, so will the
tag, so browsers will reload the asset automatically.
Certain Django components also have their own caching functionality; these are
discussed below in the sections related to those components.
Understanding laziness
======================
@ -284,11 +272,31 @@ time. Note that GZipMiddleware is currently considered a security risk, and is
vulnerable to attacks that nullify the protection provided by TLS/SSL. See the
warning in :class:`~django.middleware.gzip.GZipMiddleware` for more information.
Third-party HTTP tools
----------------------
Static files
------------
There are numerous third-party Django tools and packages available, notably
ones that are able to "minify" and compress HTML, CSS and JavaScript.
Static files, which by defintion are not dynamic, make an excellent target for
optimization gains.
:class:`~django.contrib.staticfiles.storage.CachedStaticFilesStorage`
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
By taking advantage of web browsers' caching abilities, you can
eliminate network hits entirely for a given file after the initial download.
:class:`~django.contrib.staticfiles.storage.CachedStaticFilesStorage` appends a
content-dependent tag to the filenames of :doc:`static files
</ref/contrib/staticfiles>` to make it safe for browsers to cache them
long-term without missing future changes - when a file changes, so will the
tag, so browsers will reload the asset automatically.
"Minification"
^^^^^^^^^^^^^^
Several third-party Django tools and packages provide the ability to "minify"
HTML, CSS, and JavaScript. They remove uneccessary whitespace, newlines, and
comments, and shorten variable names, and thus reduce the size of the documents
that your site publishes.
Template performance
====================
@ -313,13 +321,19 @@ Using different versions of available software
It can sometimes be worth checking whether different and better-performing
versions of the software that you're using are available.
This may be helpful, but is unlikely to solve a serious performance problem.
You won't usually gain performance advantages that are better than marginal.
These techniques are targeted at more advanced users who want to push the
boundaries of performance of an already well-optimized Django site.
However, they are not magic solutions to performance problems, and they're
unlikely to bring better than marginal gains to sites that don't already do the
more basic things the right way.
.. note::
It's worth repeating: **reaching for alternatives to software you're
already using is very rarely the answer to performance problems**.
already using is never the first answer to performance problems**. When
you reach this level of optimization, you need a formal benchmarking
solution.
Newer is often - but not always - better
----------------------------------------
@ -369,16 +383,36 @@ templating language.
Alternative software implementations
------------------------------------
It *may* be worth checking whether Python software you're using has been
It may be worth checking whether Python software you're using has been
provided in a different implementation that can execute the same code faster.
However, most Django performance problems in well-written code are typically
not to be found at the Python execution level, but rather in inefficient
database querying, caching, and templates (and if you're relying on
poorly-written Python code, your performance problems are very unlikely to be
solved by having it execute faster).
However: most performance problems in well-written Django sites aren't at the
Python execution level, but rather in inefficient database querying, caching,
and templates. If you're relying on poorly-written Python code, your
performance problems are unlikely to be solved by having it execute faster.
Avoid using C implementations of Python libraries or non-standard Python
implementations like `PyPy <http://pypy.org/>`_ in search of performance gains,
unless you are sure they are appropriate for your application. Any gains are
likely to be small, and compatibility issues are common.
Using an alternative implementation may introduce compatibility, deployment,
portability, or maintenance issues. It goes without saying that before adopting
a non-standard implementation you should ensure it provides sufficient
performance gains for your application to outweigh the potential risks.
With these caveats in mind, you should be aware of:
`PyPy <http://pypy.org/>`_
^^^^^^^^^^^^^^^^^^^^^^^^^^
`PyPy <http://pypy.org/>`_ is an implementation of Python in Python itself (the
'standard' Python implementation is in C). PyPy can offer substantial
performance gains, typically for heavyweight applications.
A key aim of the PyPy project is `compatibility
<http:://pypy.org/compat.html>`_ with existing Python APIs and libraries.
Django is compatible, but you will need to check the compatibility of other
libraries you rely on.
C implementations of Python libraries
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Some Python libraries are also implemented in C, and can be much faster. They
aim to offer the same APIs. Note that compatibility issues and behaviour
differences are not unknown (and not always immediately evident).