Date: Sun, 19 Aug 2007 01:09:17 +0200 From: Pawel Jakub Dawidek <pjd@FreeBSD.org> To: freebsd-arch@FreeBSD.org Subject: Re: Lockless uidinfo. Message-ID: <20070818230917.GI6498@garage.freebsd.pl> In-Reply-To: <20070818220756.GH6498@garage.freebsd.pl> References: <20070818120056.GA6498@garage.freebsd.pl> <20070818220756.GH6498@garage.freebsd.pl>
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[-- Attachment #1 --] 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@FreeBSD.org http://www.FreeBSD.org FreeBSD committer Am I Evil? Yes, I Am! [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.4 (FreeBSD) iD8DBQFGx3wdForvXbEpPzQRApSQAKDIUM4EV6lomKGgQouhx/RlehhVbwCgoTPf rG0yxVgxqJb74QUyxYDSZKY= =R+1E -----END PGP SIGNATURE-----
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