Re: Lockless uidinfo.

On Sun, Aug 19, 2007 at 12:07:56AM +0200, Pawel Jakub Dawidek wrote:
Two more things...

The patch below remove per-uidinfo locks:

http://people.freebsd.org/~pjd/patches/uidinfo_lockless.patch

We could upgrade from lock-free algorithm I used here to wait-free
algorithm, but we don't have atomic_fetchadd_long(). How hard will it be
to implement it?

We could then change:

do {
old = uip->ui_proccnt;
if (old + diff > max)
return (0);
} while (atomic_cmpset_long(&uip->ui_proccnt, old, old + diff) == 0);

to something like this:

if (atomic_fetchadd_long(&uip->ui_proccnt, diff) + diff > max) {
atomic_subtract_long(&uip->ui_proccnt, diff);
return (0);
}

Ok, after implementing atomic_fetchadd_long() on amd64, we get additional
6% of performance improvement:

x ./uidinfo_lockfree.txt (atomic_cmpset_long loop)
+ ./uidinfo_waitfree.txt (atomic_fetchadd_long)
+------------------------------------------------------------------------------+
| +|
| +|
|x xx xx + ++|
| |__MA___| |AM|
+------------------------------------------------------------------------------+
N Min Max Median Avg Stddev
x 5 1561566 1575987 1568964 1569767 5853.1399
+ 5 1662362 1665936 1665810 1664881.8 1541.2693
Difference at 95.0% confidence
95114.8 +/- 6241.96
6.05917% +/- 0.397636%
(Student's t, pooled s = 4279.88)

--
Pawel Jakub Dawidek http://www.wheel.pl
pjd@xxxxxxxxxxx http://www.FreeBSD.org
FreeBSD committer Am I Evil? Yes, I Am!

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