2 Copyright(c) 2010-2014 Intel Corporation. All rights reserved.
5 Redistribution and use in source and binary forms, with or without
6 modification, are permitted provided that the following conditions
9 * Redistributions of source code must retain the above copyright
10 notice, this list of conditions and the following disclaimer.
11 * Redistributions in binary form must reproduce the above copyright
12 notice, this list of conditions and the following disclaimer in
13 the documentation and/or other materials provided with the
15 * Neither the name of Intel Corporation nor the names of its
16 contributors may be used to endorse or promote products derived
17 from this software without specific prior written permission.
19 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
22 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
23 OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
24 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
25 LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
26 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
27 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
28 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
29 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
31 .. _Environment_Abstraction_Layer:
33 Environment Abstraction Layer
34 =============================
36 The Environment Abstraction Layer (EAL) is responsible for gaining access to low-level resources such as hardware and memory space.
37 It provides a generic interface that hides the environment specifics from the applications and libraries.
38 It is the responsibility of the initialization routine to decide how to allocate these resources
39 (that is, memory space, PCI devices, timers, consoles, and so on).
41 Typical services expected from the EAL are:
43 * DPDK Loading and Launching:
44 The DPDK and its application are linked as a single application and must be loaded by some means.
46 * Core Affinity/Assignment Procedures:
47 The EAL provides mechanisms for assigning execution units to specific cores as well as creating execution instances.
49 * System Memory Reservation:
50 The EAL facilitates the reservation of different memory zones, for example, physical memory areas for device interactions.
52 * PCI Address Abstraction: The EAL provides an interface to access PCI address space.
54 * Trace and Debug Functions: Logs, dump_stack, panic and so on.
56 * Utility Functions: Spinlocks and atomic counters that are not provided in libc.
58 * CPU Feature Identification: Determine at runtime if a particular feature, for example, IntelĀ® AVX is supported.
59 Determine if the current CPU supports the feature set that the binary was compiled for.
61 * Interrupt Handling: Interfaces to register/unregister callbacks to specific interrupt sources.
63 * Alarm Functions: Interfaces to set/remove callbacks to be run at a specific time.
65 EAL in a Linux-userland Execution Environment
66 ---------------------------------------------
68 In a Linux user space environment, the DPDK application runs as a user-space application using the pthread library.
69 PCI information about devices and address space is discovered through the /sys kernel interface and through kernel modules such as uio_pci_generic, or igb_uio.
70 Refer to the UIO: User-space drivers documentation in the Linux kernel. This memory is mmap'd in the application.
72 The EAL performs physical memory allocation using mmap() in hugetlbfs (using huge page sizes to increase performance).
73 This memory is exposed to DPDK service layers such as the :ref:`Mempool Library <Mempool_Library>`.
75 At this point, the DPDK services layer will be initialized, then through pthread setaffinity calls,
76 each execution unit will be assigned to a specific logical core to run as a user-level thread.
78 The time reference is provided by the CPU Time-Stamp Counter (TSC) or by the HPET kernel API through a mmap() call.
80 Initialization and Core Launching
81 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
83 Part of the initialization is done by the start function of glibc.
84 A check is also performed at initialization time to ensure that the micro architecture type chosen in the config file is supported by the CPU.
85 Then, the main() function is called. The core initialization and launch is done in rte_eal_init() (see the API documentation).
86 It consist of calls to the pthread library (more specifically, pthread_self(), pthread_create(), and pthread_setaffinity_np()).
88 .. _figure_linuxapp_launch:
90 .. figure:: img/linuxapp_launch.*
92 EAL Initialization in a Linux Application Environment
97 Initialization of objects, such as memory zones, rings, memory pools, lpm tables and hash tables,
98 should be done as part of the overall application initialization on the master lcore.
