linux-kernel - RE: [PATCH RFC V2 3/5] perf,tool: partial time support

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Date:	Mon, 13 Jul 2015 18:59:17 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Jiri Olsa <jolsa@...hat.com>
CC:	"acme@...nel.org" <acme@...nel.org>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC V2 3/5] perf,tool: partial time support

> -----Original Message-----
> From: Jiri Olsa [mailto:jolsa@...hat.com]
> Sent: Sunday, July 12, 2015 9:22 AM
> To: Liang, Kan
> Cc: acme@...nel.org; jolsa@...nel.org; namhyung@...nel.org;
> ak@...ux.intel.com; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH RFC V2 3/5] perf,tool: partial time support
> 
> On Tue, Jul 07, 2015 at 06:19:03AM -0400, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > When multiple events are sampled it may not be needed to collect fine
> > grained time stamps on all events. The sample sites are usually nearby.
> > It's enough to have time stamps on the regular reference events.
> > This patchkit adds the ability to turn off time stamps per event. This
> > in term can reduce sampling overhead and the size of the perf.data.
> 
> how is this usable in some perf example? I dont get the correlation
> between 'refference' and the rest of the events in the report time..
> 

One usable case of partial time is to work with partial callgraph to  enable
"PEBS threshold > 1" (https://lkml.org/lkml/2015/5/10/196), which can
significantly reduce the sampling overhead.
For the case as above, we intend to get callgraph in large PEBS threshold.
Timestamp is not that important. So I didn't  correlate the reference
event and the rest events here.

But it's doable. We can use oe->last_flush to set the rest
events' time. The only problem is that I cannot find a useable
partial time only case. (Partial time itself cannot save as much as partial
callgraph) So I think we do not need to implement such correlation.

I guess I will keep the code unchanged, but correct the description.
Is it OK?

Thanks,
Kan

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