gprof SELECT COUNT(*) results

I did some gprof on a simple "SELECT COUNT(*) FROM test" query on cvs tip.

Linux josh.db 2.4.29-1 #2 Tue Jan 25 17:03:33 EST 2005 i686 unknown
gcc: 2.96
gprof: 2.13.90.0.2
./configure --without-readline

There are 260k or so records in table test(i int), about 1500 pages. I
give a shared_buffers to 3000, which is enough to hold all data pages.
Other GUCs are by default. After some warmups (to make sure these pages
are in the file system buffers), I do "SELECT COUNT(*)" for 10 times of
each round, and I tested 3 rounds. The results are:

- Round 1 - %   cumulative   self              self     totaltime   seconds   seconds    calls   s/call   s/call
name16.67     0.27     0.27  2648542     0.00     0.00  LWLockAcquire13.58      0.49     0.22  2648543     0.00
0.00 LWLockRelease 8.02      0.62     0.13  5266128     0.00     0.00  LockBuffer 8.02      0.75     0.13  2621456
0.00    0.00  heapgettup
 

- Round 2 - %   cumulative   self              self     totaltime   seconds   seconds    calls   s/call   s/call
name19.14     0.31     0.31  2648542     0.00     0.00  LWLockAcquire13.58      0.53     0.22  2648543     0.00
0.00 LWLockRelease11.11      0.71     0.18  2621456     0.00     0.00  heapgettup 6.79      0.82     0.11  5266128
0.00    0.00  LockBuffer
 

- Round 3 - %   cumulative   self              self     totaltime   seconds   seconds    calls   s/call   s/call
name17.12     0.25     0.25  2648542     0.00     0.00  LWLockAcquire 8.22      0.37     0.12  2648543     0.00
0.00 LWLockRelease 7.53      0.48     0.11  2621456     0.00     0.00  heapgettup 6.85      0.58     0.10  2621440
0.00    0.00  ExecEvalConst
 

There are some variance in the results, so my question is:
(1) Are these results faithful?
(2) If so, does it indicate that LWLock needs some improvements?

Regards,
Qingqing