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plugin-development: allocation: improve content and formatting

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markdown/plugin-development/advanced/allocation.md
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@ -4,47 +4,47 @@ title: Memory allocation
# Memory allocation
Memory allocation and management is a very important topic in
Memory allocation and management are very important topics in
multimedia. High definition video uses many megabytes to store one
single frame of video. It is important to reuse the memory when possible
instead of constantly allocating and freeing the memory.
single image frame. It is important to reuse memory when possible
instead of constantly allocating and freeing it.
Multimedia systems usually use special purpose chips, such as DSPs or
GPUs to perform the heavy lifting (especially for video). These special
purpose chips have usually strict requirements for the memory that they
can operate on and how the memory is accessed.
Multimedia systems usually use special-purpose chips, such as DSPs or
GPUs to perform the heavy lifting (especially for video). These
special-purpose chips usually have strict requirements for the memory
they operate on and how it is accessed.
This chapter talks about the memory management features that GStreamer
plugins can use. We will first talk about the lowlevel `GstMemory`
object that manages access to a piece of memory. We then continue with
`GstBuffer` that is used to exchange data between plugins (and the
application) and that uses `GstMemory`. We talk about `GstMeta` that can
be placed on buffers to give extra info about the buffer and its memory.
For efficiently managing buffers of the same size, we take a look at
`GstBufferPool`. To conclude this chapter we take a look at the
GST\_QUERY\_ALLOCATION query that is used to negotiate memory management
options between elements.
This chapter talks about the memory-management features available to
GStreamer plugins. We will first talk about the lowlevel `GstMemory`
object that manages access to a piece of memory and then continue with
one of it's main users, the `GstBuffer`, which is used to exchange data
between plugins and with the application. We will also discuss the `GstMeta`.
This object can be placed on buffers to provide extra info about it an
dits memory. We will also discuss the `GstBufferPool`, which allows to
more-efficiently manage buffers of the same size.
To conclude this chapter we will take a look at the `GST_QUERY_ALLOCATION`
query, which is used to negotiate memory management options between
elements.
## GstMemory
`GstMemory` is an object that manages a region of memory. The memory
`GstMemory` is an object that manages a region of memory. This memory
object points to a region of memory of “maxsize”. The area in this
memory starting at “offset” and for “size” bytes is the accessible
region in the memory. the maxsize of the memory can never be changed
after the object is created, however, the offset and size can be
changed.
memory starting at “offset” and size “size” bytes is the accessible
memory region. After a `GstMemory` is created its maxsize can no longer
be changed, however, its "offset" and "size" can.
### GstAllocator
`GstMemory` objects are created by a `GstAllocator` object. Most
allocators implement the default `gst_allocator_alloc()` method but some
allocator might implement a different method, for example when
additional parameters are needed to allocate the specific memory.
might implement different ones, for example, when additional parameters
are needed to allocate the specific memory.
Different allocators exist for, for example, system memory, shared
memory and memory backed by a DMAbuf file descriptor. To implement
support for a new kind of memory type, you must implement a new
allocator object as shown below.
Different allocators exist for system memory, shared memory and memory
backed by a DMAbuf file descriptor. To implement support for a new kind
of memory type, you must implement a new allocator object.
### GstMemory API example
@ -54,11 +54,10 @@ access mode (read/write) must be given when mapping memory. The map
function returns a pointer to the valid memory region that can then be
accessed according to the requested access mode.
Below is an example of making a `GstMemory` object and using the
Below is an example on creating a `GstMemory` object and using the
`gst_memory_map()` to access the memory region.
``` c
[...]
GstMemory *mem;
@ -89,54 +88,53 @@ WRITEME
## GstBuffer
A `GstBuffer` is an lightweight object that is passed from an upstream
A `GstBuffer` is a lightweight object that is passed from an upstream
to a downstream element and contains memory and metadata. It represents
the multimedia content that is pushed or pull downstream by elements.
the multimedia content that is pushed to or pulled by downstream elements.
The buffer contains one or more `GstMemory` objects that represent the
data in the buffer.
A `GstBuffer` contains one or more `GstMemory` objects. These objects hold
the buffer's data.
Metadata in the buffer consists of:
- DTS and PTS timestamps. These represent the decoding and
presentation timestamps of the buffer content and is used by
synchronizing elements to schedule buffers. Both these timestamps
can be GST\_CLOCK\_TIME\_NONE when unknown/undefined.
presentation timestamps of the buffer content and are used by
synchronizing elements to schedule buffers. These timestamps
can be `GST_CLOCK_TIME_NONE` when unknown/undefined.
