cache false-sharing in lwlocks

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