Hi Clement,
Nice article... A random thought, I wonder if size1 chunks can be managed packed into size2 chunks.
Have a variable size1_active_double initially nil. First
size1 allocation, grab a size2 chunk into
size1_active_double. First half returned as the size1chunk. Second half marked at "empty". Second size1 allocation, return second half of size1_active_double
as size1chunk. Set
size1_active_double to nil. Releasing a size1chunk. Dependent on whether memory alignment can inform of which half of size2chunk is being released. Check if other half is empty ==> add it to the size2chunklist. If other half not empty, add it to one of two singlylinkedlists depending on which half is empty, the empty half used for the linkedlist. Next size1 allocations made from these lists first to fill in the empty half. Considerations * Added complexity * Remaining size1 singlylinkedlist might have minimal impact  added complexity not worth the gain. ____________ Or another random thought *grin*  and this is probably completely off base because of my limited understanding. Piggyback the size1chunk management on top of the size3chunklist. Consider one end of the size3chunklist to be "size1filledend" with references to free size1chunks, and the other end of the size3chunklist to be "size1emptyend". To release a size1chunk, move a size3chunk from the "size1emptyend" to the "size1filledend" and store the size1chunk reference there. To allocate a size1chunk, get it from the "size1filled" end of the size3chunklist and move that size3chunk to the "size1emptyend" of its list. To compact a size3chunk that refers to a size1chunk, copy that into another chunk from the size1emptyend moved to the size1filledend. I wonder if that even makes sense. Anyway, it hurts less to let the ideas roam free... cheers ben 
Hi Ben,
On Sun, Jun 17, 2018 at 4:11 PM, Ben Coman <[hidden email]> wrote:
A key design feature of Spur is that every object has at least one field to function as a forwarding pointer. Hence even zerosized objects occupy 128 bits. Each object has a 64bit header (large objects have a 128bit header). The allocation unit is 64bits. All objects must have room for a forwarding pointer. Hence the minimum sized chunk is 128bits. Hence there isn't room for two size 1 chunks inside a size2 chunk, only inside a size three chunk.
_,,,^..^,,,_ best, Eliot 
In reply to this post by Ben Coman
On Mon, 18 Jun 2018, Ben Coman wrote: > A random thought, I wonder if size1 chunks can be managed packed into size2 chunks. Or, just use the XOR trick to store each node's two links in a single pointer sized field: https://everything2.com/title/Storing+a+doublylinked+list+using+just+a+single+pointer+field Levente 
Hi Levente, > On Jun 17, 2018, at 5:36 PM, Levente Uzonyi <[hidden email]> wrote: > >> On Mon, 18 Jun 2018, Ben Coman wrote: >> >> A random thought, I wonder if size1 chunks can be managed packed into > size2 chunks. > > Or, just use the XOR trick to store each node's two links in a single pointer sized field: > https://everything2.com/title/Storing+a+doublylinked+list+using+just+a+single+pointer+field Alas, while this lovely trick does indeed encode a doublylinked list in a single field it only works for full traversals. The xor is of the two neighbours, so to get to the next one needs the prev, and to get to the prev one needs the next. So one can start from either end but not in the middle. Clément’s modification is to allow rapid removal without needing to traverse the entire list, so the xor trick is not fit for purpose here. BTW the compactor I wrote before the current one (SpurPigCompactor preceded SpurPlanningCompactor) used exactly this trick. It didn’t work for reasons unrelated to the xor trick. Just mentioning it as proof that I love the technique. > > Levente > 
Hi Eliot, On Sun, 17 Jun 2018, Eliot Miranda wrote: > > Hi Levente, > > >> On Jun 17, 2018, at 5:36 PM, Levente Uzonyi <[hidden email]> wrote: >> >>> On Mon, 18 Jun 2018, Ben Coman wrote: >>> >>> A random thought, I wonder if size1 chunks can be managed packed into >> size2 chunks. >> >> Or, just use the XOR trick to store each node's two links in a single pointer sized field: >> https://everything2.com/title/Storing+a+doublylinked+list+using+just+a+single+pointer+field > > Alas, while this lovely trick does indeed encode a doublylinked list in a single field it only works for full traversals. The xor is of the two neighbours, so to get to the next one needs the prev, and to get to the prev one needs the next. So one can start from either end but not in the middle. Clément’s modification is to allow rapid removal without needing to traverse the entire list, so the xor trick is not fit for purpose here. out, because if you always iterate over the list, you can just keep a pointer to the previous node to delete the current node. If performance is important here, random deletion can still be done in O(1) time at the cost of maintaining an external doubly linked list with each node having a pointer to the chunk and the chunk using its sole slot to point to its list node. > > BTW the compactor I wrote before the current one (SpurPigCompactor preceded SpurPlanningCompactor) used exactly this trick. It didn’t work for reasons unrelated to the xor trick. Just mentioning it as proof that I love the technique. Nice. :) Levente > >> >> Levente >> 

