Re: Cache relation sizes?

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