.. note::
Memory reservations done using the APIs provided by the rte_malloc library are also backed by pages from the hugetlbfs filesystem.
- However, physical address information is not available for the blocks of memory allocated in this way.
Xen Dom0 support without hugetbls
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
echo 50000 > pkt_io/cpu.cfs_quota_us
+Malloc
+------
+
+The EAL provides a malloc API to allocate any-sized memory.
+
+The objective of this API is to provide malloc-like functions to allow
+allocation from hugepage memory and to facilitate application porting.
+The *DPDK API Reference* manual describes the available functions.
+
+Typically, these kinds of allocations should not be done in data plane
+processing because they are slower than pool-based allocation and make
+use of locks within the allocation and free paths.
+However, they can be used in configuration code.
+
+Refer to the rte_malloc() function description in the *DPDK API Reference*
+manual for more information.
+
+Cookies
+~~~~~~~
+
+When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains
+overwrite protection fields to help identify buffer overflows.
+
+Alignment and NUMA Constraints
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The rte_malloc() takes an align argument that can be used to request a memory
+area that is aligned on a multiple of this value (which must be a power of two).
+
+On systems with NUMA support, a call to the rte_malloc() function will return
+memory that has been allocated on the NUMA socket of the core which made the call.
+A set of APIs is also provided, to allow memory to be explicitly allocated on a
+NUMA socket directly, or by allocated on the NUMA socket where another core is
+located, in the case where the memory is to be used by a logical core other than
+on the one doing the memory allocation.
+
+Use Cases
+~~~~~~~~~
+
+This API is meant to be used by an application that requires malloc-like
+functions at initialization time.
+
+For allocating/freeing data at runtime, in the fast-path of an application,
+the memory pool library should be used instead.
+
+Internal Implementation
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Data Structures
+^^^^^^^^^^^^^^^
+
+There are two data structure types used internally in the malloc library:
+
+* struct malloc_heap - used to track free space on a per-socket basis
+
+* struct malloc_elem - the basic element of allocation and free-space
+ tracking inside the library.
+
+Structure: malloc_heap
+""""""""""""""""""""""
+
+The malloc_heap structure is used to manage free space on a per-socket basis.
+Internally, there is one heap structure per NUMA node, which allows us to
+allocate memory to a thread based on the NUMA node on which this thread runs.
+While this does not guarantee that the memory will be used on that NUMA node,
+it is no worse than a scheme where the memory is always allocated on a fixed
+or random node.
+
+The key fields of the heap structure and their function are described below
+(see also diagram above):
+
+* lock - the lock field is needed to synchronize access to the heap.
+ Given that the free space in the heap is tracked using a linked list,
+ we need a lock to prevent two threads manipulating the list at the same time.
+
+* free_head - this points to the first element in the list of free nodes for
+ this malloc heap.
+
+.. note::
+
+ The malloc_heap structure does not keep track of in-use blocks of memory,
+ since these are never touched except when they are to be freed again -
+ at which point the pointer to the block is an input to the free() function.
+
+.. _figure_malloc_heap:
+
+.. figure:: img/malloc_heap.*
+
+ Example of a malloc heap and malloc elements within the malloc library
+
+
+.. _malloc_elem:
+
+Structure: malloc_elem
+""""""""""""""""""""""
+
+The malloc_elem structure is used as a generic header structure for various
+blocks of memory.
+It is used in three different ways - all shown in the diagram above:
+
+#. As a header on a block of free or allocated memory - normal case
+
+#. As a padding header inside a block of memory
+
+#. As an end-of-memseg marker
+
+The most important fields in the structure and how they are used are described below.
+
+.. note::
+
+ If the usage of a particular field in one of the above three usages is not
+ described, the field can be assumed to have an undefined value in that
+ situation, for example, for padding headers only the "state" and "pad"
+ fields have valid values.
+
+* heap - this pointer is a reference back to the heap structure from which
+ this block was allocated.
+ It is used for normal memory blocks when they are being freed, to add the
+ newly-freed block to the heap's free-list.
