[ros-users] rosbag playback speed

[Jeremy Leibs]

Sorry for the typo, I meant to say that  playback on *cturtle* invoked
the callback 2494 times.

On Fri, Jul 30, 2010 at 10:26 AM, Jeremy Leibs <leibs at willowgarage.com> wrote:
> Daniel,
>
> For very high-rate things across many channels, what Blaise is talking
> about might lead to problems.  However, I don't think it is in this
> particular case.
>
> I ran your tests and checked with rxbag and the packets seem to be
> nicely distributed as we would want.
>
> I am 99% confident that this is a problem with rosbag in boxturtle.
> The old rosbag (which actually called through to rosplay), had a lot
> of problems in the timing engine that led to this kind of bursty
> behavior on playback.  For cturtle, I rewrote the rosbag timing engine
> and I believe it is now far more robust and well-behaved.
>
> I just confirmed playback from boxturtle only invoked the callback 54
> times.  Playback on boxturtle invoked the callback 2494 times.
>
> If you can possibly move to cturtle, I believe that should solve your
> issues.  If you really need to work with boxturtle I can investigate
> the possibility of patching in some of these timing fixes.
>
> --Jeremy
>
> On Fri, Jul 30, 2010 at 10:03 AM, Daniel Grollman
> <daniel.grollman at epfl.ch> wrote:
>> Hi Blaise,
>>
>>        Thanks for this info, I hadn't thought to consider the scheduler.
>> However, opening up my bagfile with rxbag (and zooming in very close),
>> it looks like there is indeed a packet every 2ms.  I'll try to confirm
>> this with the API.
>>
>> Dan
>>
>> On 07/30/2010 06:15 PM, Blaise Gassend wrote:
>>> As far as I know, rosbag can only rely on message arrival times when it
>>> records a data stream. At 500 Hz, your messages are being sent at a rate
>>> that is fast compared with Linux's scheduling granularity, so on a
>>> loaded system, I would not expect rosbag to necessarily be scheduled
>>> each time it receives a message. Hence it will receive the messages in
>>> clumps, and during replay, messages will be played back in clumps. You
>>> can verify whether this is the case by looking at the timestamps in the
>>> bag. I think rxbag should allow you to view the timestamps graphically.
>>> The python bag API should allow you to view the timestamps directly.
>>>
>>> You might be able to improve things by puting rosbag on one of the
>>> "realtime" queues using schedtool.
>>>
>>> You can also see the scheduling happening using the kernel ftrace
>>> mechanism.
>>>
>>> On Fri, 2010-07-30 at 14:02 +0200, Daniel Grollman wrote:
>>>> Hello,
>>>>
>>>>      I've noticed some odd behavior with rosbag.  I'm generating data at
>>>> 500hz, and recording a bag.  However, when playing back the bag, only a
>>>> fraction of the messages reach my callback.  I believe that rosbag is
>>>> 'bursting' the messages, i.e., sporadically sending them at much faster
>>>> than 500 Hz, so they are then getting lost as my queue size is only 1
>>>> (which is what I want).
>>>>
>>>> I'm running ros 1.0.4, and ubuntu 9.10.  I've put together a little demo
>>>> of this issue, code is here:
>>>>
>>>> http://lasa.epfl.ch/~dang/rosbag_test.tar.gz
>>>>
>>>> Thank you,
>>>>
>>>> Dan Grollman
>>>> EPFL
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>>>
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