On Thu, 29 Apr 2021 at 12:30, Yura Sokolov <y.soko...@postgrespro.ru> wrote: > > David Rowley писал 2021-04-29 02:51: > > Another line of thought for making it go faster would be to do > > something like get rid of the hash status field from SMgrEntry. That > > could be either coded into a single bit we'd borrow from the hash > > value, or it could be coded into the least significant bit of the data > > field. A pointer to palloc'd memory should always be MAXALIGNed, > > which means at least the lower two bits are always zero. We'd just > > need to make sure and do something like "data & ~((uintptr_t) 3)" to > > trim off the hash status bits before dereferencing the pointer. That > > would make the SMgrEntry 12 bytes on a 64-bit machine. However, it > > would also mean that some entries would span 2 cache lines, which > > might affect performance a bit. > > Then data pointer will be itself unaligned to 8 bytes. While x86 is > mostly indifferent to this, doubtfully this memory economy will pay > off.
Actually, I didn't think very hard about that. The struct would still be 16 bytes and just have padding so the data pointer was aligned to 8 bytes (assuming a 64-bit machine). David
