Switch to the Jinja2 Template Engine

We've decided some time ago to remove the legacy support for the ClearSilver template engine, for Trac 1.0 (r10570). Clearsilver had its share of inconveniences, enough that we decided to switch to the nicer ​Genshi template engine in 0.11, but to be honest ClearSilver was very fast and memory lenient. While we managed to keep Genshi memory usage somewhat in control (remember #6614?), the speed was never really adequate, especially for big changesets and for displaying source files over a few thousand lines of code (see TracDev/Performance#Genshi for details).

So one solution would be to switch once again, to a template engine that would combine the advantages of Genshi (pure Python, nice templates, flexible) and ClearSilver (speed!). Such a beast seems to exist now: ​Jinja2.

Several points remain to be clarified:

• what will be the upgrade path for plugins that came to rely on ITemplateStreamFilters?
  • 127/898 plugins (14.1%) on trac-hacks.org use filter_stream()
  • → replacing ITemplateStreamFilter
• how to handle themeing? → see HtmlTemplates#Jinjaarchitecture
• should we rewrite tag builders or use lightweight string templates? → tag Fragment/Element builder has been reimplemented
• others?

See also ​this Trac-Dev discussion from 2010, which is still pertinent. Well, obviously we managed to release Genshi 0.6 since then, but the issue is a recurring one, see this recent (2016-01) ​Genshi question on Trac-Users.

Experimenting with Jinja2 (2.8)

Nothing like a few numbers to make a point ;-)

These are the timings for rendering r3871, with the diff options set to side-by-side, in place modifications, served by tracd on my development laptop. This generates a page weighing 11.5MB (Genshi) to 10.3MB (Jinja2) in size.

	Genshi				Jinja2
	stream		blob		generate		stream (5)		stream (10)		stream (100)		stream (1000)		blob
	1st	2nd	1st	2nd	1st	2nd	1st	2nd	1st	2nd	1st	2nd	1st	2nd	1st	2nd
TTFB	16600	15670	25530	24460	2020	1160	2030	1160	2070	1170	2150	1230	2280	1230	3370	2450
CD	16090	16050	387	1240	2820	2720	2730	2640	2730	2680	2470	2390	2350	2250	488	1060
Total	32690	31720	25917	25700	4840	3880	4760	3800	4800	3850	4620	3620	4630	3480	3850	3510
Rdr	—	—	23533	23273	—	—	—	—	—	—	—	—	—	—	1477	1263

Some explanations:

• Genshi (0.7 with speedups)
  • stream means we return content via Stream.serialize and send chunks as we have them
  • blob means we first generate all the content in memory with Stream.render, then send it at once
• Jinja2 (2.8 with ​speedups)
  • generate means we use Template.generate and send chunks as we have them
  • stream means we use the TemplateBuffer wrapper on the above, which groups a few chunks (given by the number in parenthesis) together before we send them; for a chunk size of 100, we get the best compromise: still a very low TTFB and a reduced Content download time; actually the sweet spot is probably between 10 and 100, and will most certainly depend on the actual content (I just tested 75 which gives 1160/2430 for example)
  • blob means we first generate all the content in memory with Template.render
• both:
  • 1st, is the time in ms for the first request, sent right after a server restart
  • 2nd, is the time in ms for the second request, sent just after the first (usually the 3rd and subsequent requests would show the same results as this 2nd request)

We measure:

• TTFB (Time to first byte), as given by Chrome network panel in the developer window
• CD (Content download), idem
• Rdr (template rendering time), mostly significant for the "blob" method otherwise it also takes the network latency into account

All values are given in milliseconds.

Note that even if the total "blob" time seems better than the total "stream" one, the lower TTFB is nevertheless a major benefit for the streaming variant, as this means the secondary requests can start earlier (and in this case, finish before the main request).

In addition, while I didn't measure precisely the memory usage, Genshi made the python.exe process jump from 109MB to 239MB while rendering the request (blob). The memory seems to be freed afterwards (there were no concurrent requests). By contrast, with Jinja2 the memory spike was 106MB to 126MB.

In summary, this means that for the big problematic pages, we can easily have a 10x speedup and more, by migrating to Jinja2, and this with a much lighter memory footprint. For smaller pages, the speed-up is between 5x to 10x as well.

Genshi to Jinja2 Migration

Some systematic comparison of the Genshi and Jinja2 template syntax can be seen in PortingFromGenshiToJinja#Changesinthetemplatesyntax (modeled after the old PortingFromClearSilverToGenshi page).

