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