+
+* prev - this pointer points to the header element/block in the memseg
+ immediately behind the current one. When freeing a block, this pointer is
+ used to reference the previous block to check if that block is also free.
+ If so, then the two free blocks are merged to form a single larger block.
+
+* next_free - this pointer is used to chain the free-list of unallocated
+ memory blocks together.
+ It is only used in normal memory blocks; on ``malloc()`` to find a suitable
+ free block to allocate and on ``free()`` to add the newly freed element to
+ the free-list.
+
+* state - This field can have one of three values: ``FREE``, ``BUSY`` or
+ ``PAD``.
+ The former two are to indicate the allocation state of a normal memory block
+ and the latter is to indicate that the element structure is a dummy structure
+ at the end of the start-of-block padding, i.e. where the start of the data
+ within a block is not at the start of the block itself, due to alignment
+ constraints.
+ In that case, the pad header is used to locate the actual malloc element
+ header for the block.
+ For the end-of-memseg structure, this is always a ``BUSY`` value, which
+ ensures that no element, on being freed, searches beyond the end of the
+ memseg for other blocks to merge with into a larger free area.
+
+* pad - this holds the length of the padding present at the start of the block.
+ In the case of a normal block header, it is added to the address of the end
+ of the header to give the address of the start of the data area, i.e. the
+ value passed back to the application on a malloc.
+ Within a dummy header inside the padding, this same value is stored, and is
+ subtracted from the address of the dummy header to yield the address of the
+ actual block header.
+
+* size - the size of the data block, including the header itself.
+ For end-of-memseg structures, this size is given as zero, though it is never
+ actually checked.
+ For normal blocks which are being freed, this size value is used in place of
+ a "next" pointer to identify the location of the next block of memory that
+ in the case of being ``FREE``, the two free blocks can be merged into one.
+
+Memory Allocation
+^^^^^^^^^^^^^^^^^
+
+On EAL initialisation, all memsegs are setup as part of the malloc heap.
+This setup involves placing a dummy structure at the end with ``BUSY`` state,
+which may contain a sentinel value if ``CONFIG_RTE_MALLOC_DEBUG`` is enabled,
+and a proper :ref:`element header<malloc_elem>` with ``FREE`` at the start
+for each memseg.
+The ``FREE`` element is then added to the ``free_list`` for the malloc heap.
+
+When an application makes a call to a malloc-like function, the malloc function
+will first index the ``lcore_config`` structure for the calling thread, and
+determine the NUMA node of that thread.
+The NUMA node is used to index the array of ``malloc_heap`` structures which is
+passed as a parameter to the ``heap_alloc()`` function, along with the
+requested size, type, alignment and boundary parameters.
+
+The ``heap_alloc()`` function will scan the free_list of the heap, and attempt
+to find a free block suitable for storing data of the requested size, with the
+requested alignment and boundary constraints.
+
+When a suitable free element has been identified, the pointer to be returned
+to the user is calculated.
+The cache-line of memory immediately preceding this pointer is filled with a
+struct malloc_elem header.
+Because of alignment and boundary constraints, there could be free space at
+the start and/or end of the element, resulting in the following behavior:
+
+#. Check for trailing space.
+ If the trailing space is big enough, i.e. > 128 bytes, then the free element
+ is split.
+ If it is not, then we just ignore it (wasted space).
+
+#. Check for space at the start of the element.
+ If the space at the start is small, i.e. <=128 bytes, then a pad header is
+ used, and the remaining space is wasted.
+ If, however, the remaining space is greater, then the free element is split.
+
+The advantage of allocating the memory from the end of the existing element is
+that no adjustment of the free list needs to take place - the existing element
+on the free list just has its size pointer adjusted, and the following element
+has its "prev" pointer redirected to the newly created element.
+
+Freeing Memory
+^^^^^^^^^^^^^^
+
+To free an area of memory, the pointer to the start of the data area is passed
+to the free function.
+The size of the ``malloc_elem`` structure is subtracted from this pointer to get
+the element header for the block.
