Hi Ben,
On Sun, Jun 17, 2018 at 4:11 PM, Ben Coman btc@openinworld.com wrote:
Hi Clement,
Nice article... https://clementbera.wordpress.com/2018/06/07/free-chunk- management-in-the-cog-vm/
A random thought, I wonder if size-1 chunks can be managed packed into size-2 chunks.
A key design feature of Spur is that every object has at least one field to function as a forwarding pointer. Hence even zero-sized objects occupy 128 bits. Each object has a 64-bit header (large objects have a 128-bit header). The allocation unit is 64-bits. All objects must have room for a forwarding pointer. Hence the minimum sized chunk is 128-bits.
Hence there isn't room for two size 1 chunks inside a size-2 chunk, only inside a size three chunk.
Have a variable size1_active_double initially nil. First size-1 allocation, grab a size-2 chunk into size1_active_double. First half returned as the size-1-chunk. Second half marked at "empty".
Second size-1 allocation, return second half of size1_active_double as size-1-chunk. Set size1_active_double to nil.
Releasing a size-1-chunk. Dependent on whether memory alignment can inform of which half of size-2-chunk is being released. Check if other half is empty ==> add it to the size-2-chunk-list.
If other half not empty, add it to one of two singly-linked-lists depending on which half is empty, the empty half used for the linked-list. Next size-1 allocations made from these lists first to fill in the empty half.
Considerations
- Added complexity
- Remaining size-1 singly-linked-list 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 size-1-chunk management on top of the size-3-chunk-list. Consider one end of the size-3-chunk-list to be "size1-filled-end" with references to free size-1-chunks, and the other end of the size-3-chunk-list to be "size1-empty-end".
To release a size-1-chunk, move a size-3-chunk from the "size1-empty-end" to the "size1-filled-end" and store the size-1-chunk reference there.
To allocate a size-1-chunk, get it from the "size1-filled" end of the size-3-chunk-list and move that size-3-chunk to the "size1-empty-end" of its list.
To compact a size-3-chunk that refers to a size-1-chunk, copy that into another chunk from the size1-empty-end moved to the size1-filled-end.
I wonder if that even makes sense. Anyway, it hurts less to let the ideas roam free...
cheers -ben