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Re: [libvirt] [PATCH v3 4/6] qemu: Implement period and quota tunable XML configuration and parsing



At 07/19/2011 09:27 PM, Adam Litke Write:
> 
> 
> On 07/18/2011 07:51 PM, Wen Congyang wrote:
>> At 07/19/2011 04:59 AM, Adam Litke Write:
>>>
>>>
>>> On 07/18/2011 04:42 AM, Wen Congyang wrote:
>>>> +int qemuSetupCgroupForVcpu(struct qemud_driver *driver, virDomainObjPtr vm)
>>>> +{
>>>> +    virCgroupPtr cgroup = NULL;
>>>> +    virCgroupPtr cgroup_vcpu = NULL;
>>>> +    qemuDomainObjPrivatePtr priv = vm->privateData;
>>>> +    int rc;
>>>> +    unsigned int i;
>>>> +    unsigned long long period = vm->def->cputune.period;
>>>> +    long long quota = vm->def->cputune.quota;
>>>> +
>>>> +    if (driver->cgroup == NULL)
>>>> +        return 0; /* Not supported, so claim success */
>>>> +
>>>> +    rc = virCgroupForDomain(driver->cgroup, vm->def->name, &cgroup, 0);
>>>> +    if (rc != 0) {
>>>> +        virReportSystemError(-rc,
>>>> +                             _("Unable to find cgroup for %s"),
>>>> +                             vm->def->name);
>>>> +        goto cleanup;
>>>> +    }
>>>> +
>>>> +    if (priv->nvcpupids == 0 || priv->vcpupids[0] == vm->pid) {
>>>> +        /* If we does not know VCPU<->PID mapping or all vcpu runs in the same
>>>> +         * thread, we can not control each vcpu.
>>>> +         */
>>>> +        if (period || quota) {
>>>> +            if (qemuCgroupControllerActive(driver, VIR_CGROUP_CONTROLLER_CPU)) {
>>>> +                if (qemuSetupCgroupVcpuBW(cgroup, period, quota) < 0)
>>>> +                    goto cleanup;
>>>> +            }
>>>> +        }
>>>> +        return 0;
>>>> +    }
>>>
>>> I found a problem above.  In the case where we are controlling quota at
>>> the domain level cgroup we must multiply the user-specified quota by the
>>> number of vcpus in the domain in order to get the same performance as we
>>> would with per-vcpu cgroups.  As written, the vm will be essentially
>>> capped at 1 vcpu worth of quota regardless of the number of vcpus.  You
>>> will also have to apply this logic in reverse when reporting the
>>> scheduler statistics so that the quota number is a per-vcpu quantity.
>>
>> When quota is 1000, and per-vcpu thread is not active, we can start
>> vm successfully. When the per-vcpu thread is active, and the num of
>> vcpu is more than 1, we can not start vm if we multiply the user-specified
>> quota. It will confuse the user: sometimes the vm can be started, but
>> sometimes the vm can not be started with the same configuration.
> 
> I am not sure I understand what you mean.  When vcpu threads are active,
> the patches work correctly.  It is only when you disable vcpu threads
> that you need to multiply the quota by the number of vcpus (since you
> are now applying it globally).  A 4 vcpu guest that is started using an
> emulator with vcpu threads active will get 4 times the cpu bandwidth as
> compared to starting the identical configuration using an emulator
> without vcpu threads.  This is because you currently apply the same
> quota setting to the full process as you were applying to a single vcpu.
> 
> I know that what I am asking for is confusing at first.  The quota value
> in a domain XML may not match up with the value actually written to the
> cgroup filesystem.  The same applies for the schedinfo API vs. cgroupfs.
>  However, my suggestion will result in quotas that match user
> expectation.  For a 4 vcpu guest with 50% cpu quota, it is more logical
> to set period=500000,quota=250000 without having to know if my qemu
> supports vcpu threads.
> 
> For example, to limit a guests to 50% CPU we would have these settings
> (when period == 500000):
> 
>                   1 VCPU    2 VCPU    4 VCPU    8 VCPU
> VCPU-threads ON   250000    250000    250000    250000
> VCPU-threads OFF  250000    500000   1000000   2000000

Ah, sorry, I understood your comment wrong :(
I will update this patch.

> 
> With VCPU threads on, the value is applied to each VCPU whereas with
> VCPU threads off it is applied globally.  This will yield roughly
> equivalent performance regardless of whether the underlying qemu process
> enables vcpu threads.
> 


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