Hi, On 2018-01-24 17:07:01 -0300, Alvaro Herrera wrote: > Andres Freund wrote: > > glibc's malloc does add a header. My half-informed suspicion is that > > most newer malloc backing allocators will have a header, because > > maintaining a shared lookup-by-address table is pretty expensive to > > maintain. A bit of metadata indicating size and/or source of the > > allocation makes using thread-local information a lot easier. > > Sounds like it'd be smart to allocate something closer to > M_MMAP_THRESHOLD (which with typical values would be about double the > amount of memory the current RAW_BUF_SIZE value), minus a few dozen > bytes to allow for palloc's and malloc's respective headers. That way > we bet for a mmap()ed allocation with minimum space wastage across all > layers.
In general there might be cases where that's worthwhile, although M_MMAP_THRESHOLD IIRC isn't a constant anymore, complicating things. The specific case this thread is discussing doesn't seem worthy of such attention though, there's afaict no actual practical problem here. > Not sure whether we want to try to minimize wastage through > clever use of malloc_usable_size() on each backend's first allocation -- > that'd probably just be overengineering. Likely also dangerous, I don't think this is a constant. Greetings, Andres Freund
