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8339648: ZGC: Division by zero in rule_major_allocation_rate #20888

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8339648: ZGC: Division by zero in rule_major_allocation_rate #20888

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JDK-8339648
MBaesken Sep 6, 2024
21fe3ca
Adjust division following suggestion by xmas
MBaesken Sep 9, 2024
6902026
add parentheses
MBaesken Sep 16, 2024
7ecdb37
adjust parentheses
MBaesken Sep 17, 2024
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3 changes: 2 additions & 1 deletion src/hotspot/share/gc/z/zDirector.cpp
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Expand Up @@ -488,7 +488,8 @@ static bool rule_major_allocation_rate(const ZDirectorStats& stats) {

// Calculate the GC cost for each reclaimed byte
const double current_young_gc_time_per_bytes_freed = double(young_gc_time) / double(reclaimed_per_young_gc);
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Could this division have the same issue?

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Yes, it could if no memory has been reclaimed at all (since the VM started). Similar issues would occur in the call to calculate_extra_young_gc_time below. And there I think the problem is even worse, because we might end up with inf - inf == -nan.

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The case where we have performed a major collection and no young collection has reclaim any memory seems like a very degenerate situation. The solution is probably to handle that case separately, and not try to adapt the current heuristics to handle the extreme values.

const double current_old_gc_time_per_bytes_freed = double(old_gc_time) / double(reclaimed_per_old_gc);
const double current_old_gc_time_per_bytes_freed = ((reclaimed_per_old_gc == 0) ? (std::numeric_limits<double>::infinity())
: (double(old_gc_time) / double(reclaimed_per_old_gc)));

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// Calculate extra time per young collection inflicted by *not* doing an
// old collection that frees up memory in the old generation.
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