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Subject: Re: [PACTH v2 0/3] Implement /proc/<pid>/totmaps
To: Michal Hocko <mhocko@kernel.org>
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From: Robert Foss <robert.foss@collabora.com>
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Date: Mon, 15 Aug 2016 12:25:10 -0400
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On 2016-08-15 09:42 AM, Michal Hocko wrote:
> On Mon 15-08-16 09:00:04, Robert Foss wrote:
>>
>>
>> On 2016-08-14 05:04 AM, Michal Hocko wrote:
>>> On Fri 12-08-16 18:04:19, robert.foss@collabora.com wrote:
>>>> From: Robert Foss <robert.foss@collabora.com>
>>>>
>>>> This series implements /proc/PID/totmaps, a tool for retrieving summarized
>>>> information about the mappings of a process.
>>>
>>> The changelog is absolutely missing the usecase. Why do we need this?
>>> Why existing interfaces are not sufficient?
>>
>> You are absolutely right, more info information is in 1/3.
>
> Patch 1 is silent about the use case as well. It is usually recommended
> to describe the motivation for the change in the cover letter.

I'll change it for v3.

>
>> But the gist of it is that it provides a faster and more convenient way of
>> accessing the information in /proc/PID/smaps.
>
> I am sorry to insist but this is far from a description I was hoping
> for. Why do we need a more convenient API? Please note that this is a
> userspace API which we will have to maintain for ever. We have made many
> mistakes in the past where exporting some information made sense at the
> time while it turned out being a mistake only later on. So let's make
> sure we will not fall into the same trap again.
>
> So please make sure you describe the use case, why the current API is
> insufficient and why it cannot be tweaked to provide the information you
> are looking for.
>

I'll add a more elaborate description to the v3 cover letter.
In v1, there was a discussion which I think presented the practical 
applications rather well:

https://lkml.org/lkml/2016/8/9/628

or the qoute from Sonny Rao pasted below:

 > The use case is to speed up monitoring of
 > memory consumption in environments where RSS isn't precise.
 >
 > For example Chrome tends to many processes which have hundreds of VMAs
 > with a substantial amount of shared memory, and the error of using
 > RSS rather than PSS tends to be very large when looking at overall
 > memory consumption.  PSS isn't kept as a single number that's exported
 > like RSS, so to calculate PSS means having to parse a very large smaps
 > file.
 >
 > This process is slow and has to be repeated for many processes, and we
 > found that the just act of doing the parsing was taking up a
 > significant amount of CPU time, so this patch is an attempt to make
 > that process cheaper.

If a reformatted version of this still isn't adequate or desirable for 
the cover-letter, please give me another heads up.

Thanks!