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Re: [libvirt] [PATCH 1/2] Get cpuMhz of virNodeGetInfo() from cpufreq/cpuinfo_max_freq, if exist



2011/2/10 Minoru Usui <usui mxm nes nec co jp>:
> Hi, Eric
>
> On Tue, 1 Feb 2011 10:26:36 +0900
> Minoru Usui <usui mxm nes nec co jp> wrote:
>
>> Hi, Eric
>>
>> On Mon, 31 Jan 2011 15:46:39 -0700
>> Eric Blake <eblake redhat com> wrote:
>>
>> > On 01/27/2011 02:51 AM, Minoru Usui wrote:
>> > > virNodeGetInfo() gets from
>> > > /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq, first.
>> > >
>> > > Signed-off-by: Minoru Usui <usui mxm nes nec co jp>
>> >
>> > I haven't looked closely at this series yet...
>> >
>> > > +    /*
>> > > +     * nodeinfo->mhz should return maximum frequency,
>> > > +     * but "cpu MHz" of /proc/cpuinfo is scaled by power saving feature.
>> > > +     * So it gets cpufreq/cpuinfo_max_freq, if possible.
>> > > +     */
>> > > +    ret = get_cpu_value(0, "cpufreq/cpuinfo_max_freq", true);
>> > > +    if (ret < 0)
>> > > + return -1;
>> > > +    else if (ret != 1) {
>> > > + /* convert unit */
>> > > + cpu_mhz = ret / 1000;
>> >
>> > But which units is this converting between, and should it truncate or
>> > round up?
>>
>> I think it divide by 1000 is collect, because my machine returns below values.
>>
>> -----------------------------------------------------------------
>> # cat /sys/devices/system/cpu/cpu?/cpufreq/cpuinfo_max_freq
>> 2333331
>> 2333331
>> 2333331
>> 2333331
>> 2333331
>> 2333331
>> 2333331
>> 2333331
>>
>> # grep 'cpu MHz' /proc/cpuinfo
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> cpu MHz         : 2333.331
>> -----------------------------------------------------------------
>>
>> On the other hand, I don't have clear opinion about truncate or round up.
>> Present implementation of linuxNodeInfoCPUPopulate() selects truncate,
>> so I implement to truncate logic.
>
> Are my explanations enough?
> If not or I'm misunderstanding something, please let me know.
>

I think we should round up in case we request something, because then
we get at least what we requested. If we truncate a request we might
get less than we requested, this might result in a bad situation.
Recently we switched from truncation to rounding up for memory and
storage request because of this.

But here we are reporting something. In that case we should truncate,
because if we would round up we could report more than there actually
is, this might result in a bad situation too.

So with rounding up request and truncating reports we are on the safe
side in both cases.

Matthias


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