Re: Let's make PostgreSQL multi-threaded

On 2023-06-07 We 17:58, Andres Freund wrote:
> 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