Hiroyuki KAMEZAWA [off-list ref] wrote:

Hi,
I testd set_bit()/__set_bit() ops, atomic and non atomic ops, on my Xeon.
I think this test is not perfect, but shows some aspect of pefromance of atomic ops.

Oh, atomic ops on a P4 hurt like hell.  Try doing this:

	time dd if=/dev/null of=foo bs=1 count=1M

on an SMP kernel and compare it with a uniproc kernel.  The difference is
large.

Certainly, executing an atomic op in a tight loop will show a lot of
difference.  But that doesn't mean that making these operations non-atomic
makes a significant difference to overall kernel performance!

But whatever - it all adds up.  The microoptimisation is fine - let's go
that way.

Result:
[root@kanex2 atomic]# nice -10 ./test-atomics
score 0 is            64011 note: cache hit, no atomic
score 1 is           543011 note: cache hit, atomic
score 2 is           303901 note: cache hit, mixture
score 3 is           344261 note: cache miss, no atomic
score 4 is          1131085 note: cache miss, atomic
score 5 is           593443 note: cache miss, mixture
score 6 is           118455 note: cache hit, dependency, noatomic
score 7 is           416195 note: cache hit, dependency, mixture

smaller score is better.
score 0-2 shows set_bit/__set_bit performance during good cache hit rate.
score 3-5 shows set_bit/__set_bit performance during bad cache hit rate.
score 6-7 shows set_bit/__set_bit performance during good cache hit
but there is data dependency on each access in the tight loop.

I _think_ the above means atomic ops are 10x more costly, yes?

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