See also PortingFromGenshiToJinja/Example for a full example presented side-by-side.

jinjachecker

Jinja2 is very helpful when it detects any kind of error, as you always end up with a meaningful backtrace. Nevertheless, it can be tedious to get the nesting of control structures right.

To help with that, there's the contrib/jinjachecker.py​ tool.

python contrib/jinjachecker.py trac/wiki/templates/jwiki_view.html

It first analyzes the Jinja2 control structures, and tries to provide some helpful diagnostics in case of nesting or stylistic errors. It also adds curly braces to the statements, so if you have an editor which can do matching of brace pairs, you can quickly spot the origin o a nesting problem.

Finally, while the indentation of the statements is free in Jinja2 templates, being consistent with it also helps to ensure a proper nesting.

# -- Jinja2 check for 'trac/wiki/templates/jwiki_view.html'
   12    EXTENDS "jlayout.html"
   18          {BLOCK title
   19            {IF title:
   21            }IF
   22          }BLOCK title
   25        {BLOCK head
   26          SET modify_perm = 'WIKI_MODIFY' in perm(page.resource)
   27          SET is_not_latest = page.exists and page.version != latest_version
   31          {IF version or page.author == 'trac':
   33          }IF
   34          {IF modify_perm:
   40          }IF
   52        }BLOCK head
   56        {BLOCK content
   57          SET modify_perm = 'WIKI_MODIFY' in perm(page.resource)
   58          SET create_perm = 'WIKI_CREATE' in perm(page.resource)
   59          SET admin_perm = 'WIKI_ADMIN' in perm(page.resource)
   60          SET is_not_latest = page.exists and page.version != latest_version
   63          {IF version:
   67                {WITH
   68                  SET version = page.version
   69                  SET author = authorinfo(page.author)
   70                  SET date = pretty_dateinfo(page.time)
   71                  SET hef = href.wiki(page.name, action='diff', version=page.version)
   72                  {TRANS version, author, date, href
   77                  }TRANS
   78                }WITH
   84          }IF
   87            {IF page.exists:
   91              SET last_modification = (page.comment and
   97              {IF not version:
  100                {WITH
  101                  SET href = href.wiki(page.name, action='diff',
  103                  SET date = pretty_dateinfo(page.time)
  104                  {TRANS href, last_modification, date
  109                  }TRANS
  110                }WITH
  116              }IF
  117            ELSE:
  119              {TRANS name = name_of(page.resource)
  124              }TRANS
  126            }IF
  129          {WITH
  130            SET alist = attachments
  131            SET compact = True
  132            SET foldable = True
  133            INCLUDE "jlist_of_attachments.html"
  134          }WITH
  136          {WITH
  137            SET delete_perm = 'WIKI_DELETE' in perm(page.resource)
  138            SET rename_perm = 'WIKI_RENAME' in perm(page.resource)
  139            {IF modify_perm or create_perm or delete_perm:
  141              {IF modify_perm or create_perm:
  145                {IF is_not_latest and modify_perm:
  148                ELIF page.exists and modify_perm:
  151                ELIF not page.exists and create_perm:
  154                  {IF templates:
  161                    {FOR t in sorted(templates):
  165                    }FOR
  168                  }IF
  169                }IF
  173              {IF page.exists:
  174                {WITH alist = attachments
  175                  INCLUDE "jattach_file_form.html"
  176                }WITH
  177              }IF
  178            }IF
  180            {IF page.exists and rename_perm:
  187            }IF
  188            {IF page.exists and delete_perm:
  193                {IF page.version == latest_version:
  195                }IF
  199            }IF
  201          }IF
  202          }WITH
  204          {IF not page.exists and higher:
  208              {FOR markup in higher:
  210              }FOR
  213          }IF
  215          {IF not page.exists and related:
  219              {FOR markup in related:
  221              }FOR
  224          }IF
  230        }BLOCK content
# -- Jinja2 OK

As a second step, jinjachecker removes the Jinja2 markup and performs a validation of the document, using lxml.

# -- HTML check for 'trac/wiki/templates/jwiki_view.html'
    1 
...
   13 
   14 <!DOCTYPE html>
   15 <html>
   16   <head>
   17     <title>
   18 
   19 
   20       ${title} ${ super() }
   21 
   22 
   23     </title>
   24 
   25 
   26 
   27 
   28 
   29     ${ super() }
   30 
...
   52 
   53   </head>
   54 
   55   <body>
   56 
   57 
   58 
   59 
   60 
   61     <div id="content" class="${classes('wiki', create=not page.exists)}">
   62 
...
  213 
  214 
  215     </div>
  216 
  217     ${ super() }
  218 
  219 
  220   </body>
  221 </html>
# -- HTML OK