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Jon Masamitsu's Weblog

  • Java
    November 1, 2011

Really? iCMS? Really?

Guest Author
When I use the term iCMS, I'm referring to the
incremental mode of CMS (-XX:+UseConcMarkSweepGC
-XX:+CMSIncrementalMode). This is a mode of
CMS where the concurrent phases (concurrent marking
and concurrent sweeping) are run in
short increments (does some work, yields and
sleeps for a while, then runs again). It was
implemented in CMS primarily for use on platforms
with a single hardware thread. The concurrent phases
are typically long (think seconds and not milliseconds).
If CMS hogged the single hardware thread for several
seconds, the application would not execute during those
several seconds and would in
effect experience a stop-the-world pause. With iCMS
the application makes progress during concurrent
phases. That of course is good. The down side
is that there is more overhead to iCMS (e.g., all that
stopping and starting), more floating garbage (objects
that die during a CMS collection that we can't tell
have died until the next collection) and more flags
to tune (just what you wanted, right). Also it's
just more complex (more things to go wrong). We put
quite a bit of effort into teaching iCMS how to do
the work in pieces and still finish the collection
before the heap fill up, but it's not perfect.
Are you using iCMS? Does your platform only have
one hardware thread? Ok, maybe two hardware threads
is reason enough if you don't want half your cycles
eaten by CMS during a long concurrent phases. But,
otherwise, I'd suggest you try without the incremental
mode. There are exceptions to every rule, but I think
you'll be happier. Really.

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