+If this header is of type ``PAD`` then the pad length is further subtracted from
+the pointer to get the proper element header for the entire block.
+
+From this element header, we get pointers to the heap from which the block was
+allocated and to where it must be freed, as well as the pointer to the previous
+element, and via the size field, we can calculate the pointer to the next element.
+These next and previous elements are then checked to see if they are also
+``FREE``, and if so, they are merged with the current element.
+This means that we can never have two ``FREE`` memory blocks adjacent to one
+another, as they are always merged into a single block.
--- /dev/null
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+# modification, are permitted provided that the following conditions
+# are met:
+#
+# - Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# - Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in
+# the documentation and/or other materials provided with the
+# distribution.
+#
+# - Neither the name of Intel Corporation nor the names of its
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+#
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"
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+ <text
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+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"
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+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
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+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas">Dummy Elements:</tspan><tspan
+ sodipodi:role="line"
+ x="690.51538"
+ y="256.02936"
+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas"
+ id="tspan13581">Size = 0</tspan><tspan
+ sodipodi:role="line"
+ x="690.51538"
+ y="275.22937"
+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas"
+ id="tspan13583">State = BUSY</tspan></text>
+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ sodipodi:linespacing="120%"><tspan
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+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas">pad</tspan></text>
+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ id="tspan13711">Malloc element header:</tspan><tspan
+ sodipodi:role="line"
+ x="429.11118"
+ y="469.04529"
+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas"
+ id="tspan13713">state = BUSY</tspan><tspan
+ sodipodi:role="line"
+ x="429.11118"
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+ id="tspan13715">size = <size></tspan><tspan
+ sodipodi:role="line"
+ x="429.11118"
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+ id="tspan13717">pad = <padsize></tspan></text>
+ <flowRoot
+ xml:space="preserve"
+ id="flowRoot13719"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:125%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;font-family:Sans"><flowRegion
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+ xml:space="preserve"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ style="font-size:16px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:start;line-height:120.00000477%;writing-mode:lr-tb;text-anchor:start;font-family:Consolas;-inkscape-font-specification:Consolas"
+ id="tspan14527">Pad element header:</tspan><tspan
+ sodipodi:role="line"
+ x="660.39099"
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+ id="tspan14533">pad = padsize</tspan></text>
+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
+ x="506.5249"
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+ <text
+ xml:space="preserve"
+ style="font-size:40px;font-style:normal;font-weight:normal;line-height:120.00000477%;letter-spacing:0px;word-spacing:0px;fill:#000000;fill-opacity:1;stroke:none;display:inline;font-family:Sans"
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+ sodipodi:linespacing="120%"><tspan
+ sodipodi:role="line"
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+ x="506.18994"
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+++ /dev/null
-.. BSD LICENSE
- Copyright(c) 2010-2014 Intel Corporation. All rights reserved.
- All rights reserved.
-
- Redistribution and use in source and binary forms, with or without
- modification, are permitted provided that the following conditions
- are met:
-
- * Redistributions of source code must retain the above copyright
- notice, this list of conditions and the following disclaimer.
- * Redistributions in binary form must reproduce the above copyright
- notice, this list of conditions and the following disclaimer in
- the documentation and/or other materials provided with the
- distribution.
- * Neither the name of Intel Corporation nor the names of its
- contributors may be used to endorse or promote products derived
- from this software without specific prior written permission.
-
- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
- OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-.. _Malloc_Library:
-
-Malloc Library
-==============
-
-The librte_malloc library provides an API to allocate any-sized memory.
-
-The objective of this library is to provide malloc-like functions to allow allocation from hugepage memory
-and to facilitate application porting.
-The *DPDK API Reference* manual describes the available functions.
-
-Typically, these kinds of allocations should not be done in data plane processing
-because they are slower than pool-based allocation and make use of locks within the allocation
-and free paths.
-However, they can be used in configuration code.
-
-Refer to the rte_malloc() function description in the *DPDK API Reference* manual for more information.
