Re: [PATCH V1 2/2] mm/damon: Add 'age' of region tracepoint support

[Xin Hao]

Hi, Park:

On 2021/11/11 下午4:20, SeongJae Park wrote:
> On Thu, 11 Nov 2021 10:04:38 +0800 Xin Hao <xhao@xxxxxxxxxxxxxxxxx> wrote:
>
> > [-- Attachment #1: Type: text/plain, Size: 8070 bytes --]
> >
> > Hi Park:
> >
> > On 2021/11/10 下午9:16, SeongJae Park wrote:
> > > On Wed, 10 Nov 2021 20:13:14 +0800 Xin Hao <xhao@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > In patch "mm/damon: add a tracepoint", it adds a
> > > > tracepoint for DAMON, it can monitor each region
> > > > for each aggregation interval, Now the region add
> > > > a new 'age' variable, some primitive would calculate
> > > > the priority of each region as a weight, there put it
> > > > into tracepoint, so we can easily track the change of
> > > > its value through perf or damon-tools.
> > > DAMON calculates the age using the address range and nr_accesses of the region,
> > > which are already in the tracepoint.  In other words, user space can calculate
> > > the age on their own.  Therefore I thought putting age in the tracepoint as
> > > adding unnecessary information, at the moment of the implementation.
> > >
> > > Of course, I would missing some use cases that need this information in the
> > > tracepoint.  Furthermore, adding just one more value in the tracepoint wouldn't
> > > incur a real issue.  But, I'd like to know why this is necessary and how much
> > > benefit it provides.  Xin, could you please share that?
> > I think these two variables nr_access &  age have different meanings,
> > the nr_access only reflect the
> >
> > period of  sample_interval,  We may be able to get the change of age
> > through continuous long-term sampling,
> >
> > But I think this is not very convenient.
> >
> > We only need to observe the change of age value a small number of times
> > to replace the continue sampling of the region.
> >
> > For example, age has been increasing to 141, but nr_access shows a value
> > of 0 at a certain time. Through this，we can
> >
> > conclude that the region has a very low nr_access value for a long time.
> I understand that you don't want to record all the traces and then process the
> huge trace data in user space in order to get the age information, because you
> want to save disk space and CPU cycles.  Is that correct?  If so, I think that
> makes sense, and it would be better to put that in the commit message.

Yes, What you said is absolutely correct, that's how I thought about it, 
I will add this part of the

information to the commit，thanks!

> Thanks,
> SJ
>
> [...]

--
Best Regards!
Xin Hao