On Mon, 18 Jun 2018, Clément Bera wrote: > There are still some unlinking, but it's less common so the single linked list for chunk of size 1 is now affordable (I don't see 75% of time spent in free chunk unlinking like before on pathological cases). > > In selective compactor, segments are compacted using unlinking but only segments mostly empty are compacted, so low amount of free chunks of size 1. The problem was in the sweep phase and we fixed it. Great. Btw, why did you decide to use a tree instead of a hash table? I had a look at the tree implementation, and it seems to be a simple binary tree. Is that correct? Levente 
Hi Levente,
On Tue, Jun 19, 2018 at 4:50 PM, Levente Uzonyi <[hidden email]> wrote:
To keep it simple. The scheme really comes from the free lists. There is either a 32 or a 64 element array with all the free chunks from size 1 to 32 or 1 to 64. So this gives fast allocation for the common case of a smallish object. Then what to do with element 0? A binary tree works well since it is populated only with larger chunks and, given that all chunks of the same size are, effectively, a single node in the tree, the tree typically isn't that large. That and some very simple balancing hacks keep it from degenerating into a list whenever I've looked at its state (although we might investigate organizing it as an AVL tree; I like AVL trees but they're a lot of work for little gain if in practice a tree doesn't degenerate). The use of 32 or 64 elements allows the use of a bitmask word to serve as a nonempty list cache, so that when the system looks for chunks bigger than needed (if a smaller list is empty) it is fast to check the list(s) at 2x, 3x, etc... A hash table needs heap space to organize and unlike the binary tree this space may need to grow if the number of free chunks is very large. I had a look at the tree implementation, and it seems to be a simple binary tree. Is that correct? Yes. A binary tree of lists. Levente and you are very welcome to experiment with alternative representations. _,,,^..^,,,_ best, Eliot 
Hi Eliot, On Tue, 19 Jun 2018, Eliot Miranda wrote: > To keep it simple. The scheme really comes from the free lists. There is either a 32 or a 64 element array with all the free chunks from size 1 to 32 or 1 to 64. So this gives fast allocation for the common case of a smallish object. Then what to do with element 0? A binary tree works well since it is populated only with larger chunks and, given that all chunks of the same size are, effectively, a single node in the tree, the tree typically isn't that large. That and some very simple balancing hacks keep it from degenerating into a list whenever I've looked at its state (although we might investigate organizing it as an AVL tree; I like AVL trees but they're a lot of work for little gain if in practice a tree doesn't degenerate). Well, random binary trees have ~1.39 x log(n) average height, so performance can be really good if the order the chunk sizes are added and removed do not unbalance the tree too much. The reason I asked about this was that I couldn't figure out why the trees were said to be AVL trees. And I would have used a hash table with open addressing over an AVL tree, because that's fairly easy to implement and has good performance, while AVL trees take a lot more effort to implement. > > The use of 32 or 64 elements allows the use of a bitmask word to serve as a nonempty list cache, so that when the system looks for chunks bigger than needed (if a smaller list is empty) it is fast to check the list(s) at 2x, 3x, etc... > > A hash table needs heap space to organize and unlike the binary tree this space may need to grow if the number of free chunks is very large. Based on your answers, I have got the impression that the performance of these data structures have negligible effect on the overall GC performance, so it's probably not worth to try to optimize them. Levente 
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