On Fri, Feb 24, 2017 at 10:59 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Jeff Janes <jeff.ja...@gmail.com> writes: > > On Thu, Feb 23, 2017 at 2:28 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > >> Maybe it's time to convert that to a doubly-linked list. > > > I don't think that would help. You would need some heuristic to guess > > whether the chunk you are looking for is near the front, or near the end. > > Uh, what? In a doubly-linked list, you can remove an element in O(1) > time, you don't need any searching. It basically becomes > item->prev->next = item->next; > item->next->prev = item->prev; > modulo possible special cases for the head and tail elements. > Currently it is walking the chain to identify which block holds the chunk in the first place, not just to get the pointer to the previous block. But I see that that could be fixed by pointer arithmetic once there is a reason to fix it. Which there isn't a reason to as long as you need to walk the chain to get the prev pointer anyway. Like this:? targetblock = (AllocBlock) (((char*)chunk) - ALLOC_BLOCKHDRSZ); > >> Although if the > >> hash code is producing a whole lot of requests that are only a bit > bigger > >> than the separate-block threshold, I'd say It's Doing It Wrong. It > should > >> learn to aggregate them into larger requests. > > > Right now it is using compiled-in 32KB chunks. Should it use something > > like max(32kb,work_mem/128) instead? > > I'd say it should double the size of the request each time. That's what > we do in most places. > I thought the design goal there was that the space in old chunks could get re-used into the new chunks in a reasonably fine-grained way. If the last chunk contains just over half of all the data, that couldn't happen. Cheers, Jeff
