Re: [PATCH 0/5] ftrace: to kill a daemon

[Steven Rostedt]

On Fri, 8 Aug 2008, Linus Torvalds wrote:
> 
> 
> On Fri, 8 Aug 2008, Jeremy Fitzhardinge wrote:
> >
> > Steven Rostedt wrote:
> > > I wish we had a true 5 byte nop. 
> > 
> > 0x66 0x66 0x66 0x66 0x90
> 
> I don't think so. Multiple redundant prefixes can be really expensive on 
> some uarchs.
> 
> A no-op that isn't cheap isn't a no-op at all, it's a slow-op.


A quick meaningless benchmark showed a slight perfomance hit.

Here's 10 runs of "hackbench 50" using the two part 5 byte nop:

run 1
Time: 4.501
run 2
Time: 4.855
run 3
Time: 4.198
run 4
Time: 4.587
run 5
Time: 5.016
run 6
Time: 4.757
run 7
Time: 4.477
run 8
Time: 4.693
run 9
Time: 4.710
run 10
Time: 4.715
avg = 4.6509


And 10 runs using the above 5 byte nop:

run 1
Time: 4.832
run 2
Time: 5.319
run 3
Time: 5.213
run 4
Time: 4.830
run 5
Time: 4.363
run 6
Time: 4.391
run 7
Time: 4.772
run 8
Time: 4.992
run 9
Time: 4.727
run 10
Time: 4.825
avg = 4.8264

# cat /proc/cpuinfo
processor	: 0
vendor_id	: AuthenticAMD
cpu family	: 15
model		: 65
model name	: Dual-Core AMD Opteron(tm) Processor 2220
stepping	: 3
cpu MHz		: 2799.992
cache size	: 1024 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca 
cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt 
rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm extapic 
cr8_legacy
bogomips	: 5599.98
clflush size	: 64
power management: ts fid vid ttp tm stc

There's 4 of these.

Just to make sure, I ran the above nop test again:

[ this is reverse from the above runs ]

run 1
Time: 4.723
run 2
Time: 5.080
run 3
Time: 4.521
run 4
Time: 4.841
run 5
Time: 4.696
run 6
Time: 4.946
run 7
Time: 4.754
run 8
Time: 4.717
run 9
Time: 4.905
run 10
Time: 4.814
avg = 4.7997

And again the two part nop:

run 1
Time: 4.434
run 2
Time: 4.496
run 3
Time: 4.801
run 4
Time: 4.714
run 5
Time: 4.631
run 6
Time: 5.178
run 7
Time: 4.728
run 8
Time: 4.920
run 9
Time: 4.898
run 10
Time: 4.770
avg = 4.757


This time it was close, but still seems to have some difference.

heh, perhaps it's just noise.

-- Steve

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