https://bugs.webkit.org/show_bug.cgi?id=85411

Reviewed by Filip Pizlo.

Opportunistic GC is punishing us severely in limited memory situations because its 
assumptions about how much time a collection will take are way out of whack when the Heap 
has been paged out by the OS. We should add a simple detection function to the Heap that 
detects if its is paged out. It will do this by iterating each block of both the MarkedSpace 
and CopiedSpace. If that operation takes longer than a fixed amount of time (e.g. 100ms), 
the function returns true. This function will only be run prior to an opportunistic 
collection (i.e. it will not run during our normal allocation-triggered collections).

In my tests, steady state was drastically improved in high memory pressure situations (i.e. 
the browser was still usable, significant reduction in SPODs). Occasionally, a normal GC
would be triggered due to pages doing things in the background, which would cause a 
significant pause. As we close pages we now cause normal collections rather than full 
collections, which prevents us from collecting all of the dead memory immediately. One 
nice way to deal with this issue might be to do incremental sweeping.


* heap/CopiedSpace.cpp:
(JSC::isBlockListPagedOut): Helper function to reduce code duplication when iterating over 
to-space, from-space, and the oversize blocks.
(JSC):
(JSC::CopiedSpace::isPagedOut): Tries to determine whether or not CopiedSpace is paged out
by iterating all of the blocks.
* heap/CopiedSpace.h:
(CopiedSpace):
* heap/Heap.cpp:
(JSC::Heap::isPagedOut): Tries to determine whether the Heap is paged out by asking the 
MarkedSpace and CopiedSpace if they are paged out.
(JSC):
* heap/Heap.h:
(Heap):
(JSC::Heap::increaseLastGCLength): Added this so that the GC timer can linearly back off 
each time it determines that the Heap is paged out.
* heap/MarkedAllocator.cpp:
(JSC::MarkedAllocator::isPagedOut): Tries to determine if this particular MarkedAllocator's
list of blocks are paged out.
(JSC):
* heap/MarkedAllocator.h:
(MarkedAllocator):
* heap/MarkedSpace.cpp:
(JSC::MarkedSpace::isPagedOut): For each MarkedAllocator, check to see if they're paged out.
* heap/MarkedSpace.h:
(MarkedSpace):
* runtime/GCActivityCallback.cpp:
(JSC::DefaultGCActivityCallback::cancel):
(JSC):
* runtime/GCActivityCallback.h:
(JSC::GCActivityCallback::cancel):
(DefaultGCActivityCallback):
* runtime/GCActivityCallbackCF.cpp: Added a constant of 100ms for the timeout in determining
whether the Heap is paged out or not.
(JSC):
(JSC::DefaultGCActivityCallbackPlatformData::timerDidFire): Added the check to see if we 
should attempt a collection based on whether or not we can iterate the blocks of the Heap in 
100ms. If we can't, we cancel the timer and tell the Heap we just wasted 100ms more trying to 
do a collection. This gives us a nice linear backoff so we're not constantly re-trying in
steady state paged-out-ness.
(JSC::DefaultGCActivityCallback::cancel): Added this function which, while currently doing 
exactly the same thing as willCollect, is more obvious as to what it's doing when we call it 
in timerDidFire.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@115915 268f45cc-cd09-0410-ab3c-d52691b4dbfc