Re: Add the ability to limit the amount of memory that can be allocated to backends.

Hello!

Earlier in this thread, the pgbench results were published, where with a strong memory limit of 100MB
a significant, about 10%, decrease in TPS was observed [1].

Using dedicated server with 12GB RAM and methodology described in [3], i performed five series
of measurements for the patches from the [2].
The series were like this:
1) unpatched 16th version at the REL_16_BETA1 (e0b82fc8e83) as close to [2] in time.
2) patched REL_16_BETA1 at e0b82fc8e83 with undefined max_total_backend_memory GUC (with default value = 0).
3) patched REL_16_BETA1 with max_total_backend_memory = 16GB
4) the same with max_total_backend_memory = 8GB
5) and again with max_total_backend_memory = 200MB

Measurements with max_total_backend_memory = 100MB were not be carried out,
with limit 100MB the server gave an error on startup:
FATAL: configured max_total_backend_memory 100MB is <= shared_memory_size 143MB
So i used 200MB to retain all other GUCs the same.

Pgbench gave the following results:
1) and 2) almost the same: ~6350 TPS. See orange and green
distributions on the attached graph.png respectively.
3) and 4) identical to each other (~6315 TPS) and a bit slower than 1) and 2) by ~0,6%.
See blue and yellow distributions respectively.
5) is slightly slower (~6285 TPS) than 3) and 4) by another 0,5%. (grey distribution)
The standard error in all series was ~0.2%. There is a raw data in the raw_data.txt.

With the best wishes,

--
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

[1] https://www.postgresql.org/message-id/3178e9a1b7acbcf023fafed68ca48d76afc07907.camel%40crunchydata.com
[2] https://www.postgresql.org/message-id/4edafedc0f8acb12a2979088ac1317bd7dd42145.camel%40crunchydata.com
[3] https://www.postgresql.org/message-id/1d3a7d8f-cb7c-4468-a578-d8a1194ea2de%40postgrespro.ru