RE: [Patch] Optimize dropping of relation buffers using dlist

Hi Amit,

Sorry for my late reply. Here are my answers for your earlier questions.

>BTW, it is not clear why the advantage for single table is not as big as multiple tables with the Truncate command
I guess it's the amount of table blocks caused this difference. For single table I tested the amount of block is
threshold.
For multiple tables I test the amount of block is a value(like: one or dozens or hundreds) which far below threshold.
The closer table blocks to the threshold, the less advantage raised.

I tested below 3 situations of 50 tables when shared buffers=20G / 100G.
1. For multiple tables which had one or dozens or hundreds blocks(far below threshold) per table, we got significant
improve,like [1]. 
 
2. For multiple tables which has half threshold blocks per table, advantage become less, like [2].
3. For multiple tables which has threshold blocks per table, advantage become more less, like [3].

[1].  247 blocks per table
s_b    master        patched        %reg((patched-master)/patched)
----------------------------------------------------
20GB     1.109        0.108        -927%
100GB    3.113        0.108        -2782%

[2].  NBuffers/256/2 blocks per table
s_b    master        patched        %reg
----------------------------------------------------
20GB     2.012        1.210        -66%
100GB    10.226        6.4        -60%

[3].  NBuffers/256 blocks per table
s_b    master        patched        %reg
----------------------------------------------------
20GB     3.868        2.412        -60%
100GB    14.977        10.591        -41%

>Can you share your exact test steps for any one of the tests? Also, did you change autovacumm = off for these tests?
Yes, I configured streaming replication environment as Kirk did before.
autovacumm = off. 
full_page_writes = off. 
checkpoint_timeout = 30min

Test steps: 
e.g. shared_buffers=20G, NBuffers/512, table blocks= 20*1024*1024/8/512=5120 . table size(kB)= 20*1024*1024/512=40960kB

1. (Master) create table test(id int, v_ch varchar, v_ch1 varchar); 
2. (Master) insert about 40MB data to table.
3. (Master) delete from table (all rows of table) 
4. (Standby) To test with failover, pause the WAL replay on standby server.
   SELECT pg_wal_replay_pause();
5. (Master) VACUUM;
6. (Master) Stop primary server. pg_ctl stop -D $PGDATA -w 7. (Standby) Resume wal replay and promote standby. (get the
recoverytime from this step)
 

Regards
Tang