RFR: 8229189: Improve JFR leak profiler tracing to deal with discontiguous heaps

Erik Österlund erik.osterlund at oracle.com
Wed Aug 7 08:18:43 UTC 2019


The JFR leak profiler has marking bit maps that assume a contiguous Java 
heap. ZGC is discontiguous, and therefore does not work with JFR. If one 
tried to use the JFR leak profiler with ZGC, it would allocate a bit map 
for the multi-terabyte "reserved region", even though perhaps only 64 MB 
is used, spread out across this address space. That is one of the reason 
the leakprofiler is turned off for ZGC.

In order to enable leakprofiler support on ZGC, the tracing must also 
use the Access API instead of raw oop loads. But that is outside the 
scope of this RFE; here we deal only with the discontiguous address 
space problem.

My solution involves implementing a segmented bit map, that makes no 
assumptions about the layout of the Java heap. Given an address, it 
looks up a bitmap fragment from a hash table, given the high order bits 
of a pointer. If there is no such fragment, it is populated to the 
table. The low order bits (shifted by LogMinObjAlignmentInBytes) are 
used to find the bit in the bit map for marking an object in the traversal.

In order to not cause regressions in the speed, some optimizations have 
been made:

1) The table uses & instead of % to lookup buckets, ensuring the table 
is always a power of two size.
2) The hot paths get inlined.
3) There is a cache for the last fragment, as the probability of two 
subsequent bit accesses for two objects found during tracing in the heap 
do not cross the set up fragment granule (64 MB heap memory) boundary. 
This is something G1 exploits for the cross region check, and the same 
general idea is applied here. The code also asks first if a bit is 
marked and then marks it, as two calls. The cache + inlining allows the 
compiler to lookup the fragment only once for the two operations.
4) Keep the table sparse.

As a result, no regressions that are outside of the noise can be noticed 
with this new more GC-agnostic approach.




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