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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