Lockless uidinfo.

[Pawel Jakub Dawidek]

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 at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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