Hi, On Fri, Sep 06, 2024 at 03:03:17PM +0000, Bertrand Drouvot wrote: > The struct size is 1040 Bytes and that's much more reasonable than the size > needed for per backend I/O stats in v1 (about 16KB).
One could think that's a high increase of shared memory usage with a high number of connections (due to the fact that it's implemented as "fixed amount" stats based on the max number of backends). So out of curiosity, I did some measurements with: dynamic_shared_memory_type=sysv shared_memory_type=sysv max_connections=20000 On my lab, ipcs -m gives me: - on master key shmid owner perms bytes nattch status 0x00113a04 51347487 postgres 600 1149394944 6 0x4bc9f2fa 51347488 postgres 600 4006976 6 0x46790800 51347489 postgres 600 1048576 2 - with v3 applied key shmid owner perms bytes nattch status 0x00113a04 51347477 postgres 600 1170227200 6 0x08e04b0a 51347478 postgres 600 4006976 6 0x74688c9c 51347479 postgres 600 1048576 2 So, with 20000 max_connections (not advocating that's a reasonable number in all the cases), that's a 1170227200 - 1149394944 = 20 MB increase of shared memory (expected with 20K max_connections and the new struct size of 1040 Bytes) as compare to master which is 1096 MB. It means that v3 produces about 2% shared memory increase with 20000 max_connections. Out of curiosity with max_connections=100000, then: - on master: ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00113a04 52953134 postgres 600 5053915136 6 0x37abf5ce 52953135 postgres 600 20006976 6 0x71c07d5c 52953136 postgres 600 1048576 2 - with v3: ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00113a04 52953144 postgres 600 5157945344 6 0x7afb24de 52953145 postgres 600 20006976 6 0x2695af58 52953146 postgres 600 1048576 2 So, with 100000 max_connections (not advocating that's a reasonable number in all the cases), that's a 5157945344 - 5053915136 = 100 MB increase of shared memory (expected with 100K max_connections and the new struct size of 1040 Bytes) as compare to master which is about 4800 MB. It means that v3 produces about 2% shared memory increase with 100000 max_connections. Then, based on those numbers, I think that the "fixed amount" approach is a good one as 1/ the amount of shared memory increase is "relatively" small and 2/ most probably provides performance benefits as compare to the "non fixed" approach. Regards, -- Bertrand Drouvot PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
