I am just analysing a heap dump (god bless the
-XX:+HeapDumpOnOutOfMemoryError flag) of a recent application
cache due
to
an OutOfMemoryError ("GC overhead limit exceeded" to be precise).
Using
jhat, the "175456 instances of class
org.apache.wicket.util.concurrent.ConcurrentHashMap$Entry"
immediately
got
my attention. While looking through the 107 instance of
ConcurrentHashMap, I
found one *really* big one: Localizer.cache has a hash table
length of
262144, each of its 32 segments with about 5300 entries, where a
hash key
is
a string, sometimes longer than 500 charactes, similar to (see
Localizer.getCacheKey(String,Component)):
fooTitle.bar-
org.apache.wicket.markup.html.link.BookmarkablePageLink:fooLink-
org.apache.wicket.markup.html.panel.Fragment:track-
org.apache.wicket.markup.html.list.ListItem:14-
my.company.FooListPanel$1:fooList-my.company.FooListPanel:foos-
org.apache.wicket.markup.html.list.ListItem:0-
my.company.BarListPanel$1:bars-my.company.FooListPanel:panel-
my.company.boxes.BodyBox:2-
org.apache.wicket.markup.repeater.RepeatingView:body-
my.company.layout.Border:border-my.company.pages.music.FoobarPage:
43-de-null
Those numbers pretty much convinced me: The localizer cache has
blown
away
my application.
Looking at this hash keys, I suspect the following problem: those
strings
are constructed from the "position" of a localized String on a page,
which
is quite a bad thing if you use nested list views or repeating
views to
construct your page. For instance, I have a panel with a long
(pageable)
list of entries, might be > 5000 entries which might appear on
various
positions in a repeating view I use as a container for most of my
pages.
Let's say there are 5 possible positions, this would cause 2500
thousand
cached entries, each with a key of 300+ characters plus some more
characters
for the cached message - feel free to do the maths. From a quick
estimate
I'd say: No wonder, this has blown away my app.
As a quick fix, I'd suggest to regularly clear the localizer
cache, use a
more sophisticated cache (that expires old entries once in a
while!!) or
to
disable the cache completely. However, don't try to overwrite
Localizer.newCache() and clear the cache regularly: clearCache()
will
replace your cache with a ConcurrentHashMap (not using
Localizer.newCache()). However, quite unlikely, that this will
happen as
newCache() is private anyway ;) I am going to add some code to
clear the
cache regularly.
Best regards, Stefan
PS: I'll also create a JIRA issue, but I am really short on time
right
now.
-----
-------
Stefan Fußenegger
http://talk-on-tech.blogspot.com // looking for a nicer domain ;)
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