TieredThresholdPolicy::select_task implements a special case of downgrading.

[quote]
  c. 0 -> (3->2) -> 4.
     In this case we enqueue a method for compilation at level 3, but the C1 queue is long enough
     to enable the profiling to fully occur at level 0. In this case we change the compilation level
    of the method to 2 while the request is still in-queue, because it'll allow it to run much faster
    without full profiling while c2 is compiling
[/quote]

A method might downgrade from level-3(CompLevel_full_profile) to level-2(CompLevel_limited_profile). After then, C1 will compile the method and a nmethod will be populated in the code cache.

Because the level-3 method is not seen, the application might trigger level-3 compilation again. CompileBroker::compile_method fails to detect the pre-existing nmethod because comp_level doesn't match. As a result, it will repeat the above procedure.

We observe that some applications will execute this loop thousands of times. 2 negative impacts occur:
1. higher CPU utility for C1 compilation
2. code cache bloat or even overflow

OSR is worse. Because all osr-compiled nmethods are stored in an InstanceKlass in the form of a linked-list, InstanceKlass::lookup_osr_nmethod will degenerate dramatically because of same method recompilation.