Messages in this thread | | | Date | Mon, 2 Feb 2015 22:38:51 -0800 | From | Andrew Morton <> | Subject | Re: [RFC] change non-atomic bitops method |
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On Tue, 3 Feb 2015 13:42:45 +0800 "Wang, Yalin" <Yalin.Wang@sonymobile.com> 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.
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