Re: Let's make PostgreSQL multi-threaded

Hi,

On 2023-06-07 08:53:24 -0400, Robert Haas wrote:

Now, Andres is not a man who accepts a tax on performance of any size
without a fight, so his "really expensive" might turn out to resemble my
"pretty cheap." However, if widespread use of TLS is too expensive and we
have to start rewriting code to not depend on global variables, that's going
to be more of a problem. If we can get by with doing such rewrites only in
performance-critical places, it might not still be too bad. Personally, I
think the degree of dependence that PostgreSQL has on global variables is
pretty excessive and I don't think that a certain amount of refactoring to
reduce it would be a bad thing. If it turns into an infinite series of
hastily-written patches to rejigger every source file we have, though, then
I'm not really on board with that.

I think a lot of such rewrites would be a good idea, even if we right now all
agree to swear we'll never go to threads.  Not having any sort of grouping of
global variables makes it IMO considerably harder to debug. I can easily ask
somebody to print out a variable pointing to a struct describing the state of
a subsystem. I can't really do that for 50 variables.

And once you do that, I think you reduce the TLS cost substantially. The
variable pointing to the struct is already likely in a register. Whereas each
individual variable being in TLS makes the job harder for the compiler.

I could certainly get on board with a project to tame the use of global variables.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com