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Please help: 97% CPU cycles are wasted in disk I/O wait on FC4/FC5?



Hi, 

 Just found a strange things happened on my FC4/FC5
boxes: around 90% CPU times are wasted in disk I/O
wait when I doing a simple "dd if=/dev/sdb
of=/dev/null >/dev/null 2>&1 &" test. vmstat shows the
CPU cycles when there are I/O waiting. And at these
times ssh sessions and keyboard response is pretty
slow.

On old FC1 and Redhat 9 I did't see this problem, I
configured 2.4 kernels to accept other Interrupts
during disk I/O to for much beter interactive
responses. How can I do it again on 2.6 kernels?

The following is my test results. Please have a look
and help.


[root testnode01 ~]# vmstat 2 100
procs -----------memory---------- ---swap--
-----io---- --system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi   
bo   in    cs us sy id wa
 1  0    160  29792 2626332 556320    0    0  1849   
18  695   148  1  1 96  2
 0  0    160  29304 2626368 556284    0    0   110   
42  286    47 14  0 83  3
 0  0    160  34880 2626396 550100    0    0   156   
44  289    68  9  2 84  5

[root testnode01 ~]# dd if=/dev/sdc of=/dev/null 2>&1
&
[1] 9933
[root testnode01 ~]# vmstat 2 100
procs -----------memory---------- ---swap--
-----io---- --system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi   
bo   in    cs us sy id wa
 1  2    160  20284 2747752 441056    0    0  1853   
18  694   148  1  1 96  2
 0  1    160  20496 2865276 321480    0    0 59062    
0  499   530 11 13  0 77
 0  1    160  20604 2981924 202780    0    0 58496   
46  493   518  4 13  0 83
 0  1    160  20508 3099396  83256    0    0 59008   
24  499   528  3 14  0 83
 0  1    160  21372 3156688  23912    0    0 59008    
0  497   519  4 13  0 83

[root testnode01 ~]# cat /proc/loadavg
0.51 0.23 0.10 2/84 9937
[root testnode01 ~]# uname -a
Linux testnode01.example.com 2.6.15.6_64_v3tcp #1 SMP
Fri Mar 17 16:59:19 PST 2006 x86_64 x86_64 x86_64
GNU/Linux
[root testnode01 ~]# cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 3
model name      :               Intel(R) Pentium(R) 4
CPU 3.60GHz
stepping        : 4
cpu MHz         : 3600.302
cache size      : 1024 KB
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : 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 ht tm syscall lm constant_tsc pni
monitor ds_cpl cid xtpr
bogomips        : 7211.15
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 48 bits virtual
power management:

 



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