99 The creation and initialization functions for these objects are not multi-thread safe.
100 However, once initialized, the objects themselves can safely be used in multiple threads simultaneously.
102 Multi-process Support
103 ~~~~~~~~~~~~~~~~~~~~~
105 The Linuxapp EAL allows a multi-process as well as a multi-threaded (pthread) deployment model.
107 :ref:`Multi-process Support <Multi-process_Support>` for more details.
109 Memory Mapping Discovery and Memory Reservation
110 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
112 The allocation of large contiguous physical memory is done using the hugetlbfs kernel filesystem.
113 The EAL provides an API to reserve named memory zones in this contiguous memory.
114 The physical address of the reserved memory for that memory zone is also returned to the user by the memory zone reservation API.
118 Memory reservations done using the APIs provided by rte_malloc are also backed by pages from the hugetlbfs filesystem.
123 The EAL uses the /sys/bus/pci utilities provided by the kernel to scan the content on the PCI bus.
124 To access PCI memory, a kernel module called uio_pci_generic provides a /dev/uioX device file
125 and resource files in /sys
126 that can be mmap'd to obtain access to PCI address space from the application.
127 The DPDK-specific igb_uio module can also be used for this. Both drivers use the uio kernel feature (userland driver).
129 Per-lcore and Shared Variables
130 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
134 lcore refers to a logical execution unit of the processor, sometimes called a hardware *thread*.
136 Shared variables are the default behavior.
137 Per-lcore variables are implemented using *Thread Local Storage* (TLS) to provide per-thread local storage.
142 A logging API is provided by EAL.
143 By default, in a Linux application, logs are sent to syslog and also to the console.
144 However, the log function can be overridden by the user to use a different logging mechanism.
146 Trace and Debug Functions
147 ^^^^^^^^^^^^^^^^^^^^^^^^^
149 There are some debug functions to dump the stack in glibc.
150 The rte_panic() function can voluntarily provoke a SIG_ABORT,
151 which can trigger the generation of a core file, readable by gdb.
153 CPU Feature Identification
154 ~~~~~~~~~~~~~~~~~~~~~~~~~~
156 The EAL can query the CPU at runtime (using the rte_cpu_get_features() function) to determine which CPU features are available.
158 User Space Interrupt Event
159 ~~~~~~~~~~~~~~~~~~~~~~~~~~
161 + User Space Interrupt and Alarm Handling in Host Thread
163 The EAL creates a host thread to poll the UIO device file descriptors to detect the interrupts.
164 Callbacks can be registered or unregistered by the EAL functions for a specific interrupt event
165 and are called in the host thread asynchronously.
166 The EAL also allows timed callbacks to be used in the same way as for NIC interrupts.
170 In DPDK PMD, the only interrupts handled by the dedicated host thread are those for link status change
171 (link up and link down notification) and for sudden device removal.
176 The receive and transmit routines provided by each PMD don't limit themselves to execute in polling thread mode.
177 To ease the idle polling with tiny throughput, it's useful to pause the polling and wait until the wake-up event happens.
178 The RX interrupt is the first choice to be such kind of wake-up event, but probably won't be the only one.
180 EAL provides the event APIs for this event-driven thread mode.
181 Taking linuxapp as an example, the implementation relies on epoll. Each thread can monitor an epoll instance
182 in which all the wake-up events' file descriptors are added. The event file descriptors are created and mapped to
183 the interrupt vectors according to the UIO/VFIO spec.
184 From bsdapp's perspective, kqueue is the alternative way, but not implemented yet.
186 EAL initializes the mapping between event file descriptors and interrupt vectors, while each device initializes the mapping
187 between interrupt vectors and queues. In this way, EAL actually is unaware of the interrupt cause on the specific vector.
188 The eth_dev driver takes responsibility to program the latter mapping.
192 Per queue RX interrupt event is only allowed in VFIO which supports multiple MSI-X vector. In UIO, the RX interrupt
193 together with other interrupt causes shares the same vector. In this case, when RX interrupt and LSC(link status change)
194 interrupt are both enabled(intr_conf.lsc == 1 && intr_conf.rxq == 1), only the former is capable.
