Hi, On 2018-11-07 11:40:22 +1300, Thomas Munro wrote: > PostgreSQL likes to probe the size of relations with lseek(SEEK_END) a > lot. For example, a fully prewarmed pgbench -S transaction consists > of recvfrom(), lseek(SEEK_END), lseek(SEEK_END), sendto(). I think > lseek() is probably about as cheap as a syscall can be so I doubt it > really costs us much, but it's still a context switch and it stands > out when tracing syscalls, especially now that all the lseek(SEEK_SET) > calls are gone (commit c24dcd0cfd).
I'd really really like to see some benchmarking before embarking on a more complex scheme. I aesthetically dislike those lseeks, but ... > If we had a different kind of buffer mapping system of the kind that > Andres has described, there might be a place in shared memory that > could track the size of the relation. Even if we could do that, I > wonder if it would still be better to do a kind of per-backend > lock-free caching, like this: Note that the reason for introducing that isn't primarily motivated by getting rid of the size determining lseeks, but reducing the locking for extending *and* truncating relations. Greetings, Andres Freund
