On Tue, 3 Feb 2015 13:42:45 +0800 "Wang, Yalin" [off-list ref] wrote:

...

#ifdef CHECK_BEFORE_SET
			if (p[i] != times)
#endif

...

----
One run on CPU0, reader thread run on CPU1,
Test result:
sudo ./cache_test
reader:8.426228173
8.672198335

With -DCHECK_BEFORE_SET
sudo ./cache_test_check
reader:7.537036819
10.799746531

You aren't measuring the right thing.  You should compare

	if (p[i] != x)
		p[i] = x;

versus

	p[i] = x;

and you should do this for two cases:

a) p[i] == x

b) p[i] != x


The first code sequence will be slower when (p[i] != x) and faster when
(p[i] == x).


Next, we should instrument the kernel to work out the frequency of
set_bit on an already-set bit.

It is only with both these ratios that we can work out whether the
patch is a net gain.  My suspicion is that set_bit on an already-set
bit is so rare that the patch will be a loss.