- The duration of the buffer contents. This duration can be
GST\_CLOCK\_TIME\_NONE when unknown/undefined.
`GST_CLOCK_TIME_NONE` when unknown/undefined.
- Media specific offsets and offset\_end. For video this is the frame
number in the stream and for audio the sample number. Other
definitions for other media exist.
- Media-specific `offset` and `offset_end` values. For video this is the
frame number in the stream, for audio, the sample number. Other media
might use different definitions.
- Arbitrary structures via `GstMeta`, see below.
### GstBuffer writability
### Writability
A buffer is writable when the refcount of the object is exactly 1,
A `GstBuffer` is writable when the refcount of the object is exactly 1,
meaning that only one object is holding a ref to the buffer. You can
only modify anything in the buffer when the buffer is writable. This
means that you need to call `gst_buffer_make_writable()` before changing
the timestamps, offsets, metadata or adding and removing memory blocks.
only modify the buffer when it is writable. This means that you need to
call `gst_buffer_make_writable()` before changing the timestamps,
offsets, metadata or adding and removing memory blocks.
### GstBuffer API examples
### API examples
You can create a buffer with `gst_buffer_new ()` and then add memory
objects to it or you can use a convenience function
`gst_buffer_new_allocate ()` which combines the two. It's also possible
to wrap existing memory with `gst_buffer_new_wrapped_full () ` where you
can give the function to call when the memory should be freed.
You can create a `GstBuffer` with `gst_buffer_new ()` and then you can
add memory objects to it. You can alternatively use the convenience function
`gst_buffer_new_allocate ()` to perform both operations at once. It's also possible
to wrap existing memory with `gst_buffer_new_wrapped_full ()` and specify the
function to call when the memory should be freed.
You can access the memory of the buffer by getting and mapping the
You can access the memory of a `GstBuffer` by getting and mapping the
`GstMemory` objects individually or by using `gst_buffer_map ()`. The
latter merges all the memory into one big block and then gives you a
pointer to this block.
pointer to it.
Below is an example of how to create a buffer and access its memory.
``` c
[...]
GstBuffer *buffer;
GstMemory *mem;
@ -164,25 +162,23 @@ Below is an example of how to create a buffer and access its memory.
gst_buffer_unref (buffer);
[...]
```
## GstMeta
With the `GstMeta` system you can add arbitrary structures on buffers.
With the `GstMeta` system you can add arbitrary structures to buffers.
These structures describe extra properties of the buffer such as
cropping, stride, region of interest etc.
cropping, stride, region of interest, etc.
The metadata system separates API specification (what the metadata and
its API look like) and the implementation (how it works). This makes it
possible to make different implementations of the same API, for example,
its API look like) and its implementation (how it works). This makes it
possible to have different implementations of the same API, for example,
depending on the hardware you are running on.
### GstMeta API example
### API example
After allocating a new buffer, you can add metadata to the buffer with
the metadata specific API. This means that you will need to link to the
After allocating a new `GstBuffer`, you can add metadata to it with
the metadata-specific API. This means that you will need to link to the
header file where the metadata is defined to use its API.
By convention, a metadata API with name `FooBar` should provide two
@ -196,7 +192,6 @@ Let's have a look at the metadata that is used to specify a cropping
region for video frames.
``` c
#include <gst/video/gstvideometa.h>
[...]
@ -211,15 +206,12 @@ region for video frames.
meta->width = 120;
meta->height = 80;
[...]
```
An element can then use the metadata on the buffer when rendering the
frame like this:
``` c
#include <gst/video/gstvideometa.h>
[...]
@ -236,9 +228,6 @@ frame like this:
_render_frame (buffer);
}
[...]
```
### Implementing new GstMeta
@ -248,21 +237,20 @@ and use it on buffers.
#### Define the metadata API
First we need to define what our API will look like and we will have to
register this API to the system. This is important because this API
First we need to define what our API will look like and we have to
register this API to the system. This is important because the API
definition will be used when elements negotiate what kind of metadata
they will exchange. The API definition also contains arbitrary tags that
give hints about what the metadata contains. This is important when we
see how metadata is preserved when buffers pass through the pipeline.
see how metadata is preserved as buffers pass through the pipeline.
If you are making a new implementation of an existing API, you can skip
this step and move on to the implementation step.
this step and move directly to the implementation.
First we start with making the `my-example-meta.h` header file that will
contain the definition of the API and structure for our metadata.
