rts: Do not traverse nursery for dead closures in LDV profile
authorMatthew Pickering <matthewtpickering@gmail.com>
Thu, 6 Jun 2019 13:07:06 +0000 (14:07 +0100)
committerMarge Bot <ben+marge-bot@smart-cactus.org>
Thu, 27 Jun 2019 14:42:29 +0000 (10:42 -0400)
commit07cffc49d031df71702be3bf9006e0ad006414e2
treeee392cff58057155beb76c3d7f609dd464e90cc8
parented4cbd932651bb2a2673e62aad4e5cc6492c6735
rts: Do not traverse nursery for dead closures in LDV profile

It is important that `heapCensus` and `LdvCensusForDead` traverse the
same areas.

`heapCensus` increases the `not_used` counter which tracks how many
closures are live but haven't been used yet.

`LdvCensusForDead` increases the `void_total` counter which tracks how
many dead closures there are.

The `LAG` is then calculated by substracting the `void_total` from
`not_used` and so it is essential that `not_used >= void_total`. This
fact is checked by quite a few assertions.

However, if a program has low maximum residency but allocates a lot in
the nursery then these assertions were failing (see #16753 and #15903)
because `LdvCensusForDead` was observing dead closures from the nursery
which totalled more than the `not_used`. The same closures were not
counted by `heapCensus`.

Therefore, it seems that the correct fix is to make `LdvCensusForDead`
agree with `heapCensus` and not traverse the nursery for dead closures.

Fixes #16100 #16753 #15903 #8982
rts/LdvProfile.c