jdk-sandbox: comparison src/hotspot/share/memory/metaspace/chunkManager.cpp

equal deleted inserted replaced

-:960e372a6065
+:5e9e519e5dc9
 log_debug(metaspace)("ChunkManager %s: get chunk: max " CHKLVL_FORMAT " (" SIZE_FORMAT "),"
 "preferred " CHKLVL_FORMAT " (" SIZE_FORMAT ").",
 _name, max_level, chklvl::word_size_for_level(max_level),
 pref_level, chklvl::word_size_for_level(pref_level));
+// When handing a new chunk to the caller, we must balance the need to not let free space go to waste
+// with the need to keep fragmentation low.
+// Every request comes with a preferred chunk size and the minimum chunk size, which is the bare minimum
+// needed to house the metadata allocation which triggered this chunk allocation.
+//
+// Ideally, we have a free chunk of the preferred size just laying around and hand that one out.
+// Realistically, we are often faced with the choice of either taking a larger chunk and split it
+// or taking a smaller chunk than the preferred size. Both has pros and cons: splitting a larger chunk
+// reduces the number of large chunks available for large allocations, and, increases fragmentation.
+// Using a smaller chunk may also increase fragmentation (beside being inefficient at some point) if
+// e.g. a normal class loader - which lives normally off 64K chunks - eats from the bowl of small chunk
+// loaders, which prefer 4K or 1K chunks.
 // 1) Attempt to find a free chunk of exactly the pref_level level
 c = remove_first_chunk_at_level(pref_level);
-// 2) Failing that, we are also willing to accept a chunk half that size, but nothing less for now...
+// 2) Failing that, we are also willing to accept a smaller chunk, but only up to a limit
+//    (for now, half the preferred size)...
 if (c == NULL && pref_level < max_level) {
 c = remove_first_chunk_at_level(pref_level + 1);
 }
-// 3) Failing that, attempt to find a free chunk of larger size and split it to get the ideal size...
+// 3) Failing that, attempt to find a free chunk of larger size and split it to the preferred size...
 if (c == NULL) {
 for (chklvl_t lvl = pref_level - 1; lvl >= chklvl::ROOT_CHUNK_LEVEL; lvl --) {
 c = remove_first_chunk_at_level(lvl);
 if (c != NULL) {
 // Split chunk; add splinters to freelist
 break;
 }
 }
 }
-// 4) Failing that, before we start allocating a new root chunk, lets really scrape the barrel. Any
+// 4) Failing that, before we give up and get a new root chunk, lets really scrape the barrel. Any
-//    smaller chunk is acceptable provided it fits the minimal size....
+//    smaller chunk is acceptable now...
 if (c == NULL) {
-for (chklvl_t lvl = pref_level + 1; lvl <= max_level; lvl ++) {
+for (chklvl_t lvl = pref_level + 2; lvl <= max_level; lvl ++) {
 c = remove_first_chunk_at_level(lvl);
 if (c != NULL) {
 break;
 }
 }
 }
-// 4) Failing that, attempt to allocate a new root chunk from the connected virtual space.
+// 5) Failing all that, allocate a new root chunk from the connected virtual space.
 if (c == NULL) {
 // Tracing
 log_debug(metaspace)("ChunkManager %s: need new root chunk.", _name);
 return NULL;
 }
 assert(c->level() == chklvl::LOWEST_CHUNK_LEVEL, "Not a root chunk?");
-// Split this root chunk to the desired chunk size.
+// Split this root chunk to the desired chunk size. Splinters are added to freelist.
 if (pref_level > c->level()) {
 c = split_chunk_and_add_splinters(c, pref_level);
 }
 }

branch	stuefe-new-metaspace-branch
changeset 58717	5e9e519e5dc9
parent 58716	960e372a6065