Some new info from Reddit suggests Sony is using hbm2 combined with gddr4 and is extremely pleased with outcome for games,12gb gddr 4 and 8 gb hbm2 is reserved for games and 4 gb gddr4 for os :
24 GB RAM in total (20 GB usable by games)
8 GB in form of 2 * 4-Hi stacks HBM2
Sony got "amazing" deal for HBM
in part due to them buying up bad chips from other customers which can't run higher then 1.6 Gbps while keeping 1.2v.
HBM is expected to scale down in price a lot more than GDDR6 over the console lifetime
Samsung, Micron and SK Hynix already shifting part of their capacity towards HBM due to falling NAND prices
Sony will be one of the first high volume customers of TSMCs InFO_MS when mass production starts later this year (normal InFo already used by Apple in their iPhone)
InFO_MS brings down the cost compared to traditional silicon interposers - has thermal and performance advantage as well
InFO_MS allows them to drive their 1.6 Gbps chips @ 1.7 Gbps (435 GB/sec.) without having to increase the voltage above 1.2v
HBM is more power efficient compared to GDDR6 - the savings were invested into more GPU power
additional 16 GB in form of DDR4 @ 256 bit for 102.4 GB/sec.
4 GB reserved for OS, the remaining 12 GB usable by games
memory automatically managed by HBCC and appears as 20 GB to the developers
HBCC manages streaming of game data from storage as well
developers can use the API to take control if they choose and manage the memory and storage streaming themselves
memory solution alleviates problems found in PS4
namely that CPU bandwidth reduces GPU bandwidth disproportionately
2 stacks of HBM have 512 banks (more banks = fewer conflicts and higher utilization)
GDDR6 better than GDDR5 and GDDR5x in that regard but still less banks than HBM
at the same time trying to keep CPU memory access to slower DDR4
very satisfied with decision to use two kinds of memory for price to performance reasons
allowed them to go below ~50 GFLOPs per GB/sec. bandwidth but still keep above 40 GFLOPs per GB/sec.
He also mentions ps4 refresh is coming around September for 199 on 7nm but not pro.pro stays the same .