Add mutex test.

Author: bmomjian

doc/TODO.detail/performance

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+From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
+To: "'pgsql-hackers@postgresql.org'" <pgsql-hackers@postgresql.org>
+Subject: [HACKERS] Using POSIX mutex-es
+Date: Wed, 1 Aug 2001 12:04:24 -0700 
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+Status: OR
+
+1. Just changed
+	TAS(lock) to pthread_mutex_trylock(lock)
+	S_LOCK(lock) to pthread_mutex_lock(lock)
+	S_UNLOCK(lock) to pthread_mutex_unlock(lock)
+(and S_INIT_LOCK to share mutex-es between processes).
+
+2. pgbench was initialized with scale 10.
+   SUN WS 10 (512Mb), Solaris 2.6 (I'm unable to test on E4500 -:()
+   -B 16384, wal_files 8, wal_buffers 256,
+   checkpoint_segments 64, checkpoint_timeout 3600
+   50 clients x 100 transactions
+   (after initialization DB dir was saved and before each test
+    copyed back and vacuum-ed).
+
+3. No difference.
+   Mutex version maybe 0.5-1 % faster (eg: 37.264238 tps vs 37.083339 tps).
+
+So - no gain, but no performance loss "from using pthread library"
+(I've also run tests with 1 client), at least on Solaris.
+
+And so - looks like we can use POSIX mutex-es and conditional variables
+(not semaphores; man pthread_cond_wait) and should implement light lmgr,
+probably with priority locking.
+
+Vadim
+
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