linux-kernel - Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

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Date:   Tue, 4 Feb 2020 14:42:55 +0530
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH 0/3] pseries: Track and expose idle PURR and SPURR ticks

On 12/6/19 2:44 PM, Naveen N. Rao wrote:
> Naveen N. Rao wrote:
>> Hi Nathan,
>>
>> Nathan Lynch wrote:
>>> Hi Kamalesh,
>>>
>>> Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com> writes:
>>>> On 12/5/19 3:54 AM, Nathan Lynch wrote:
>>>>> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
>>>>>>
>>>>>> Tools such as lparstat which are used to compute the utilization need
>>>>>> to know [S]PURR ticks when the cpu was busy or idle. The [S]PURR
>>>>>> counters are already exposed through sysfs.  We already account for
>>>>>> PURR ticks when we go to idle so that we can update the VPA area. This
>>>>>> patchset extends support to account for SPURR ticks when idle, and
>>>>>> expose both via per-cpu sysfs files.
>>>>>
>>>>> Does anything really want to use PURR instead of SPURR? Seems like we
>>>>> should expose only SPURR idle values if possible.
>>>>>
>>>>
>>>> lparstat is one of the consumers of PURR idle metric
>>>> (https://groups.google.com/forum/#!topic/powerpc-utils-devel/fYRo69xO9r4). Agree, on the argument that system utilization metrics based on SPURR
>>>> accounting is accurate in comparison to PURR, which isn't proportional to
>>>> CPU frequency.  PURR has been traditionally used to understand the system
>>>> utilization, whereas SPURR is used for understanding how much capacity is
>>>> left/exceeding in the system based on the current power saving mode.
>>>
>>> I'll phrase my question differently: does SPURR complement or supercede
>>> PURR? You seem to be saying they serve different purposes. If PURR is
>>> actually useful rather then vestigial then I have no objection to
>>> exposing idle_purr.
>>
>> SPURR complements PURR, so we need both. SPURR/PURR ratio helps provide an indication of the available headroom in terms of core resources, at maximum frequency.
> 
> Re-reading this today morning, I realize that this isn't entirely accurate. SPURR alone is sufficient to understand core resource utilization.
> 
> Kamalesh is using PURR to display non-normalized utilization values (under 'actual' column), as reported by lparstat on AIX. I am not entirely sure if it is ok to derive these based on the SPURR busy/idle ratio.

Both idle_purr and idle_spurr complement each other and we need to expose both of them.
It will improve the accounting accuracy of tools currently consuming system-wide PURR
and/or SPURR numbers to report system usage.  Deriving one from another, from my
experience makes it hard for tools or any custom scripts to give an accurate system view.
One tool I am aware of is lparstat, which uses PURR based metrics.

-- 
Kamalesh