linux-kernel - Re: [RFC PATCH 00/11] trace-cmd: Support the feature recording trace data of guests on the host

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Date:	Mon, 26 Aug 2013 10:22:29 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
Cc:	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org, yrl.pp-manager.tt@...achi.com
Subject: Re: [RFC PATCH 00/11] trace-cmd: Support the feature recording
 trace data of guests on the host

On Mon, 26 Aug 2013 10:46:38 +0900
Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com> wrote:

> > The --date option is used because the two machines are not in sync with
> > the trace time stamp. What the date option does, is to sync the
> > timestamp up with the gettimeofday and the output reports that. This
> > allows the two boxes to report information that is relatively close to
> > how the two interacted.
> 
> Oh, I didn't know the --date option.
> As you mentioned, we can merge trace data in chronological order by
> using --date option if the times of those machines are synchronized by
> NTP.
> 
> > If the guest and the host have the same clock, then the --date option
> > is not needed and the two should be able to be merged normally.
> 
> No, we can not assure that the guest and the host have the same clock
> even if it is running on the same physical machine, because both kernel
> doesn't share it, there is some difference between them. So, we still
> need time synchronizing guest-host by NTP and --date option.
> 
> However, there are cases that times of those machines cannot be
> synchronized. For example, although multiple users can run guests on
> virtualization environments (e.g. multi-tenant cloud hosting), there
> are no guarantee that they use the same NTP server. Moreover, even if
> the times are synchronized, trace data cannot exactly be merged because
> the NTP-synchronized time granularity may not be enough fine for
> sorting guest-host switching events.

Right, unless there's some other means no synchronize between boxes,
this is currently the best we have.

> 
> > Also, I haven't released it yet (will soon), but trace-cmd handles
> > multiple buffers too. That is, with the multiple buffers that ftrace
> > has, it will create and read from them as well as report them.
> 
> Is it commit ID d56f30679f9811a91ed471c8e081cc7ffbed1e62?
> We can download the feature from your git repository.

Yep.

-- Steve
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