# `•memory` — Memory

`•memory x` gives a record of what this thread holds, and of what `x`
keeps alive. `•memory 0` is the usual spelling for the counts alone.

- `records` counts the live record nodes: the linear calls that reverse
  mode records while `∂` runs. Each `∂` frees its record when it
  returns, so the count is 0 between programs.
- `pending` counts the live pending arrays, whose items wait for their
  first read.
- `bytes` counts the bytes that `x` keeps alive: its items, the base of
  a view, the arrays that a pending array reads, the parts of a value
  that `∂` is differentiating, and the nodes of a record. An allocation
  that `x` reaches twice counts once.

`records` and `pending` count the nodes that this thread made, even when
another thread holds one. `•memory` reads no items, so it never forces a
pending array, and it accepts values that `∂` is differentiating.

``` bpl
{+/⍵×⍵}∂ 1 2 3     ⍝ 2 4 6
•memory 0          ⍝ ["records":0ₓ "pending":0ₓ "bytes":0ₓ]
```

A reshape is a view, which keeps all of its base’s items alive:

``` bpl
v←⍳1000ₓ
m←2 500⍴v
(•memory m).bytes  ⍝ 2000ₓ
```

[`•explain`](explain.qmd) reports what happened while an expression ran.