I have been observing a program which has about 4mb of live data but retains over 700 megablocks. The `memInventory` indicated that the majority of blocks were being retained in the free list. I dumped out the contents of the free list and could see a lot of megabocks which were 251/252 free. Then I dumped out the contents of the nursery as well and checked which mblock each nursery block was in. You can see that the nursery has ended up consisting of a small number of blocks from a large number of different megablocks. ``` [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] [("free",249),("nurs",3)] .... repeated 500 times [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] [("free",251),("nurs",1)] ``` The reason we end up with this problem in the first place is because the `allocatePinned` function will steal 1/2 blocks from the nursery. This leads to a lot of resize operations just increasing the size of the nursery by 1/2 blocks which just get random blocks from the free list. * Freshly reallocate the nursery periodically (expensive if we do it every time) * Refuse to resize the nursery by a small amount, so to make sure that you upper bound the number of mblocks which could participate in the nursery. * Don't steal nursery blocks in allocatePinned (You could still get fragmentation issues here but it will be nowhere near as bad) This is what the free list looks like after stopping `allocatePinned` from stealing blocks from the nursery. No fragmentation! ``` [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("nurs",252)] [("free",38),("gen1",189)] 220:F(48):G1(44):N(128) [("free",70),("gen1",161)] [("free",71),("gen1",157)] [("free",90),("gen1",48)] [("free",134),("gen1",116)] [("free",147),("gen1",98)] [("free",188),("gen1",60)] [("free",194),("gen1",37)] [("free",199),("gen1",52)] [("free",203),("gen1",39)] [("free",219),("gen1",31)] [("free",220),("gen1",31)] [("free",222),("gen1",29)] [("free",223),("gen1",28)] [("free",224),("gen1",27)] [("free",225),("gen1",26)] [("free",227),("gen1",16)] [("free",231),("gen1",20)] [("free",233),("gen1",18)] [("free",234)] [("free",235),("gen1",16)] [("free",237),("gen1",14)] [("free",237),("gen1",14)] [("free",238),("gen1",13)] [("free",239)] [("free",242),("gen1",9)] [("free",242)] [("free",244),("gen1",8)] [("free",246),("gen1",5)] [("free",246),("gen1",5)] [("free",247)] [("free",247)] [("free",248)] [("free",248)] [("free",248)] [("free",250)] [("free",250)] [("free",250)] [("free",250)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] [("free",251)] ```