-
-Cookies
--------
-
-When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains overwrite protection fields
-to help identify buffer overflows.
-
-Alignment and NUMA Constraints
-------------------------------
-
-The rte_malloc() takes an align argument that can be used to request a memory area
-that is aligned on a multiple of this value (which must be a power of two).
-
-On systems with NUMA support, a call to the rte_malloc() function will return memory
-that has been allocated on the NUMA socket of the core which made the call.
-A set of APIs is also provided, to allow memory to be explicitly allocated on a NUMA socket directly,
-or by allocated on the NUMA socket where another core is located,
-in the case where the memory is to be used by a logical core other than on the one doing the memory allocation.
-
-Use Cases
----------
-
-This library is needed by an application that requires malloc-like functions at initialization time,
-and does not require the physical address information for the individual memory blocks.
-
-For allocating/freeing data at runtime, in the fast-path of an application,
-the memory pool library should be used instead.
-
-If a block of memory with a known physical address is needed,
-e.g. for use by a hardware device, a memory zone should be used.
-
-Internal Implementation
------------------------
-
-Data Structures
-~~~~~~~~~~~~~~~
-
-There are two data structure types used internally in the malloc library:
-
-* struct malloc_heap - used to track free space on a per-socket basis
-
-* struct malloc_elem - the basic element of allocation and free-space tracking inside the library.
-
-Structure: malloc_heap
-^^^^^^^^^^^^^^^^^^^^^^
-
-The malloc_heap structure is used in the library to manage free space on a per-socket basis.
-Internally in the library, there is one heap structure per NUMA node,
-which allows us to allocate memory to a thread based on the NUMA node on which this thread runs.
-While this does not guarantee that the memory will be used on that NUMA node,
-it is no worse than a scheme where the memory is always allocated on a fixed or random node.
-
-The key fields of the heap structure and their function are described below (see also diagram above):
-
-* mz_count - field to count the number of memory zones which have been allocated for heap memory on this NUMA node.
- The sole use of this value is, in combination with the numa_socket value,
- to generate a suitable, unique name for each memory zone.
-
-* lock - the lock field is needed to synchronize access to the heap.
- Given that the free space in the heap is tracked using a linked list,
- we need a lock to prevent two threads manipulating the list at the same time.
-
-* free_head - this points to the first element in the list of free nodes for this malloc heap.
-
-.. note::
-
- The malloc_heap structure does not keep track of either the memzones allocated,
- since there is little point as they cannot be freed.
- Neither does it track the in-use blocks of memory,
- since these are never touched except when they are to be freed again -
- at which point the pointer to the block is an input to the free() function.
-
-.. _figure_malloc_heap:
-
-.. figure:: img/malloc_heap.*
-
- Example of a malloc heap and malloc elements within the malloc library
-
-
-Structure: malloc_elem
-^^^^^^^^^^^^^^^^^^^^^^
-The malloc_elem structure is used as a generic header structure for various blocks of memory in a memzone.
-It is used in three different ways - all shown in the diagram above:
-
-#. As a header on a block of free or allocated memory - normal case
-
-#. As a padding header inside a block of memory
-
-#. As an end-of-memzone marker
-
-The most important fields in the structure and how they are used are described below.
-
-.. note::
-
- If the usage of a particular field in one of the above three usages is not described,
- the field can be assumed to have an undefined value in that situation, for example,
- for padding headers only the "state" and "pad" fields have valid values.
-
-* heap - this pointer is a reference back to the heap structure from which this block was allocated.
- It is used for normal memory blocks when they are being freed,
- to add the newly-freed block to the heap's free-list.
-
-* prev - this pointer points to the header element/block in the memzone immediately behind the current one.
- When freeing a block, this pointer is used to reference the previous block to check if that block is also free.
- If so, then the two free blocks are merged to form a single larger block.
-
-* next_free - this pointer is used to chain the free-list of unallocated memory blocks together.
- Again, it is only used in normal memory blocks - on malloc() to find a suitable free block to allocate,
- and on free() to add the newly freed element to the free-list.
