Use Cycles32 in CpuCache::ResizeInfo to reduce metadata footprint - #444
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TCMalloc's CpuCache::ResizeInfo previously stored 64-bit cycle timestamps (std::atomic<int64_t>) for last_miss_cycles across 2 size-class arrays of kNumClasses=258 and for last_reclaim. This consumed 16 bytes per size class per CPU plus an 8-byte reclaim timestamp per CPU. Replaces last_miss_cycles timestamps with Cycles32, reducing per-CPU metadata by 8 bytes per size-class array element, and removes last_reclaim entirely from ResizeInfo. This is safe, because in CpuCache::ResizeInfo, last_miss_cycles is read-only diagnostic telemetry consumed exclusively by CpuCache::GetStats(); it does not influence allocator correctness, slab resizing, or cache eviction. Right-shifting CycleClock::Now() by kShift = 20 bits (~1M cycles, ~0.35 ms granularity at 3 GHz) extends the 32-bit wraparound epoch to ~17.3 days before overflow occurs, while unsigned 32-bit subtraction still calculates exact elapsed durations across wraparound boundaries. Furthermore, CpuCache::Reclaim now resets last_miss_cycles to 0 on reclaimed CPU caches, reducing any 32-bit overflow risk on idle CPUs. PiperOrigin-RevId: 958568123
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Use Cycles32 in CpuCache::ResizeInfo to reduce metadata footprint
TCMalloc's CpuCache::ResizeInfo previously stored 64-bit cycle timestamps
(std::atomic<int64_t>) for last_miss_cycles across 2 size-class arrays of
kNumClasses=258 and for last_reclaim. This consumed 16 bytes per size class per
CPU plus an 8-byte reclaim timestamp per CPU.
Replaces last_miss_cycles timestamps with Cycles32, reducing per-CPU metadata by
8 bytes per size-class array element, and removes last_reclaim entirely from
ResizeInfo.
This is safe, because in CpuCache::ResizeInfo, last_miss_cycles is read-only
diagnostic telemetry consumed exclusively by CpuCache::GetStats(); it does not
influence allocator correctness, slab resizing, or cache eviction.
Right-shifting CycleClock::Now() by kShift = 20 bits (~1M cycles, ~0.35 ms
granularity at 3 GHz) extends the 32-bit wraparound epoch to ~17.3 days before
overflow occurs, while unsigned 32-bit subtraction still calculates exact
elapsed durations across wraparound boundaries. Furthermore, CpuCache::Reclaim
now resets last_miss_cycles to 0 on reclaimed CPU caches, reducing any 32-bit
overflow risk on idle CPUs.