Re: [Xen-users] 99% iowait on one core in 8 core processor

[Matthias <matthias.kannenberg@xxxxxxxxxxxxxx>]

Hi,

I experianced the same.

I figured the reason behind this has nothing to do with what (v)cpus
you assign to the domUs. In fact, I think that the real reason is that
all I/O emulation from the domUs (like HDD, Network and Stuff) is all
handled by CPU0 of dom0, even in a multi-core-architecture.

You can test this with trying something I/O intensive like a hdd
benchmark in the domU and you will see the dom0-cpu0 utilization
rising.

Using PVHVM or GPLPV-Driver only reduces the amount of utilization but
it is still handled by cpu0 only. Due to that fact i leave cpu0 to my
dom0 alone and only assign the other cpu-cores to the domUs.


Note: This is only an assumption I made after seeing my machines
behaviour. If the above isn't true, please correct me.


2012/7/11 Rajesh Kumar <rajesh@xxxxxxxx>:
> Hi All,
>
> We have a xen server and using 8 core processor.
>
> I can see that there is 99% iowait on only  core 0.
>
>
> 02:28:49 AM  CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal   
> %idle    intr/s
> 02:28:54 AM  all    0.00    0.00    0.00   12.65    0.00    0.02    2.24   
> 85.08   1359.88
> 02:28:54 AM    0    0.00    0.00    0.00   96.21    0.00    0.20    3.19    
> 0.40    847.11
> 02:28:54 AM    1    0.00    0.00    0.00    6.41    0.00    0.00    9.42   
> 84.17    219.56
> 02:28:54 AM    2    0.00    0.00    0.00    0.00    0.00    0.00    0.00  
> 100.00      2.59
> 02:28:54 AM    3    0.00    0.00    0.00    0.00    0.00    0.00    2.12   
> 97.88     76.25
> 02:28:54 AM    4    0.00    0.00    0.00    0.00    0.00    0.00    1.20   
> 98.80    118.56
> 02:28:54 AM    5    0.00    0.00    0.00    0.00    0.00    0.00    0.00  
> 100.00      3.59
> 02:28:54 AM    6    0.00    0.00    0.00    0.00    0.00    0.00    2.02   
> 97.98     89.62
> 02:28:54 AM    7    0.00    0.00    0.00    0.00    0.00    0.00    0.20   
> 99.80      2.59
>
> 02:28:54 AM  CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal   
> %idle    intr/s
> 02:28:59 AM  all    0.00    0.00    0.00   12.48    0.00    0.00    2.78   
> 84.74   1317.43
> 02:28:59 AM    0    0.00    0.00    0.00   98.80    0.00    0.00    0.80    
> 0.40    885.17
> 02:28:59 AM    1    0.00    0.00    0.00    0.00    0.00    0.00   11.38   
> 88.62    151.30
> 02:28:59 AM    2    0.00    0.00    0.00    0.00    0.00    0.00    0.20   
> 99.80      2.81
> 02:28:59 AM    3    0.00    0.00    0.00    0.00    0.00    0.00    7.21   
> 92.79     94.79
> 02:28:59 AM    4    0.00    0.00    0.00    0.00    0.00    0.00    2.20   
> 97.80    170.34
> 02:28:59 AM    5    0.00    0.00    0.00    0.00    0.00    0.00    0.00  
> 100.00      4.41
> 02:28:59 AM    6    0.00    0.00    0.00    0.00    0.00    0.00    0.00  
> 100.00      5.81
> 02:28:59 AM    7    0.00    0.00    0.00    0.00    0.00    0.00    0.00  
> 100.00      2.81
>
>
> I have even tries changing the cpus mapped to the DomU with out no effect.
>
> Name                              CPU  CPU Affinity
> 4pulse                              1           1
> 2music                             3            3
> Domain-0                        0               0
> Domain-0                        1               1
> Domain-0                        2               2
> Domain-0                        3               3
> Domain-0                        4               4
> Domain-0                        5               5
> Domain-0                        6               6
> Domain-0                        7               7
> analshah                         6              any cpu
> arunvelayudhan            7             any cpu
> backup                            7             any cpu
> crickruns                         3             1-3
> crickruns                         2             1-3
> crickruns                         1             1-3
> crickruns                         2             1-3
> crickruns                         1             1-3
> crickruns                         1             1-3
> dedicatedjv                     7               any cpu
> yeluthu                             4           3-5
> yeluthu                             3           3-5
> yeluthu                             3           3-5
> yeluthu                             3           3-5
> yeluthu                             3           3-5
> yeluthu                             3           3-5
> freshnfresh                      3              any cpu
> monitoring                       7              any cpu
> reporter                                  6             5-7
> reporter                          7             5-7
> reporter                          6             5-7
> reporter                          6             5-7
> reporter                          7             5-7
> reporter                                  7             5-7
> reporter                          5             5-7
> reporter                          7             5-7
> radio03                            7            any cpu
> saampeter                       2               1-2
> saampeter                       2               1-2
>
>
> Thanks,
> Rajesh
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