[lttng-dev] Limitations on subbuf_size / num_subbuf?

[Mathieu Desnoyers]

* Alexandre Montplaisir (alexmonthy at voxpopuli.im) wrote:
> On 13-04-18 01:23 PM, Mathieu Desnoyers wrote:
> > * Alexandre Montplaisir (alexmonthy at voxpopuli.im) wrote:
> >> On 13-04-18 01:07 PM, Mathieu Desnoyers wrote:
> >>> * Alexandre Montplaisir (alexmonthy at voxpopuli.im) wrote:
> >>>> On 13-04-18 02:11 AM, Amit Margalit wrote:
> >>>>> Hi,
> >>>>>
> >>>>> Thank for the quick response. Here is the missing data:
> >>>>>
> >>>>> UST
> >>>>> 2.6.32.12-205 (I had to make a tiny patch to make it compile)
> >>>>> MemTotal:       24628852 kB
> >>>>> lttng-ust-2.1.2
> >>>>> lttng-tools-2.1.1
> >>>>> lttng-modules-2.1.1
> >>>>> babeltrace-1.1.0
> >>>>> userspace-rcu-0.7.6
> >>>>>
> >>>>> Which patch for 4MB?
> >>>> Hi,
> >>>>
> >>>> Not sure if it's specifically this one that Matthew was talking about,
> >>>> but we have such a patch in the PPA packages. See:
> >>>> http://bazaar.launchpad.net/~lttng/lttng-tools/packaging-daily/view/head:/patches/0002-Increase-default-subbuffer-size-to-4MB.patch
> >>> First, it should be noted that lttng enable-channel allow overriding the
> >>> subbuffer size.
> >> True that. But the value needs to be a power of two (which is not
> >> immediately obvious to new users). And if it's not exactly a power of
> >> two, it gets rejected, instead of simply rounding to the closest one. We
> >> have a patch for that too. ;)
> > I've never seen it on lttng-dev. Please share it, don't be shy :)
> 
> Granted, it wasn't on lttng-dev. But an initial version was posted on
> Redmine at:
> https://bugs.lttng.org/issues/228

Yep, and I thoroughly reviewed it, and no new version has been sent
since then.

> 
> >
> >>> It should be noted that lttng-modules 2.2 is going to output even _more_
> >>> events, so I don't think just making the buffers larger by default is
> >>> the right approach.
> >> If it's gonna output more events, don't we want larger sizes?
> > No, because at some point, you want the default to take into account
> > typical HW configuration limitations. Usually, systems should not be
> > expected to be dedicated to tracing: whatever memory the tracer reserves
> > is less memory available for page cache and application, and therefore
> > more impact on the system.
> 
> I think 4 MB isn't anywhere close to hardware limitations (at least for
> desktop users, which are the main audience of the Ubuntu PPA packages).

4 MB * nr cpus * nr processes traced can be closer to HW limitations,
even of desktops.

Thanks,

Mathieu

> 
> 
> Cheers,
> Alex
> 
> >>> I think we need to port the "loglevel" from UST to lttng-modules ASAP,
> >>> and assign a "debug" loglevel to events we don't care about by default.
> >> This sounds like a good idea.
> >>
> >> My main concern was that most users don't read the man pages, don't
> >> fiddle with custom flags, etc. They just run enable-event -a, then start
> >> tracing. In this setup, they should not expect dropped events.
> > Yep. Having loglevels should take care of that: debug-level events would
> > not be enabled unless a specific loglevel is specified.
> >
> > Thanks,
> >
> > Mathieu
> >
> >> Cheers,
> >> Alex
> >>
> >>> Thoughts ?
> >>>
> >>> Thanks,
> >>>
> >>> Mathieu
> >>>
> >>>>> Thanks,
> >>>>>
> >>>>> Amit Margalit
> >>>>> IBM XIV - Storage Reinvented
> >>>>> XIV-NAS Development Team
> >>>>> Tel. 03-689-7774
> >>>>> Fax. 03-689-7230
> >>>>>
> >>>>>
> >>>>>
> >>>>> From:   Matthew Khouzam <matthew.khouzam at ericsson.com>
> >>>>> To:     <lttng-dev at lists.lttng.org>
> >>>>> Date:   04/15/2013 05:24 PM
> >>>>> Subject:        Re: [lttng-dev] Limitations on subbuf_size / num_subbuf?
> >>>>>
> >>>>>
> >>>>>
> >>>>> Not at all a n00b question, but could you give some more info? what is 
> >>>>> your version of LTTng tools, are you using UST or kernel tracing? How much 
> >>>>> ram is on your system? Kernel version? 
> >>>>>
> >>>>> I actually have the patched lttng that has standard subbuffer sizes of 4 
> >>>>> mb, so I have not personnaly seen that problem. But the first thing I 
> >>>>> would do is upgrade to the latest stable.
> >>>>>
> >>>>> On 13-04-14 06:49 AM, Amit Margalit wrote:
> >>>>> Hello, 
> >>>>>
> >>>>> Sorry for the noob question. I seem to be unable to define the total of 
> >>>>> subbuf_size * num_subbuf higher than 4MB. I am getting ~30% discarded 
> >>>>> tracepoint data. 
> >>>>>
> >>>>> Any assistance would be greatly appreciated. 
> >>>>>
> >>>>> Amit Margalit 
> >>>>> IBM XIV - Storage Reinvented 
> >>>>> XIV-NAS Development Team 
> >>>>> Tel. 03-689-7774 
> >>>>> Fax. 03-689-7230 
> >>>> _______________________________________________
> >>>> lttng-dev mailing list
> >>>> lttng-dev at lists.lttng.org
> >>>> http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com