性能和优化

本文档概述了一些技术和工具，这些技术和工具可以帮助您更有效地运行Django代码——更快，并且使用更少的系统资源。

介绍

通常，首先要考虑的是编写工作的代码，它的逻辑功能与产生预期输出所需的逻辑功能相同。然而，有时，这并不足以使代码像人们所希望的那样高效地工作。

在这种情况下，需要的是一些东西——在实践中，通常是一组东西——以提高代码的性能，而不影响或只影响其行为。

一般方法

你在为什么而优化？

清楚地理解你所说的“绩效”是什么很重要，因为它不仅仅是一个指标。

提高速度可能是程序最明显的目标，但有时可能会寻求其他性能改进，例如降低内存消耗或减少对数据库或网络的要求。

一个领域的改进通常会提高另一个领域的性能，但并不总是如此；有时甚至会牺牲另一个领域的性能。例如，一个程序速度的提高可能会导致它使用更多的内存。更糟糕的是，如果速度提高太过内存不足，以致于系统开始耗尽内存，那么你所做的弊大于利。

还有其他的权衡。你自己的时间是一个宝贵的资源，比CPU时间更宝贵。一些改进可能太难实现，或者可能影响代码的可移植性或可维护性。并非所有的性能改进都值得付出努力。

所以，你需要知道你的目标是什么样的性能改进，你也需要知道你有一个很好的理由去瞄准那个方向——而且你需要：

性能标竿

仅仅猜测或假设代码中存在效率低下的原因是没有好处的。

Django工具

`django-debug-toolbar <https://github.com/jazzband/django-debug-toolbar/>`_是一个非常方便的工具，它可以深入了解您的代码正在做什么以及花费了多少时间。特别是它可以显示您的页面生成的所有SQL查询，以及每个查询所用的时间。

第三方面板也可用于工具栏，可以（例如）报告缓存性能和模板呈现时间。

第三方服务

There are a number of free services that will analyze and report on theperformance of your site's pages from the perspective of a remote HTTP client,in effect simulating the experience of an actual user.

These can't report on the internals of your code, but can provide a usefulinsight into your site's overall performance, including aspects that can't beadequately measured from within Django environment. Examples include:

• Yahoo's Yslow
• Google PageSpeed
  There are also several paid-for services that perform a similar analysis,including some that are Django-aware and can integrate with your codebase toprofile its performance far more comprehensively.

Get things right from the start

Some work in optimization involves tackling performance shortcomings, but someof the work can simply be built in to what you'd do anyway, as part of the goodpractices you should adopt even before you start thinking about improvingperformance.

In this respect Python is an excellent language to work with, because solutionsthat look elegant and feel right usually are the best performing ones. As withmost skills, learning what "looks right" takes practice, but one of the mostuseful guidelines is:

Work at the appropriate level

Django offers many different ways of approaching things, but just because it'spossible to do something in a certain way doesn't mean that it's the mostappropriate way to do it. For example, you might find that you could calculatethe same thing - the number of items in a collection, perhaps - in aQuerySet, in Python, or in a template.

However, it will almost always be faster to do this work at lower rather thanhigher levels. At higher levels the system has to deal with objects throughmultiple levels of abstraction and layers of machinery.

That is, the database can typically do things faster than Python can, which cando them faster than the template language can:

# QuerySet operation on the database
# fast, because that's what databases are good at
my_bicycles.count()
 
# counting Python objects
# slower, because it requires a database query anyway, and processing
# of the Python objects
len(my_bicycles)
 
# Django template filter
# slower still, because it will have to count them in Python anyway,
# and because of template language overheads
{{ my_bicycles|length }}

Generally speaking, the most appropriate level for the job is the lowest-levelone that it is comfortable to code for.

注解

The example above is merely illustrative.

Firstly, in a real-life case you need to consider what is happening beforeand after your count to work out what's an optimal way of doing it in thatparticular context. The database optimization documents describes acase where counting in the template would be better.

Secondly, there are other options to consider: in a real-life case, {{
my_bicycles.count }}, which invokes the QuerySet count() methoddirectly from the template, might be the most appropriate choice.

Caching

Often it is expensive (that is, resource-hungry and slow) to compute a value,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 acomprehensive caching framework, as well as other smaller pieces of cachingfunctionality.

The caching framework

Django's caching framework offers very significantopportunities for performance gains, by saving dynamic content so that itdoesn't need to be calculated for each request.

For convenience, Django offers different levels of cache granularity: you cancache the output of specific views, or only the pieces that are difficult toproduce, or even an entire site.

Implementing caching should not be regarded as an alternative to improving codethat's performing poorly because it has been written badly. It's one of thefinal steps towards producing well-performing code, not a shortcut.

cached_property

It's common to have to call a class instance's method more than once. Ifthat function is expensive, then doing so can be wasteful.

Using the cached_property decorator saves thevalue returned by a property; the next time the function is called on thatinstance, it will return the saved value rather than re-computing it. Note thatthis only works on methods that take self as their only argument and thatit changes the method to a property.

Certain Django components also have their own caching functionality; these arediscussed below in the sections related to those components.

Understanding laziness

Laziness is a strategy complementary to caching. Caching avoidsrecomputation by saving results; laziness delays computation until it'sactually required.

Laziness allows us to refer to things before they are instantiated, or evenbefore it's possible to instantiate them. This has numerous uses.

For example, lazy translation can be used before thetarget language is even known, because it doesn't take place until thetranslated string is actually required, such as in a rendered template.

Laziness is also a way to save effort by trying to avoid work in the firstplace. That is, one aspect of laziness is not doing anything until it has to bedone, because it may not turn out to be necessary after all. Laziness cantherefore have performance implications, and the more expensive the workconcerned, the more there is to gain through laziness.

