Allow allocNursery() to allocate single blocks (#7257)

Forcing large allocations here can creates serious fragmentation in
some cases, and since the large allocations are only a small
optimisation we should allow the nursery to hoover up small blocks
before allocating large chunks.

1 files changed, 7 insertions, 1 deletions

diff --git a/rts/sm/BlockAlloc.c b/rts/sm/BlockAlloc.c
index f0f6fb551c..9f04c68a70 100644
--- a/rts/sm/BlockAlloc.c
+++ b/rts/sm/BlockAlloc.c
@@ -393,12 +393,18 @@ finish:
 // Allocate a chunk of blocks that is at least min and at most max
 // blocks in size. This API is used by the nursery allocator that
 // wants contiguous memory preferably, but doesn't require it.  When
-// memory is fragmented we might have lots of large chunks that are
+// memory is fragmented we might have lots of chunks that are
 // less than a full megablock, so allowing the nursery allocator to
 // use these reduces fragmentation considerably.  e.g. on a GHC build
 // with +RTS -H, I saw fragmentation go from 17MB down to 3MB on a
 // single compile.
 //
+// Further to this: in #7257 there is a program that creates serious
+// fragmentation such that the heap is full of tiny <4 block chains.
+// The nursery allocator therefore has to use single blocks to avoid
+// fragmentation, but we make sure that we allocate large blocks
+// preferably if there are any.
+//
 bdescr *
 allocLargeChunk (W_ min, W_ max)
 {