Django template rendering speed

Question

i'm having a performance issue while rendering a django template with a prefetch queryset witch is not really large (~1000 objects in production environment / ~200 objects in developement environment) but has several level of nesting.

Here is the prefetch :

stories = Story.objects.select_related(
            'commande',
            'commande__societe',
            'commande__plateforme',
            'client',).prefetch_related(
            Prefetch('crm_message_related',
                     queryset=Message.objects.select_related(
                         'societe',
                         'plateforme',
                         'type_message').order_by('-date_received')),
            Prefetch('commande__crm_feedback_related'),
            Prefetch('commande__crm_guarantee_related'),
            Prefetch('commande__soyouz_item_related',
                     queryset=Item.objects.select_related(
                         'etat', 'produit', 'produit__produit_enhanced', )),
            Prefetch('commande__tagged_items__tag'),
            Prefetch('commande__soyouz_item_related__tagged_items__tag'),
            Prefetch('commande__soyouz_item_related__soyouz_annulationclient_related'),
            Prefetch('commande__soyouz_item_related__soyouz_achat_related'),
            Prefetch('commande__soyouz_item_related__soyouz_achat_related__soyouz_receptionachat_related'),
            Prefetch('commande__soyouz_item_related__soyouz_achat_related__soyouz_annulationachat_related'),
            Prefetch('soyouz_action_related'),
            Prefetch('soyouz_action_related__receptionmessage__message',
                     to_attr='receptionmessages',
                     queryset=Message.objects.order_by(
                         '-date_received').select_related(
                         'societe', 'type_message', 'plateforme'))
        ).order_by(
            '-commande__date_commande',
            '-date_created'
        ).all().distinct()

The SQL run smoothly, thats not the issue.

The issue is when i try to render my template witch is basicaly

{% for story in stories %}
    {% with items=story.commande.soyouz_item_related.all %}
    {% for item in items %}
        {% for tag in item.tagged_items.all %}
            <button>{{ tag.tag }}</button>
        {% endfor %}
        {{ item.titre|safe|truncatechars:50 }}
    {% endfor %}
{% endfor %}

It takes about 10 sec to display the page in production environment.

I profiled it in my development environment with django-template-timing (https://github.com/orf/django-debug-toolbar-template-timings) and indeed:

name            Times  Tot Time    Queries   Queries Time
stories.html    1      2814,1 ms   0         0 ms

3 seconds for 3 loops with 200 data (in development environment) ? It seems a lot.

Any ideas to improve the speed of template rendering ?

Thanks a lot!

Have you tried with `DEBUG = False`? Django's template language is not known for its speed, but this seems excessive, and I don't believe for-loops are particularly slow. Just to be sure: are you talking about 200 stories, tags, total items? — knbk, Jun 30 '15 at 10:25
in dev : 200 stories - 1 item per story - 0 tag per item in prod : 1000 stories - 1 or 2 item per story - 1 tag per item — Tigrou, Jun 30 '15 at 11:53

score 7 · Answer 1 · edited May 23 '17 at 11:54

Although django-template-timing reports the rendering time to be 2.8s, I suspect most of the time is consumed in executing the complex SQL query.

In Django, QuerySets are evaluated lazily. This means, the statement you posted Story.objects.select_related(...).distinct() does NOT execute the query in database. The SQL gets executed later. Since you didn't post all the code before rendering the template, I am not sure if the QuerySet gets evalueted before rendering. If not, it may be executed when iterating stories in the template:

{% for story in stories %}

If this is the case, then maybe improving your SQL can reduce the time.

You can check if SQL execution is the culprit by inserting this before rendering the template:

stories = [story for story in stories]

This iteration gets the SQL executed before rendering. After this, if django-template-timing reports a much shorter rendering time, you know SQL is the problem.

If it is indeed a template-rendering performance issue, there are some alternatives:

Use a more performant template engine, like Jinja2.
Chances are there is room for improving rendering performance with Django template: https://stackoverflow.com/a/26592207/954376.

We found the best solution : jinja + paginate when possible – chocobn69 Jul 03 '15 at 08:05 — chocobn69, Jul 03 '15 at 08:05

score 1 · Answer 2 · answered Jun 30 '15 at 11:42

1

Use a custom tag and generate the output outside the template. That will help avoid the django template overhead.

answered Jun 30 '15 at 11:42

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Django template rendering speed

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