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The Struct That Grew By Half

Read the counters before the options. Nothing here is labelled with the answer.

The report

We added one boolean flag to a particle struct and memory use for the particle array jumped by about 30%. One boolean. The array is 20 million elements and we are now over our memory budget. Someone thinks the language runtime is boxing the field.

Before and after the flag was added

// before — 24 bytes per particle
struct Particle {
    double x        // 8 bytes, 8-byte aligned
    double y        // 8 bytes
    uint32 id       // 4 bytes
}                   // + 4 bytes trailing padding = 24

// after — 32 bytes per particle
struct Particle {
    double x        // 8 bytes
    bool   active   // 1 byte  <-- added here
    double y        // 8 bytes
    uint32 id       // 4 bytes
}

CountersSIMULATED

sizeof(Particle)	24 → 32 bytes	The struct grew by 8 bytes after adding a 1-byte field.
array footprint (20M)	480 MB → 640 MB	Total memory grew in proportion to the per-element size.
particles per 64-byte line	2.67 → 2.0	Fewer elements now fit within each unit of transfer.
L1-dcache-load-misses (scan)	1.6% → 2.1%	The sequential scan misses somewhat more often than before.

Field offsets reported by the compiler for the "after" layout

offset  size  field
     0     8  x
     8     1  active
     9     7  <padding>
    16     8  y
    24     4  id
    28     4  <padding>
total 32 bytes, alignment 8

What is the hardware doing?

Padding: Why Your Struct Is Bigger Than Its Fields Alignment: Why Addresses Are Not Arbitrary Memory Moves in Lines, Not Variables Array of Structs, or Struct of Arrays?Data-Oriented Design, Without the Dogma What `arr[i]` Actually Compiles To