LWLock statistics collector

Hi,

Here is a patch to collect statistics of LWLocks.
The following information are available on pg_stat_lwlocks view:
- sh_call : number of locked for share mode
- sh_wait : number of blocked for share mode
- ex_call : number of locked for exclusive mode
- ex_wait : number of blocked for exclusive mode

This feature is for developers, so available only if LWLOCK_STAT is defined
(default off). Otherwise, stab functions are installed.

There is room for discussion.
- Should not collect information for shared buffers?
If not, we can reduce consumption of memory for stats.
- Information for LWLockConditionalAcquire()
It is not gathered now because of the size limitation of LWLockStat
struct. I intended the total size of LWLock to be less than 64bytes.
- Information for spinlocks
Number of sleeps in s_lock() might be useful for analyzing
spinlock-level lock contentions.
- Documentation
Where to document such a feature for only developers?
...

Comments welcome.

# SELECT * FROM pg_stat_lwlocks;
kind | lwlock | sh_call | sh_wait | ex_call | ex_wait
------+----------------------------+---------+---------+---------+---------
0 | BufFreelistLock | 0 | 0 | 1237 | 0
1 | ShmemIndexLock | 0 | 0 | 437 | 0
2 | OidGenLock | 0 | 0 | 0 | 0
3 | XidGenLock | 80086 | 9 | 8174 | 0
4 | ProcArrayLock | 162491 | 59 | 16231 | 38
5 | SInvalLock | 163423 | 0 | 180 | 0
6 | FreeSpaceLock | 0 | 0 | 399 | 0
7 | WALInsertLock | 0 | 0 | 67214 | 247
8 | WALWriteLock | 0 | 0 | 8028 | 12
9 | ControlFileLock | 0 | 0 | 16 | 0
10 | CheckpointLock | 0 | 0 | 0 | 0
11 | CheckpointStartLock | 8028 | 0 | 0 | 0
12 | CLogControlLock | 22449 | 9 | 8028 | 4
13 | SubtransControlLock | 32731 | 0 | 4 | 0
14 | MultiXactGenLock | 0 | 0 | 0 | 0
15 | MultiXactOffsetControlLock | 0 | 0 | 0 | 0
16 | MultiXactMemberControlLock | 0 | 0 | 0 | 0
17 | RelCacheInitLock | 0 | 0 | 0 | 0
18 | BgWriterCommLock | 0 | 0 | 1185 | 0
19 | TwoPhaseStateLock | 0 | 0 | 0 | 0
20 | TablespaceCreateLock | 0 | 0 | 0 | 0
21 | BtreeVacuumLock | 0 | 0 | 12 | 0
22 | BufMappingLock | 322414 | 1 | 315 | 0
23 | LockMgrLock | 0 | 0 | 207585 | 6927
24 | BufferIO | 0 | 0 | 2295 | 0
25 | BufferContent | 522714 | 362 | 83375 | 324
26 | CLogBuffer | 0 | 0 | 0 | 0
27 | SubTransBuffer | 0 | 0 | 0 | 0
28 | MultiXactOffsetBuffer | 0 | 0 | 0 | 0
29 | MultiXactMemberBuffer | 0 | 0 | 0 | 0
(30 rows)

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center