[lttng-dev] Limitations on subbuf_size / num_subbuf?

[Alexandre Montplaisir]

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

>
>>> 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).


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 
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