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

Hi Amit, Kirk

>One idea could be to remove "nBlocksToInvalidate <
>BUF_DROP_FULL_SCAN_THRESHOLD" part of check "if (cached &&
>nBlocksToInvalidate < BUF_DROP_FULL_SCAN_THRESHOLD)" so that it always
>use optimized path for the tests. Then use the relation size as
>NBuffers/128, NBuffers/256, NBuffers/512 for different values of
>shared buffers as 128MB, 1GB, 20GB, 100GB.

I followed your idea to remove check and use different relation size for different shared buffers as 128M,1G,20G,50G(my
environmentcan't support 100G, so I choose 50G).
 
According to results, all three thresholds can get optimized, even NBuffers/128 when shared_buffers > 128M.
IMHO, I think NBuffers/128 is the maximum relation size we can get optimization in the three thresholds, Please let me
knowif I made something wrong. 
 

Recovery after vacuum test results as below ' Optimized percentage' and ' Optimization details(unit: second)' shows:
(512),(256),(128): means relation size is NBuffers/512, NBuffers/256, NBuffers/128
%reg: means (patched(512)- master(512))/ master(512)

Optimized percentage:
shared_buffers    %reg(512)    %reg(256)    %reg(128)
-----------------------------------------------------------------
128M        0%        -1%        -1%
1G         -65%        -49%        -62%
20G         -98%        -98%        -98%
50G         -99%        -99%        -99%

Optimization details(unit: second):
shared_buffers    master(512)    patched(512)    master(256)    patched(256)    master(128)    patched(128)

-----------------------------------------------------------------------------------------------------------------------------
128M        0.108        0.108        0.109        0.108        0.109        0.108
1G        0.310         0.107         0.410         0.208         0.811         0.309
20G         94.493         1.511         188.777     3.014         380.633     6.020
50G        537.978        3.815        867.453        7.524        1559.076    15.541

Test prepare:
Below is test table amount for different shared buffers. Each table size is 8k, so I use table amount = NBuffers/(512
or256 or 128):
 
shared_buffers    NBuffers    NBuffers/512    NBuffers/256    NBuffers/128
-------------------------------------------------------------------------------------------
128M        16384        32        64        128
1G        131072        256        512        1024
20G        2621440           5120        10240        20480
50G        6553600           12800        25600        51200

Besides, I also did single table performance test.
Still, NBuffers/128 is the max relation size which we can get optimization.

Optimized percentage:
shared_buffers    %reg(512)    %reg(256)    %reg(128)
-----------------------------------------------------------------
128M        0%        0%        -1%
1G         0%        1%        0%
20G         0%        -24%        -25%
50G         0%        -24%        -20%

Optimization details(unit: second):
shared_buffers    master(512)    patched(512)    master(256)    patched(256)    master(128)    patched(128)

-----------------------------------------------------------------------------------------------------------------------------
128M        0.107        0.107        0.108        0.108        0.108        0.107
1G        0.108         0.108         0.107         0.108         0.108         0.108
20G        0.208         0.208         0.409         0.309         0.409         0.308
50G        0.309         0.308         0.408         0.309         0.509         0.408

Any question on my test results is welcome.

Regards,
Tang