-
-* state - This field can have one of three values: "Free", "Busy" or "Pad".
- The former two, are to indicate the allocation state of a normal memory block,
- and the latter is to indicate that the element structure is a dummy structure at the end of the start-of-block padding
- (i.e. where the start of the data within a block is not at the start of the block itself, due to alignment constraints).
- In this case, the pad header is used to locate the actual malloc element header for the block.
- For the end-of-memzone structure, this is always a "busy" value, which ensures that no element,
- on being freed, searches beyond the end of the memzone for other blocks to merge with into a larger free area.
-
-* pad - this holds the length of the padding present at the start of the block.
- In the case of a normal block header, it is added to the address of the end of the header
- to give the address of the start of the data area i.e.
- the value passed back to the application on a malloc.
- Within a dummy header inside the padding, this same value is stored,
- and is subtracted from the address of the dummy header to yield the address of the actual block header.
-
-* size - the size of the data block, including the header itself.
- For end-of-memzone structures, this size is given as zero, though it is never actually checked.
- For normal blocks which are being freed,
- this size value is used in place of a "next" pointer to identify the location of the next block of memory
- (so that if it too is free, the two free blocks can be merged into one).
-
-Memory Allocation
-~~~~~~~~~~~~~~~~~
-
-When an application makes a call to a malloc-like function,
-the malloc function will first index the lcore_config structure for the calling thread,
-and determine the NUMA node idea of that thread.
-That is used to index the array of malloc_heap structures,
-and the heap_alloc () function is called with that heap as parameter,
-along with the requested size, type and alignment parameters.
-
-The heap_alloc() function will scan the free_list for the heap,
-and attempt to find a free block suitable for storing data of the requested size,
-with the requested alignment constraints.
-If no suitable block is found - for example, the first time malloc is called for a node,
-and the free-list is NULL - a new memzone is reserved and set up as heap elements.
-The setup involves placing a dummy structure at the end of the memzone
-to act as a sentinel to prevent accesses beyond the end
-(as the sentinel is marked as BUSY, the malloc library code will never attempt to reference it further),
-and a proper element header at the start of the memzone.
-This latter header identifies all space in the memzone, bar the sentinel value at the end,
-as a single free heap element, and it is then added to the free_list for the heap.
-
-Once the new memzone has been set up, the scan of the free-list for the heap is redone,
-and on this occasion should find the newly created,
-suitable element as the size of memory reserved in the memzone is set to be
-at least the size of the requested data block plus the alignment -
-subject to a minimum size specified in the DPDK compile-time configuration.
-
-When a suitable, free element has been identified, the pointer to be returned to the user is calculated,
-with the space to be provided to the user being at the end of the free block.
-The cache-line of memory immediately preceding this space is filled with a struct malloc_elem header:
-if the remaining space within the block is small e.g. <=128 bytes,
-then a pad header is used, and the remaining space is wasted.
-If, however, the remaining space is greater than this, then the single free element block is split into two,
-and a new, proper, malloc_elem header is put before the returned data space.
-[The advantage of allocating the memory from the end of the existing element is that
-in this case no adjustment of the free list needs to take place -
-the existing element on the free list just has its size pointer adjusted,
-and the following element has its "prev" pointer redirected to the newly created element].
-
-Freeing Memory
-~~~~~~~~~~~~~~
-
-To free an area of memory, the pointer to the start of the data area is passed to the free function.
-The size of the malloc_elem structure is subtracted from this pointer to get the element header for the block.
-If this header is of type "PAD" then the pad length is further subtracted from the pointer
-to get the proper element header for the entire block.
-
-From this element header, we get pointers to the heap from which the block came -- and to where it must be freed,
-as well as the pointer to the previous element, and, via the size field,
-we can calculate the pointer to the next element.
-These next and previous elements are then checked to see if they too are free,
-and if so, they are merged with the current elements.
-This means that we can never have two free memory blocks adjacent to one another,
-they are always merged into a single block.
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