Re: Lockless uidinfo.

On Sat, Aug 18, 2007 at 04:35:42PM -0700, Jeff Roberson wrote:
On Sun, 19 Aug 2007, Pawel Jakub Dawidek wrote:
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)

How does this effect the single-threaded performance? Do you attribute
this to atomic fetchadd being cheaper than atomic cmpset? What is your
processor?

CPU: Intel(R) Xeon(R) CPU E5310 @ 1.60GHz (1597.65-MHz
K8-class CPU)
Origin = "GenuineIntel" Id = 0x6f7 Stepping = 7
Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
Features2=0x4e33d<SSE3,RSVD2,MON,DS_CPL,VMX,TM2,SSSE3,CX16,xTPR,PDCM,DCA>
AMD Features=0x20100800<SYSCALL,NX,LM>
AMD Features2=0x1<LAHF>
Cores per package: 4

Ok, I changed the code to something like this:

long old;
int diff, loops;

atomic_add_int(&uidinfo_cnt1, 1);
if (diff > 0) {
loops = 0;
do {
loops++;
old = uip->ui_sbsize;
if (old + diff > max)
return (0);
} while (atomic_cmpset_long(&uip->ui_sbsize, old, old + diff) == 0);
if (loops > 1)
atomic_add_int(&uidinfo_cnt2, loops);
} else {
atomic_add_long(&uip->ui_sbsize, (long)diff);
}

This allows me to see how many additional loops I do, because with
lock-free version we still can have contention and loop, that's why
wait-free version is superior.

Actually I was a bit surprised with the results:

debug.uidinfo.cnt1: 88746008
debug.uidinfo.cnt2: 31296304

(Running 8 processes.)

Which means, because of contention, we do 31296304 additional atomic
operations, which is about 30% more.

--
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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