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 environment can'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 know if 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 or 256 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