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.

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)
 {

rts/sm/Storage.c

@@ -437,16 +437,12 @@ allocNursery (bdescr *tail, W_ blocks)
     // tiny optimisation (~0.5%), but it's free.
 
     while (blocks > 0) {
-        if (blocks >= BLOCKS_PER_MBLOCK / 4) {
-            n = stg_min(BLOCKS_PER_MBLOCK, blocks);
-            bd = allocLargeChunk(16, n); // see comment with allocLargeChunk()
-            // NB. we want a nice power of 2 for the minimum here
-            n = bd->blocks;
-        } else {
-            bd = allocGroup(blocks);
-            n = blocks;
-        }
-
+        n = stg_min(BLOCKS_PER_MBLOCK, blocks);
+        // allocLargeChunk will prefer large chunks, but will pick up
+        // small chunks if there are any available.  We must allow
+        // single blocks here to avoid fragmentation (#7257)
+        bd = allocLargeChunk(1, n);
+        n = bd->blocks;
         blocks -= n;
 
         for (i = 0; i < n; i++) {