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From: Sonny Rao <sonnyrao@chromium.org>
Date: Mon, 19 Sep 2016 17:29:56 -0700
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Subject: Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps
To: Robert Foss <robert.foss@collabora.com>
Cc: Michal Hocko <mhocko@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Sep 19, 2016 at 5:27 PM, Robert Foss <robert.foss@collabora.com> wrote:
>
>
> On 2016-09-19 03:32 PM, Michal Hocko wrote:
>>
>> On Mon 19-09-16 11:16:31, Robert Foss wrote:
>>>
>>> On 2016-09-14 05:12 AM, Michal Hocko wrote:
>>>>
>>>> On Tue 13-09-16 13:27:39, Sonny Rao wrote:
>>
>> [...]
>>>>>
>>>>> Given that smaps
>>>>> doesn't provide this in a straightforward way, what do you think is
>>>>> the right way to provide this information?
>>>>
>>>>
>>>> I would be tempted to sneak it into /proc/<pid>/statm because that looks
>>>> like a proper place but getting this information is not for free
>>>> performance wise so I am not really sure something that relies on this
>>>> file would see unexpected stalls. Maybe this could be worked around by
>>>> some caching... I would suggest to check who is actually using this file
>>>> (top/ps etc...)
>>>
>>>
>>> What would this caching look like? Can any information be re-used between
>>> vma walks?
>>
>>
>> yes basically return the same value if called within HZ or something
>> similar. But that assumes that statm latency really matters and it is
>> called often enough.
>
>
> Any single application querying more often than HZ, would presumably do so
> for accuracy reasons.
> However for multiple applications that combined query more often than HZ,
> this would most definitely be halpful in terms of performance.
>
> @Sonny, does chromiumos fall into the first or second category?

It's a single application -- and it definitely doesn't query at HZ --
especially given how long it takes to gather the data :-)