On Thu, Sep 22, 2022 at 11:46 AM John Naylor <john.nay...@enterprisedb.com> wrote: > One thing I want to try soon is storing fewer than 16/32 etc entries, so that the whole node fits comfortably inside a power-of-two allocation. That would allow us to use aset without wasting space for the smaller nodes, which would be faster and possibly would solve the fragmentation problem Andres referred to in
> https://www.postgresql.org/message-id/20220704220038.at2ane5xkymzzssb%40awork3.anarazel.de While calculating node sizes that fit within a power-of-two size, I noticed the current base node is a bit wasteful, taking up 8 bytes. The node kind only has a small number of values, so it doesn't really make sense to use an enum here in the struct (in fact, Andres' prototype used a uint8 for node_kind). We could use a bitfield for the count and kind: uint16 -- kind and count bitfield uint8 shift; uint8 chunk; That's only 4 bytes. Plus, if the kind is ever encoded in a pointer tag, the bitfield can just go back to being count only. Here are the v6 node kinds: node4: 8 + 4 +(4) + 4*8 = 48 bytes node16: 8 + 16 + 16*8 = 152 node32: 8 + 32 + 32*8 = 296 node128: 8 + 256 + 128/8 + 128*8 = 1304 node256: 8 + 256/8 + 256*8 = 2088 And here are the possible ways we could optimize nodes for space using aset allocation. Parentheses are padding bytes. Even if my math has mistakes, the numbers shouldn't be too far off: node3: 4 + 3 +(1) + 3*8 = 32 bytes node6: 4 + 6 +(6) + 6*8 = 64 node13: 4 + 13 +(7) + 13*8 = 128 node28: 4 + 28 + 28*8 = 256 node31: 4 + 256 + 32/8 + 31*8 = 512 (XXX not good) node94: 4 + 256 + 96/8 + 94*8 = 1024 node220: 4 + 256 + 224/8 + 220*8 = 2048 node256: = 4096 The main disadvantage is that node256 would balloon in size. -- John Naylor EDB: http://www.enterprisedb.com
