[[PageOutline(2-3,Contents)]] = 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 [http://genshi.edgewall.org/ Genshi template engine] in 0.11, but ClearSilver was **very** fast and memory lenient. While we managed to keep Genshi memory usage somewhat in control (#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 template engine once again, to one that would combine the advantages of Genshi (pure Python, nice templates, flexible) and ClearSilver (speed!). Such a beast seems to exist now: **[http://jinja.pocoo.org/2/documentation/ Jinja2]**. == Overview of activities and progress There's an experimental branch which supports this proposal: [source:cboos.git@jinja2] \\ (mirror available in [https://github.com/cboos/trac.git github] - [[Image(https://travis-ci.org/cboos/trac.svg?branch=jinja2,link=https://travis-ci.org/cboos/trac,valign=bottom)]] on Travis, [[Image(https://ci.appveyor.com/api/projects/status/kqgv4awct01hsl7t/branch/jinja2,link=https://ci.appveyor.com/project/cboos/trac/branch/jinja2,valign=bottom)]] on AppVeyor) [=#Status Status] of the branch (2016-03-20): - ported **66%** of the Genshi templates, the automated tests should stays green - DONE clarify upgrade path for plugins that came to rely on `ITemplateStreamFilter`s? 127/898 plugins (14.1%) on trac-hacks.org use `filter_stream()` -> see [PortingFromGenshiToJinja#ReplacingITemplateStreamFilter replacing ITemplateStreamFilter] * DONE clarify how to handle themeing? -> see HtmlTemplates#Jinjaarchitecture * DONE rewrite tag builders ~~or use lightweight string templates~~? -> [PortingFromGenshiToJinja#tag tag] `Fragment`/`Element` builder API has been reimplemented - TODO accesskey support - TODO hack `ITemplateStreamFilter` support for Jinja2 templates - TODO site.html replacement, for example try to reproduce t.e.o customizations * others? See also [googlegroups:trac-dev:fc8d8c0447140110 this Trac-Dev discussion] from 2010, which is still pertinent. While we managed to release Genshi 0.6 since then, the issue is a recurring one, see this recent (2016-01) [gmessage:trac-users:PYqQ4UDRnl8/wg8lQzrGDAAJ Genshi question] on Trac-Users. The topic is now (Feb / March 2016) again discussed on [gdiscussion:trac-dev:KqWPQWuZ63k trac-dev]. The Jinja2 template engine has the peripheral benefit of being used by the Django community: [gdiscussion:trac-dev/KqWPQWuZ63k/GPfda0_PDgAJ]. == Experimenting with Jinja2 (2.8) Nothing like a few numbers to make a point. These are the timings for rendering !r3871 (//don't try this one here, please//), with the diff options set to side-by-side, in place modifications, served by tracd on my development laptop. This generates a page weighing from 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 [http://www.pocoo.org/projects/markupsafe/ 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 parentheses) 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. In this case, it can 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 another experiment, I used the [pypi:memory_profiler] on Windows, which provided the following results when rendering a big changeset, for a rendered page weighing 4.48MB (it was a side-by-side diff): {{{ Genshi xhtml: 293.612 total (load=0.017, generate=0.017, filter=0.000, render=293.578) Filename: d:\Trac\repos\trunk\trac\web\chrome.py Line # Mem usage Increment Line Contents ================================================ 1283 101.6 MiB 0.0 MiB @profile 1284 def genshi(): 1285 101.6 MiB 0.0 MiB buffer = StringIO() 1286 101.6 MiB 0.0 MiB t5 = time.time() 1287 101.6 MiB 0.0 MiB stream.render(method, doctype=doctype, out=buffer, 1288 153.2 MiB 51.6 MiB encoding='utf-8') 1289 158.0 MiB 4.9 MiB gs = buffer.getvalue().translate(_translate_nop, 1290 158.0 MiB 0.0 MiB _invalid_control_chars) 1291 158.0 MiB 0.0 MiB t6 = time.time() 1292 158.0 MiB 0.0 MiB show_times('Genshi', t2 - t1, t4 - t3, t5a - t4a, t6 - t5, 1293 158.1 MiB 0.0 MiB method) 1294 158.1 MiB 0.0 MiB return gs }}} vs. {{{ Line # Mem usage Increment Line Contents ================================================ 1255 101.6 MiB 0.0 MiB @profile 1256 def jinja(mode='render'): 1257 101.6 MiB 0.0 MiB if jtemplate: 1258 101.6 MiB 0.0 MiB j5 = time.time() 1259 101.6 MiB 0.0 MiB if mode == 'render': 1260 111.1 MiB 9.4 MiB js = jtemplate.render(jdata) 1261 111.1 MiB 0.0 MiB j5a = time.time() 1262 115.4 MiB 4.3 MiB js = js.encode('utf-8') \ 1263 115.4 MiB 0.0 MiB .translate(_translate_nop, 1264 106.7 MiB -8.7 MiB _invalid_control_chars) 1265 106.7 MiB 0.0 MiB j6 = time.time() 1266 106.7 MiB 0.0 MiB show_times('Jinja2', j2 - j1, 0, j5a - j5, j6 - j5a, 1267 106.7 MiB 0.0 MiB 'html') 1268 106.7 MiB 0.0 MiB return js }}} 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 (that page was modeled after the old PortingFromClearSilverToGenshi page). See also PortingFromGenshiToJinja/Example for a full example presented side-by-side. [=#jinjachecker] To facilitate the creation of error-free Jinja2 templates for HTML (or XML), we also wrote a utility called [PortingFromGenshiToJinja/Checker jinjachecker], which helps troubleshoot the most common nesting problems.