[GC] Avoid leaving fragmented slabs behind us when refilling the thread bins.

Author: deadalnix

sdlib/d/gc/arena.d

@@ -126,6 +126,7 @@ public:
 		ubyte sizeClass,
 		void** top,
 		void** bottom,
+		void** requested,
 		size_t slotSize,
 	) shared {
 		// TODO: in contracts
@@ -134,8 +135,8 @@ public:
 		import d.gc.slab;
 		assert(slotSize == binInfos[sizeClass].slotSize, "Invalid slot size!");
 
-		return bins[sizeClass]
-			.batchAllocate(&filler, emap, sizeClass, top, bottom, slotSize);
+		return bins[sizeClass].batchAllocate(&filler, emap, sizeClass, top,
+		                                     bottom, requested, slotSize);
 	}
 
 	uint batchFree(ref CachedExtentMap emap, const(void*)[] worklist,

sdlib/d/gc/bin.d

@@ -27,6 +27,7 @@ struct Bin {
 		ubyte sizeClass,
 		void** top,
 		void** bottom,
+		void** requested,
 		size_t slotSize,
 	) shared {
 		import d.gc.sizeclass;
@@ -38,6 +39,9 @@ struct Bin {
 		assert(bottom < top, "Invalid stack boundaries!");
 		assert((top - bottom) < uint.max, "Invalid stack size!");
 
+		assert(bottom < requested && requested <= top,
+		       "Invalid requested slot count!");
+
 		/**
 		 * When we run out of slab with free space, we allocate a fresh slab.
 		 * However, while we do so, another thread may have returned slabs to
@@ -112,8 +116,20 @@ struct Bin {
 		assert(freshSlab is null);
 		assert(progressed);
 
-		freshSlab = filler.allocSlab(emap, sizeClass);
+		/**
+		 * We want to avoid leaving partially filled slabs behind us,
+		 * so, before allocating a new slab, we make sure that it'll
+		 * either be required to meet the requested number of slot,
+		 * or that it'll be completely filled.
+		 */
+		assert(bottom <= insert && insert < top, "Invalid insertion point!");
+		auto navailable = top - insert;
 		auto nslots = binInfos[sizeClass].nslots;
+		if (requested <= insert && navailable <= nslots) {
+			return insert;
+		}
+
+		freshSlab = filler.allocSlab(emap, sizeClass);
 		assert(freshSlab is null || freshSlab.nfree == nslots);
 
 		progressed = false;

sdlib/d/gc/tbin.d

@@ -78,16 +78,14 @@ public:
 		auto nfill = state.getFill(nmax);
 		assert(nfill > 0);
 
-		/**
-		 * TODO: We should pass available in addition to nfill to batchAllocSmall.
-		 *       This would ensure batchAllocSmall has some wiggle room to provide
-		 *       as many slots as possible without allocating new slabs.
-		 */
 		auto insert = _head - nfill;
 		assert(available <= insert);
 
-		auto filled =
-			arena.batchAllocSmall(emap, sizeClass, _head, insert, slotSize);
+		auto requested = insert + (nfill >> 1) + 1;
+		assert(insert < requested && requested <= _head);
+
+		auto filled = arena.batchAllocSmall(emap, sizeClass, _head, insert,
+		                                    requested, slotSize);
 		state.onRefill();
 
 		/**