Profiling results of an almost empty page with it's skin (panel Recent
Changes removed):
Keep in mind this is a page that loads in less than 200ms.
But it's interesting because it is currently the incompressible work
in the core once you fixed all you can in the logic.
We do see things like:
getXWikiPreferences 26%
getSkinFile 14%
getVelocityEngineCacheKey 13%
hasAccessLevel 12%
If I'm correct these are independent from one each other and could be
improved using some caching (at least for getSkinFile and
getXWikiPreferences)
And these three methods are called all the time including in more
complex scripts.
createURL particularly uses XWikiPreferences a lot which calls
getDocument and getXObject which calls TreeMap (this last part is
inefficient because we look for the XWikiPrefs object for the right
translation).
For sure if we transform the getXWikiPreferences to call some internal
structures created once instead of relying on the XWikiDocument there
will be a big win.
Also get should look at if we can optimize getDocument and getXObject
which are heavily used everywhere.
I believe the getSkinFile functions also should be optimized (maybe
only if a specific "production" preference is activated).
hasAccessLevel is also interesting especially since here we are only
checking Admin right which is quite light. For another user would cost
much more.
Ludovic
Le 06/03/11 10:41, Ludovic Dubost a écrit :
Le 06/03/11 04:18, Sergiu Dumitriu a écrit :
On 03/06/2011 12:37 AM, Ludovic Dubost wrote:
Interesting I did some simple instrumentation of
#template and for the
page Sandbox.WebHome we get:
(results here
http://dev.xwiki.org/xwiki/bin/view/Design/PageLoadTimeReport30SnapShot1)
Time frequentlyUsedDocs.vm: 3
Time deprecatedVars.vm: 1
Time xwikivars.vm: 11
Time layoutExtraVars.vm: 1
Time layoutvars.vm: 7
Time colorThemeInit.vm: 2
Time stylesheets.vm: 5
Time analytics.vm: 0
Time javascript.vm: 9
Time htmlheader.vm: 36
Time menuview.vm: 19
Time global.vm: 3
Time header.vm: 4
Time startpage.vm: 78
Time contentmenu.vm: 6
Time frequentlyUsedDocs.vm: 1
Time deprecatedVars.vm: 1
Time xwikivars.vm: 7
Time hierarchy.vm: 25
Time titlevars.vm: 2
Time shortcuts.vm: 2
Time contentview.vm: 37
Time frequentlyUsedDocs.vm: 1
Time deprecatedVars.vm: 1
Time xwikivars.vm: 7
Time documentTags.vm: 12
Time frequentlyUsedDocs.vm: 1
Time deprecatedVars.vm: 1
Time xwikivars.vm: 9
Time commentsinline.vm: 12
Time docextra.vm: 15
Time leftpanels.vm: 1
Time rightpanels.vm: 50
Time footer.vm: 2
Time htmlfooter.vm: 0
Time endpage.vm: 54
Time view.vm: 216
in Firebug the page loads in 10ms more than view.vm
As we can see:
- the panels (quick links and recent changes) cost 50ms -> 25%
- startpage cost 78ms -> 30%
- breadcrumb cost 25ms -> 12%
- some templates are repeated (on repeat is dur to AJAX, the other
not)
- we have 37 templates called
If we implement caching in panels, breadcrumb and part of the start
page
we could win 33% of the general time of the skin.
If we win 1ms per template run, we can win 15% of the general time of
the skin.
The results on the home page (2 to 3 seconds), show that we ought to
look at dynamic code of course as the main slow-down. A panel with a
list of changes or of categories is way more costly than the whole
skin.
The dashboard page is even more costly.
A long Syntax 2.0 page is also quite costly.
So implementing caches on all this is a good way to keep
performance good.
I think this is not the right approach. Caching always
introduces
surprises. Image we cache the "recently viewed" panel. The user views
some documents, but that panel doesn't show them, but insists on
displaying things from 5 minutes ago. Buggy feature...
Imagine we cache the homepage, and I go and create a new "product", and
go to the homepage and don't see it there. What do I do? Panic? Say
it's
a bug and call the IT guy only to look like a fool later when I try to
show it? Report a bug to those developers only to have it closed as
"won't fix, duplicate of the other 30 issues reported this month"?
Of course caching needs to be use with intelligence and should be
used only on content that you can invalidate the cache for.
But the numbers shown higher up, show that a significant portion of
the load time is actually in certain scripts (panels or home page)
that are costly by nature and quite almost the same from one run to
another.
It's not something that is problematic by essence but this is
something an administrator needs to be aware of.
It's known that an entry point should be fast. For instance Google
does not use a dynamic page for it's search box.
Having a cost of 2 seconds to display the boxes from our home page is
a killer.
Another thing that it shows is that indeed there is a lot of time in
many little things. 37 templates, thousands of calls to the prefs and
to the message API (you forgot that one).
Personally, I think that most of the costs come
from three main points:
- checkAccess is too slow
- getXWikiPreference is too slow
- there's no way to just get some document metadata like the title
without loading the full document from the database
We should focus on these three for a start.
But I might be wrong as well; the best way to work on performance is to
start a profiler, find the hot spots, and tinker them until they stop
being a problem.
That's not always true. I've run profilers on XWiki and profilers are
good to look at contentions and specific code that is called a lot
under load.
It does not tell you if your code is necessary or not. A good
instrumentation of the "logic" of your page is interesting too.
In my view all performance information is interesting as it allows to
gain information about our application.
What is really important is to separate the use case. There is no
point in profiling the home page of XWiki since it's load time is
probably not XWiki core apis time, but it's the logic that is used in
the home page itself.
So it's important to know exactly what you profile.
Either:
1/ You profile a scenario to find out which pages should be improved
version A: no load
version B: with load
2/ You profile your skin alone
3/ You try to profile the core with and without load
(but that last one is complicated because there are many ways to use
the core).
In my view, "load testing" is not even what we are looking at now.
It's something different to look at load, CPU and memory consumption
than looking at raw speed of pages. On this aspect I noticed during
load testing that XWikiDocument cloning is called too much (on
getObject() and therefore by livetables) and has increased in cost
with component reference injection in BaseProperty. We can probably
measure that an XWikiPreference cloning is quite expensive.
Now I agree that checkAccess is interesting (in my tests I'm Admin so
we bypass most of the code there). getXWikiPreference is probably not
that slow, but is called too much. I would look at the message api
too, and the skin finding code.
In general I would think about all the stuff that is actually NOT
changing once you got out of development mode. XWiki is very
powerfull and everything is very dynamic. However once you have put
your wiki in production, you don't necessarly need this dynamicity
and you are paying the code of it.
Maybe we could have an option which says "production mode" in the
admin which activates long term caches for everything that is
configurable in the Wiki:
- skin
- prefs
- translations
What I mean by long term caches, is not even looking at if this data
has changed. Doing this won't reduce a page load from 4 seconds to
0.5 seconds because when you are at 4 seconds it usually means there
is logic that needs to work with more data, but it will more reduce
the currently incompressible part from something like 200ms to 100ms
or even 50ms, and therefore would have a lot of effect on the
footprint of the web server. It won't fix the time of loading of JS
and CSS which is quite significant too.
Now the big question is what to focus on in priority. That's one the
objectives of the investigation process. See what makes more sense to
focus on.
I'll try to list all the areas that can be looked at. Clearly the
current document puts a bit to core part aside.
Ludovic
Caches work well for mostly static pages, not for
highly dynamic
scripts, and these scripts are the ones that cost the most. Caching
plain wiki documents will save too little.
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