the preparing of content before execution is interesting idea, that can be used more. The almost queries and plans are not too big, so when the size of content is not too big - less than 1MB, then can be used one DSM for all backends.
When size of content is bigger than limit, then DSM will be allocated specially for this content. The pointer to DSM and offset can be stored in requested process slot. The reading and writing to requested slot should be protected by spinlock, but it should block only two related processes for short time (copy memory).
Sorry, I don't think this will fly.
The whole idea is that a backend publishes the plan of a query just before running it and it doesn't care which other backend(s) might be reading it, how many times and in which order. The only required locking (implicit) is contained in the code for dsm_attach/detach().
I didn't propose too different solution. There is only one difference - sharing DSM for smaller data. It is similar to using usual shared memory.
Does this mean implementing some sort of allocator on top of the shared memory segment? If so, how are you going to prevent fragmentation?
yes, simple memory allocator is necessary in this case. But it should be really simple - you can allocate only fixed size blocks - 10KB, 100KB and 1MB from separate buffers. So the fragmentation is not possible.
