On 12.06.2023 3:23 PM, Pavel Borisov wrote:
Is the following true or not?
1. If we switch processes to threads but leave the amount of session
local variables unchanged, there would be hardly any performance gain.
2. If we move some backend's local variables into shared memory then
the performance gain would be very near to what we get with threads
having equal amount of session-local variables.
In other words, the overall goal in principle is to gain from less
memory copying wherever it doesn't add the burden of locks for
concurrent variables access?
Regards,
Pavel Borisov,
Supabase
IMHO both statements are not true.
Switching to threads will cause less context switch overhead (because
all threads are sharing the same memory space and so preserve TLB.
How big will be this advantage? In my prototype I got ~10%. But may be
it is possible to fin workloads when it is larger.
Postgres backend is "thick" not because of large number of local variables.
It is because of local caches: catalog cache, relation cache, prepared
statements cache,...
If they are not rewritten, then backend still may consume a lot of
memory even if it will be thread rather then process.
But threads simplify development of global caches, although it can be
done with DSM.