``` c
#include <gst/gst.h>
typedef struct _MyExampleMeta MyExampleMeta;
@ -279,24 +267,21 @@ GType my_example_meta_api_get_type (void);
#define gst_buffer_get_my_example_meta(b) \
((MyExampleMeta*)gst_buffer_get_meta((b),MY_EXAMPLE_META_API_TYPE))
```
The metadata API definition consists of the definition of the structure
that holds a gint and a string. The first field in the structure must be
`GstMeta`.
that holds a `gint` and a string. The first field in the structure must be
a `GstMeta`.
We also define a `my_example_meta_api_get_type ()` function that will
register out metadata API definition. We also define a convenience macro
`gst_buffer_get_my_example_meta ()` that simply finds and returns the
register our metadata API definition and a convenience
`gst_buffer_get_my_example_meta ()` macro that simply finds and returns the
metadata with our new API.
Next let's have a look at how the `my_example_meta_api_get_type ()`
function is implemented in the `my-example-meta.c` file.
Let's have a look at how the `my_example_meta_api_get_type ()`
function is implemented in the `my-example-meta.c` file:
``` c
#include "my-example-meta.h"
GType
@ -311,26 +296,24 @@ my_example_meta_api_get_type (void)
}
return type;
}
```
As you can see, it simply uses the `gst_meta_api_type_register ()`
function to register a name for the api and some tags. The result is a
new pointer GType that defines the newly registered API.
function to register a name and some tags for the API. The result is a
new `GType` pointer that defines the newly registered API.
#### Implementing a metadata API
Next we can make an implementation for a registered metadata API GType.
The implementation detail of a metadata API are kept in a `GstMetaInfo`
structure that you will make available to the users of your metadata API
Next we can make an implementation for a registered metadata API `GType`.
The implementation details of a metadata API are kept in a `GstMetaInfo`
structure that you make available to the users of your metadata API
implementation with a `my_example_meta_get_info ()` function and a
convenience `MY_EXAMPLE_META_INFO` macro. You will also make a method to
convenience `MY_EXAMPLE_META_INFO` macro. You also provide a method to
add your metadata implementation to a `GstBuffer`. Your
`my-example-meta.h` header file will need these additions:
``` c
[...]
/* implementation */
@ -340,15 +323,12 @@ const GstMetaInfo *my_example_meta_get_info (void);
MyExampleMeta * gst_buffer_add_my_example_meta (GstBuffer *buffer,
gint age,
const gchar *name);
```
Let's have a look at how these functions are implemented in the
`my-example-meta.c` file.
``` c
[...]
static gboolean
@ -417,12 +397,10 @@ gst_buffer_add_my_example_meta (GstBuffer *buffer,
return meta;
}
```
`gst_meta_register ()` registers the implementation details, like the
API that you implement and the size of the metadata structure along with
API that you implement and the size of the metadata structure, alongside
methods to initialize and free the memory area. You can also implement a
transform function that will be called when a certain transformation
(identified by the quark and quark specific data) is performed on a
@ -438,36 +416,34 @@ lists of reusable buffers. Essential for this object is that all the
buffers have the same properties such as size, padding, metadata and
alignment.
A bufferpool object can be configured to manage a minimum and maximum
amount of buffers of a specific size. A bufferpool can also be
configured to use a specific `GstAllocator` for the memory of the
buffers. There is support in the bufferpool to enable bufferpool
specific options, such as adding `GstMeta` to the buffers in the pool or
such as enabling specific padding on the memory in the buffers.
A `GstBufferPool` can be configured to manage a minimum and maximum
amount of buffers of a specific size. It can also be configured to use a
specific `GstAllocator` for the memory of the buffers. There is also
support in the bufferpool to enable bufferpool specific options, such as
adding `GstMeta` to the pool's buffers or enabling specific padding on the
buffers' memory.
A Bufferpool can be inactivate and active. In the inactive state, you
A `GstBufferPool` can be either inactivate or active. In the inactive state, you
can configure the pool. In the active state, you can't change the
configuration anymore but you can acquire and release buffers from/to
the pool.
In the following sections we take a look at how you can use a
bufferpool.
In the following sections we take a look at how you can use a `GstBufferPool`.
### GstBufferPool API example
### API example
Many different bufferpool implementations can exist; they are all
subclasses of the base class `GstBufferPool`. For this example, we will
assume we somehow have access to a bufferpool, either because we created
it ourselves or because we were given one as a result of the ALLOCATION
query as we will see below.
There can be many different `GstBufferPool` implementations; they are all
subclasses of the `GstBufferPool` base class. For this example, we will
assume we somehow have access to a buffer pool, either because we created
it ourselves or because we were given one as a result of the `ALLOCATION`
query, as we will see below.
