Re: Scaling shared buffer eviction

Data with LWLOCK_STATS
----------------------------------------------
BufMappingLocks

PID 7245 lwlock main 38: shacq 41117 exacq 34561 blk 36274 spindelay 101
PID 7310 lwlock main 39: shacq 40257 exacq 34219 blk 25886 spindelay 72
PID 7308 lwlock main 40: shacq 41024 exacq 34794 blk 20780 spindelay 54
PID 7314 lwlock main 40: shacq 41195 exacq 34848 blk 20638 spindelay 60
PID 7288 lwlock main 41: shacq 84398 exacq 34750 blk 29591 spindelay 128
PID 7208 lwlock main 42: shacq 63107 exacq 34737 blk 20133 spindelay 81
PID 7245 lwlock main 43: shacq 278001 exacq 34601 blk 53473 spindelay 503
PID 7307 lwlock main 44: shacq 85155 exacq 34440 blk 19062 spindelay 71
PID 7301 lwlock main 45: shacq 61999 exacq 34757 blk 13184 spindelay 46
PID 7235 lwlock main 46: shacq 41199 exacq 34622 blk 9031 spindelay 30
PID 7324 lwlock main 46: shacq 40906 exacq 34692 blk 8799 spindelay 14
PID 7292 lwlock main 47: shacq 41180 exacq 34604 blk 8241 spindelay 25
PID 7303 lwlock main 48: shacq 40727 exacq 34651 blk 7567 spindelay 30
PID 7230 lwlock main 49: shacq 60416 exacq 34544 blk 9007 spindelay 28
PID 7300 lwlock main 50: shacq 44591 exacq 34763 blk 6687 spindelay 25
PID 7317 lwlock main 50: shacq 44349 exacq 34583 blk 6861 spindelay 22
PID 7305 lwlock main 51: shacq 62626 exacq 34671 blk 7864 spindelay 29
PID 7301 lwlock main 52: shacq 60646 exacq 34512 blk 7093 spindelay 36
PID 7324 lwlock main 53: shacq 39756 exacq 34359 blk 5138 spindelay 22

This data shows that after patch, there is no contention
for BufFreeListLock, rather there is a huge contention around
BufMappingLocks.  I have checked that HEAD also has contention
around BufMappingLocks.

As per my analysis till now, I think reducing contention around
BufFreelistLock is not sufficient to improve scalability, we need
to work on reducing contention around BufMappingLocks as well.