Python provides a number of tools for lazy evaluation, particularly through thegenerator and generator expression constructs. It's worthreading up on laziness in Python to discover opportunities for making use oflazy patterns in your code.

Laziness in Django

Django is itself quite lazy. A good example of this can be found in theevaluation of QuerySets. QuerySets are lazy.Thus a QuerySet can be created, passed around and combined with otherQuerySets, without actually incurring any trips to the database to fetchthe items it describes. What gets passed around is the QuerySet object, notthe collection of items that - eventually - will be required from the database.

On the other hand, certain operations will force the evaluation of aQuerySet. Avoiding the premature evaluation ofa QuerySet can save making an expensive and unnecessary trip to thedatabase.

Django also offers a keep_lazy() decorator.This allows a function that has been called with a lazy argument to behavelazily itself, only being evaluated when it needs to be. Thus the lazy argument- which could be an expensive one - will not be called upon for evaluationuntil it's strictly required.

Databases

Database optimization

Django's database layer provides various ways to help developers get the bestperformance from their databases. The database optimization documentation gathers together links to the relevantdocumentation and adds various tips that outline the steps to take whenattempting to optimize your database usage.

Other database-related tips

Enabling Persistent connections can speed up connections to thedatabase accounts for a significant part of the request processing time.

This helps a lot on virtualized hosts with limited network performance, for example.

HTTP performance

中间件

Django comes with a few helpful pieces of middlewarethat can help optimize your site's performance. They include:

ConditionalGetMiddleware

Adds support for modern browsers to conditionally GET responses based on theETag and Last-Modified headers. It also calculates and sets an ETag ifneeded.

GZipMiddleware

Compresses responses for all modern browsers, saving bandwidth and transfertime. Note that GZipMiddleware is currently considered a security risk, and isvulnerable to attacks that nullify the protection provided by TLS/SSL. See thewarning in GZipMiddleware for more information.

会话

Using cached sessions

Using cached sessions may be a way to increaseperformance by eliminating the need to load session data from a slower storagesource like the database and instead storing frequently used session data inmemory.

Static files

Static files, which by definition are not dynamic, make an excellent target foroptimization gains.

ManifestStaticFilesStorage

By taking advantage of web browsers' caching abilities, you caneliminate network hits entirely for a given file after the initial download.

ManifestStaticFilesStorage appends acontent-dependent tag to the filenames of static files to make it safe for browsers to cache themlong-term without missing future changes - when a file changes, so will thetag, 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 unnecessary whitespace, newlines, andcomments, and shorten variable names, and thus reduce the size of the documentsthat your site publishes.

Template performance

Note that:

• using {% block %} is faster than using {% include %}
• heavily-fragmented templates, assembled from many small pieces, can affectperformance

The cached template loader

Enabling the cached template loader often improves performancedrastically, as it avoids compiling each template every time it needs to berendered.

Using different versions of available software

It can sometimes be worth checking whether different and better-performingversions of the software that you're using are available.

These techniques are targeted at more advanced users who want to push theboundaries of performance of an already well-optimized Django site.

However, they are not magic solutions to performance problems, and they'reunlikely to bring better than marginal gains to sites that don't already do themore basic things the right way.

注解

It's worth repeating: reaching for alternatives to software you'realready using is never the first answer to performance problems. Whenyou reach this level of optimization, you need a formal benchmarkingsolution.

Newer is often - but not always - better

It's fairly rare for a new release of well-maintained software to be lessefficient, but the maintainers can't anticipate every possible use-case - sowhile being aware that newer versions are likely to perform better, don'tsimply assume that they always will.

This is true of Django itself. Successive releases have offered a number ofimprovements across the system, but you should still check the real-worldperformance of your application, because in some cases you may find thatchanges mean it performs worse rather than better.

Newer versions of Python, and also of Python packages, will often performbetter too - but measure, rather than assume.

注解

Unless you've encountered an unusual performance problem in a particularversion, you'll generally find better features, reliability, and securityin a new release and that these benefits are far more significant than anyperformance you might win or lose.

Alternatives to Django's template language

For nearly all cases, Django's built-in template language is perfectlyadequate. However, if the bottlenecks in your Django project seem to lie in thetemplate system and you have exhausted other opportunities to remedy this, athird-party alternative may be the answer.

Jinja2 can offer performance improvements,particularly when it comes to speed.

Alternative template systems vary in the extent to which they share Django'stemplating language.

注解

If you experience performance issues in templates, the first thing to dois to understand exactly why. Using an alternative template system mayprove faster, but the same gains may also be available without going tothat trouble - for example, expensive processing and logic in yourtemplates could be done more efficiently in your views.

Alternative software implementations

It may be worth checking whether Python software you're using has beenprovided in a different implementation that can execute the same code faster.

However: most performance problems in well-written Django sites aren't at thePython execution level, but rather in inefficient database querying, caching,and templates. If you're relying on poorly-written Python code, yourperformance problems are unlikely to be solved by having it execute faster.

Using an alternative implementation may introduce compatibility, deployment,portability, or maintenance issues. It goes without saying that before adoptinga non-standard implementation you should ensure it provides sufficientperformance gains for your application to outweigh the potential risks.

With these caveats in mind, you should be aware of:

PyPy

PyPy is an implementation of Python in Python itself(the 'standard' Python implementation is in C). PyPy can offer substantialperformance gains, typically for heavyweight applications.

A key aim of the PyPy project is compatibility with existing Python APIs and libraries.Django is compatible, but you will need to check the compatibility of otherlibraries you rely on.

C implementations of Python libraries

Some Python libraries are also implemented in C, and can be much faster. Theyaim to offer the same APIs. Note that compatibility issues and behaviordifferences are not unknown (and not always immediately evident).