[lttng-dev] Limitations on subbuf_size / num_subbuf?

[Alexandre Montplaisir]

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. ;)

> 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?

>
> 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.

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