On 2020-Feb-16, Lars Aksel Opsahl wrote: > On a server with 32 cores and 250 GB memory, with CentOS 7 and kernel > 4.4.214-1.el7.elrepo.x86_64, I try to run 30 parallel threads using > dblink. (https://github.com/larsop/postgres_execute_parallel) . I have > tried to disconnect and reconnect in the dblink code and that did not > help.
I think one issue is that pg_clog has 128 buffers (per commit 5364b357fb1) while subtrans only has 32. It might be productive to raise the number of subtrans buffers (see #define NUM_SUBTRANS_BUFFERS in src/include/access/subtrans.h; requires a recompile.) Considering that each subtrans entry is 16 times larger than clog (2 bits vs. 4 bytes), you'd require 2048 subtrans buffers to cover the same XID range without I/O if my math is right. That's only 16 MB ... though slru.c code might not be prepared to deal with that many buffers. Worth some experimentation, I guess. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
