From 07cffc49d031df71702be3bf9006e0ad006414e2 Mon Sep 17 00:00:00 2001
From: Matthew Pickering <matthewtpickering@gmail.com>
Date: Thu, 6 Jun 2019 14:07:06 +0100
Subject: [PATCH] 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 | 23 -----------------------
1 file changed, 23 deletions(-)
diff --git a/rts/LdvProfile.c b/rts/LdvProfile.c
index 608961e24678..c918be55a879 100644
--- a/rts/LdvProfile.c
+++ b/rts/LdvProfile.c
@@ -172,28 +172,6 @@ processHeapForDead( bdescr *bd )
}
}
-/* --------------------------------------------------------------------------
- * Calls processHeapClosureForDead() on every *dead* closures in the nursery.
- * ----------------------------------------------------------------------- */
-static void
-processNurseryForDead( void )
-{
- StgPtr p;
- bdescr *bd;
-
- if (MainCapability.r.rNursery == NULL)
- return;
-
- for (bd = MainCapability.r.rNursery->blocks; bd != NULL; bd = bd->link) {
- p = bd->start;
- while (p < bd->free) {
- while (p < bd->free && !*p) p++; // skip slop
- if (p >= bd->free) break;
- p += processHeapClosureForDead((StgClosure *)p);
- }
- }
-}
-
/* --------------------------------------------------------------------------
* Calls processHeapClosureForDead() on every *dead* closures in the closure
* chain.
@@ -233,7 +211,6 @@ LdvCensusForDead( uint32_t N )
//
barf("Lag/Drag/Void profiling not supported with -G1");
} else {
- processNurseryForDead();
for (g = 0; g <= N; g++) {
processHeapForDead(generations[g].old_blocks);
processChainForDead(generations[g].large_objects);
--
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