Re: [Xen-users] Disk iops performance scalability

[John Morrison <john@xxxxxxxxxxxxx>]

Hi,

It was a typo in the config file (cpus=) that caused the drop in iops. Now I 
get 145k consistently.
The other issue still remains, very low ioping output, especially as the first 
comes back in 1ms then the rest take 6 times as long


Thanks

John
On 19 Mar 2013, at 09:07, Adi Kriegisch <adi@xxxxxxxxxxxxxxx> wrote:

> Hi!
> 
>> Seeing a drop-off in iops when more vcpu's are added:-
>> 
>> 3.8.2 kernel/xen-4.2.1/single domU/LVM backend/8GB RAM domU/2GB RAM dom0
>> 
>> dom0_max_vcpus=2 dom0_vcpus_pin
>> 
>> domU  8 cores  fio result 145k iops
> [...]
>> domU 14 cores fio result 81k iops
> [...]
>> The initial ioping response is good, then a lot of latency with later ones. 
>> Any ideas ?
> There are several possible optimizations:
> * At first I'd try to set the I/O-Scheduler for all DomUs to 'noop'; there is
>  no benefit in scheduling I/Os twice.
> * Second thing I'd try is to give Dom0 more scheduler weight
>  (xm sched-credit -d 0 -w 512).
> Now do the benchmarks again and try to find the bottlenecks by using
> dstat/iostat/... to find out what is going on. Probably rebalancing
> interrupts on the Dom0 may help (either manual or with the help of
> irqbalance).
> There are several other optimizations possible, but most of them depend on
> your usecase (like increasing RAM on Dom0 for caching, changeing readahead
> settings in Dom0 for the storage backend to better fit the I/O requests or
> adding more CPUs to Dom0 to be able to better handle the storage backend
> and things like that.)
> 
> Please post your results!
> 
> -- Adi


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