[External] Re: Managing reference count of GstBuffers created in GstBaseSrc

[Dass, Reena Collins]

I also queried the different flags for the buffer created in GstPushSrc vs the ones created by GstTransform. The flags are all the same after creation, but for the buffer created by GstPushSrc GST_BUFFER_FLAG_DISCONT  is set after its been pushed the first time down the chain.

-----Original Message-----
From: gstreamer-devel <gstreamer-devel-bounces at lists.freedesktop.org> On Behalf Of Dass, Reena Collins via gstreamer-devel
Sent: Tuesday, October 25, 2022 9:57 PM
To: Tim-Philipp Müller <t.i.m at zen.co.uk>; gstreamer-devel at lists.freedesktop.org
Cc: Dass, Reena Collins <Reena.Dass2 at collins.com>
Subject: RE: [External] Re: Managing reference count of GstBuffers created in GstBaseSrc

Yes I meant NOT the one created by GstPushSrc. 
These elements in my pipeline are custom element based on the Gstreamer base classes.
I did verify that  each gst_buffer_map() has a corresponding gst_buffer_unmap(). 
I am noticing this behavior only with the buffer created in the GstPushSrc.


-----Original Message-----
From: Tim-Philipp Müller <t.i.m at zen.co.uk>
Sent: Tuesday, October 25, 2022 7:13 PM
To: gstreamer-devel at lists.freedesktop.org
Cc: Dass, Reena Collins <Reena.Dass2 at collins.com>
Subject: [External] Re: Managing reference count of GstBuffers created in GstBaseSrc

On Tue, 2022-10-25 at 20:09 +0000, Dass, Reena Collins via gstreamer- devel wrote:

Hi Reena,

> I have a pipeline that has a GstPushSrc, some GstTransforms 
> andGstBaseSink.
>  
> This pipeline runs continuously. I noticed the GstBuffers created in 
> the GstTransform modules have their reference count decremented when 
> they are pushed to the next element in the pipeline but the one 
> created in GstPushSrc. As a result the memory usage keeps on 
> increasing with time.  I could not find documentation anywhere that 
> talks about how and why the buffers reference counting is done.
>  
> I would appreciate it if somebody has some insight or could point me 
> to some documentation that talks about the reference counting.

Did you mean "but _not_ the one created in GstPushSrc"?

Generally speaking, when a buffer is passed from one element to the next, the (possibly shared) ownership of the buffer (i.e. the ref) is passed from one element to the next. The reason for this is that the writability of the buffer is linked to the refcount of the
buffer+memories attached to it. If the buffer is shared (refcount >= 2)
an element can't just write into the existing buffer because that would affect the other owner(s). But if the refcount is == 1 the receiving element knows it can do in-place transforms without copying should it wish to do so.

Are these elements custom element or standard GStreamer ones?

If the GstBaseTransform does an in-place transform (or passthrough) of the input buffer from the GstPushSrc, it will pass ownership on to the next element (sink?) after doing so, in which case the sink would be responsible for unreffing it when the time comes. Sinks often keep the last buffer around until the next one arrives (can be disabled with enable-last-sample=false).

If the GstBaseTransform does a transform from the input buffer into a new output buffer it is responsible for unreffing the old buffer when it's done.

There are also things like buffer pools, in which case buffers get recycled into the pool when they're about to be destroyed.

In any case, if you use the base classes they should take care of everything for you in this respect already really.

If there are custom elements involved, perhaps they're missing a
gst_buffer_unmap() call somewhere after a gst_buffer_map()?

Cheers
 Tim