The bufferpool is initially in the inactive state so that we can
configure it. Trying to configure a bufferpool that is not in the
The `GstBufferPool` is initially in the inactive state so that we can
configure it. Trying to configure a `GstBufferPool` that is not in the
inactive state will fail. Likewise, trying to activate a bufferpool that
is not configured will fail.
is not configured will also fail.
``` c
GstStructure *config;
[...]
@ -486,17 +462,16 @@ is not configured will fail.
[...]
```
The configuration of the bufferpool is maintained in a generic
The configuration of a `GstBufferPool` is maintained in a generic
`GstStructure` that can be obtained with `gst_buffer_pool_get_config()`.
Convenience methods exist to get and set the configuration options in
this structure. After updating the structure, it is set as the current
configuration in the bufferpool again with
configuration in the `GstBufferPool` again with
`gst_buffer_pool_set_config()`.
The following options can be configured on a bufferpool:
The following options can be configured on a `GstBufferPool`:
- The caps of the buffers to allocate.
@ -505,7 +480,7 @@ The following options can be configured on a bufferpool:
padding.
- The minimum and maximum amount of buffers in the pool. When minimum
is set to \> 0, the bufferpool will pre-allocate this amount of
is set to `\> 0`, the bufferpool will pre-allocate this amount of
buffers. When maximum is not 0, the bufferpool will allocate up to
maximum amount of buffers.
@ -527,7 +502,6 @@ point on you can use `gst_buffer_pool_acquire_buffer ()` to retrieve a
buffer from the pool, like this:
``` c
[...]
GstFlowReturn ret;
@ -538,14 +512,12 @@ buffer from the pool, like this:
goto pool_failed;
[...]
```
It is important to check the return value of the acquire function
because it is possible that it fails: When your element shuts down, it
will deactivate the bufferpool and then all calls to acquire will return
GST\_FLOW\_FLUSHNG.
`GST_FLOW_FLUSHING`.
All buffers that are acquired from the pool will have their pool member
set to the original pool. When the last ref is decremented on the
@ -560,14 +532,14 @@ WRITEME
## GST\_QUERY\_ALLOCATION
The ALLOCATION query is used to negotiate `GstMeta`, `GstBufferPool` and
The `ALLOCATION` query is used to negotiate `GstMeta`, `GstBufferPool` and
`GstAllocator` between elements. Negotiation of the allocation strategy
is always initiated and decided by a srcpad after it has negotiated a
format and before it decides to push buffers. A sinkpad can suggest an
allocation strategy but it is ultimately the source pad that will decide
based on the suggestions of the downstream sink pad.
The source pad will do a GST\_QUERY\_ALLOCATION with the negotiated caps
The source pad will do a `GST_QUERY_ALLOCATION` with the negotiated caps
as a parameter. This is needed so that the downstream element knows what
media type is being handled. A downstream sink pad can answer the
allocation query with the following results:
@ -575,7 +547,7 @@ allocation query with the following results:
- An array of possible `GstBufferPool` suggestions with suggested
size, minimum and maximum amount of buffers.
- An array of GstAllocator objects along with suggested allocation
- An array of `GstAllocator` objects along with suggested allocation
parameters such as flags, prefix, alignment and padding. These
allocators can also be configured in a bufferpool when this is
supported by the bufferpool.
@ -585,16 +557,15 @@ allocation query with the following results:
what kind of metadata is supported downstream before it places that
metadata on buffers.
When the GST\_QUERY\_ALLOCATION returns, the source pad will select from
When the `GST_QUERY_ALLOCATION` returns, the source pad will select from
the available bufferpools, allocators and metadata how it will allocate
buffers.
### ALLOCATION query example
Below is an example of the ALLOCATION query.
Below is an example of the `ALLOCATION` query.
``` c
#include <gst/video/video.h>
#include <gst/video/gstvideometa.h>
#include <gst/video/gstvideopool.h>
@ -638,19 +609,20 @@ Below is an example of the ALLOCATION query.
gst_buffer_pool_set_active (pool, TRUE);
[...]
```
This particular implementation will make a custom `GstVideoBufferPool`
object that is specialized in allocating video buffers. You can also
enable the pool to put `GstVideoMeta` metadata on the buffers from the
pool doing `gst_buffer_pool_config_add_option (config,
GST_BUFFER_POOL_OPTION_VIDEO_META)`.
pool doing:
``` c
gst_buffer_pool_config_add_option (config, GST_BUFFER_POOL_OPTION_VIDEO_META)
```
### The ALLOCATION query in base classes
In many baseclasses you will see the following virtual methods for
In many base classes you will see the following virtual methods for
influencing the allocation strategy:
- `propose_allocation ()` should suggest allocation parameters for the