Hi,
LWLockPadded is either 16 or 32 bytes, so modern systems (e.g. Core2
or AMD Opteron [1]) with cacheline size of 64 bytes can get
false-sharing in lwlocks.
I changed LWLOCK_PADDED_SIZE in src/backend/storage/lmgr/lwlock.c to
64, and ran sysbench OLTP read-only benchmark, and got a slight
improvement in throughput:
Hardware: single-socket Core 2, quad-core, Q6600 @ 2.40GHz
Software: Linux 2.6.28-17, glibc 2.9, gcc 4.3.3
PostgreSQL: 8.5alpha3
sysbench parameters: sysbench --num-threads=4 --max-requests=0
--max-time=120 --oltp-read-only=on --test=oltp
original: 3227, 3243, 3243
after: 3256, 3255, 3253
So there is a speedup of 1.005 or what other people usually call it, a
0.5% improvement.
However, it's a single socket machine, so all the cache traffic does
not need to go off-chip. Can someone with a multi-socket machine help
me run some test so that we can get a better idea of how this change
(patch attached) performs in bigger systems??
Thanks,
Rayson
P.S. And I just googled and found similar discussions about padding
LWLOCK_PADDED_SIZE, but the previous work was done on an IBM POWER
system, and the benchmark used was apachebench. IMO, the setup was too
complex to measure a small performance improvement in PostgreSQL.
[1] Performance Guidelines for AMD Athlon™ 64 and AMD Opteron™ ccNUMA
Multiprocessor Systems Application Note
Index: src/backend/storage/lmgr/lwlock.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/storage/lmgr/lwlock.c,v
retrieving revision 1.55
diff -u -4 -r1.55 lwlock.c
--- src/backend/storage/lmgr/lwlock.c 2 Jan 2010 16:57:52 -0000 1.55
+++ src/backend/storage/lmgr/lwlock.c 11 Jan 2010 16:40:03 -0000
@@ -57,9 +57,11 @@
*
* LWLock is between 16 and 32 bytes on all known platforms, so these two
* cases are sufficient.
*/
-#define LWLOCK_PADDED_SIZE (sizeof(LWLock) <= 16 ? 16 : 32)
+/* #define LWLOCK_PADDED_SIZE (sizeof(LWLock) <= 16 ? 16 : 32) */
+
+#define LWLOCK_PADDED_SIZE 64
typedef union LWLockPadded
{
LWLock lock;
--
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