Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

[Shailabh Nagar <nagar@watson.ibm.com>]

Paul Jackson wrote:

> Shailabh wrote:
>  
>
> > The overhead of creating cpusets just for this
> > reason seems excessive when the need is only to
> > reduce the number of sockets to monitor
> >    
>
> As I reread this thread, some of my ancient interactions with process
> accounting come to mind again.
>
> K.I.S.S. - keep it simple, I'm telling myself.
>
> I'm also thinking that since this is a system wide stat tool, it
> wants to minimize interactions with other mechanisms.
>
> Hog tying cpusets and process accounting together seems just
> plain weird, and risks imposing conflicting demands on the cpuset
> configuration of a system.
>
> Please be so kind as to forget I suggested that ;).
>  
What suggestion are you talking about  :-)

> How about a simple way to disable collection on specified CPUs.
>  

> Collecting this sort of data makes sense for certain managed system
> situations, where one chooses to spend some portion of the system
> tracking the rest of it.
>
> Collecting it may put an intolerable performance impact on pedal to
> the metal maximum performance beasts running on dedicated cpus/nodes.
>
>
> I propose a per-cpu boolean flag to disable collection.
>
> If this flag is set on the cpu on which a task happens to be when
> exiting, then we just drop that data on the floor, silently, with no
> accumulation, as quickly as we can, avoiding any system-wide locks.
>
> Then I could run a managed job mix, collecting accounting data, on
> some nodes, while running dedicated performance beasts on other nodes,
> without the accounting interfering with the performance beasts.
>  
Doing enablement/disablement on a per-CPU basis seems to fit the cpuset 
framework where
jobs are closely tied to CPUs.

Otherwise, from a generic taskstats perspective, having the CPU of exit 
determine the output
of exit related data seems a bit arbitrary.

> Independently, the cpuset friendly customers could make use of cpusets
> to help manage which jobs were on which cpus, so that they collected
> their accounting data as desired.  But no need for the accounting
> system to be aware of that, past the state of its per-cpu flag.
>
> Such a flag reduces the need for further (over) designing this to
> handle the extreme case.  

> If someone has such an extreme case, they
> can turn off collecting on some cpus, to get a handle on the situation.
>  
Hmm ? Again a very cpuset'ish solution where turning off collection on a 
set of cpus will mean only
a known set of tasks (aggregated under a job) get affected. In general, 
this seems like a terrible
way of doing flow control.....just pick some tasks and shut their data 
output out (admittedly thats
what we're doing today when data gets dropped on overflow but I guess 
the aim here is to do
better)

> This could be done as a variant of your idea for multiple
> TASKSTATS_LISTEN_GROUP's.  Essentially, for now, we would have two
> GROUP's - one that drops the data on the floor, and one that collects
> it.  Each cpu is in either one or the other group.  Later on, when the
> need arises, we add support for more GROUP's that can actually collect
> data.
>  
Sorry...don't like this proposal much but others may differ.

--Shailabh

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