196 The RX interrupt are controlled/enabled/disabled by ethdev APIs - 'rte_eth_dev_rx_intr_*'. They return failure if the PMD
197 hasn't support them yet. The intr_conf.rxq flag is used to turn on the capability of RX interrupt per device.
199 + Device Removal Event
201 This event is triggered by a device being removed at a bus level. Its
202 underlying resources may have been made unavailable (i.e. PCI mappings
203 unmapped). The PMD must make sure that on such occurrence, the application can
204 still safely use its callbacks.
206 This event can be subscribed to in the same way one would subscribe to a link
207 status change event. The execution context is thus the same, i.e. it is the
208 dedicated interrupt host thread.
210 Considering this, it is likely that an application would want to close a
211 device having emitted a Device Removal Event. In such case, calling
212 ``rte_eth_dev_close()`` can trigger it to unregister its own Device Removal Event
213 callback. Care must be taken not to close the device from the interrupt handler
214 context. It is necessary to reschedule such closing operation.
219 The EAL PCI device blacklist functionality can be used to mark certain NIC ports as blacklisted,
220 so they are ignored by the DPDK.
221 The ports to be blacklisted are identified using the PCIe* description (Domain:Bus:Device.Function).
226 Locks and atomic operations are per-architecture (i686 and x86_64).
228 Memory Segments and Memory Zones (memzone)
229 ------------------------------------------
231 The mapping of physical memory is provided by this feature in the EAL.
232 As physical memory can have gaps, the memory is described in a table of descriptors,
233 and each descriptor (called rte_memseg ) describes a contiguous portion of memory.
235 On top of this, the memzone allocator's role is to reserve contiguous portions of physical memory.
236 These zones are identified by a unique name when the memory is reserved.
238 The rte_memzone descriptors are also located in the configuration structure.
239 This structure is accessed using rte_eal_get_configuration().
240 The lookup (by name) of a memory zone returns a descriptor containing the physical address of the memory zone.
242 Memory zones can be reserved with specific start address alignment by supplying the align parameter
243 (by default, they are aligned to cache line size).
244 The alignment value should be a power of two and not less than the cache line size (64 bytes).
245 Memory zones can also be reserved from either 2 MB or 1 GB hugepages, provided that both are available on the system.
251 DPDK usually pins one pthread per core to avoid the overhead of task switching.
252 This allows for significant performance gains, but lacks flexibility and is not always efficient.
254 Power management helps to improve the CPU efficiency by limiting the CPU runtime frequency.
255 However, alternately it is possible to utilize the idle cycles available to take advantage of
256 the full capability of the CPU.
258 By taking advantage of cgroup, the CPU utilization quota can be simply assigned.
259 This gives another way to improve the CPU efficiency, however, there is a prerequisite;
260 DPDK must handle the context switching between multiple pthreads per core.
262 For further flexibility, it is useful to set pthread affinity not only to a CPU but to a CPU set.
264 EAL pthread and lcore Affinity
265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
267 The term "lcore" refers to an EAL thread, which is really a Linux/FreeBSD pthread.
268 "EAL pthreads" are created and managed by EAL and execute the tasks issued by *remote_launch*.
269 In each EAL pthread, there is a TLS (Thread Local Storage) called *_lcore_id* for unique identification.
270 As EAL pthreads usually bind 1:1 to the physical CPU, the *_lcore_id* is typically equal to the CPU ID.
272 When using multiple pthreads, however, the binding is no longer always 1:1 between an EAL pthread and a specified physical CPU.
273 The EAL pthread may have affinity to a CPU set, and as such the *_lcore_id* will not be the same as the CPU ID.
274 For this reason, there is an EAL long option '--lcores' defined to assign the CPU affinity of lcores.
275 For a specified lcore ID or ID group, the option allows setting the CPU set for that EAL pthread.
278 --lcores='<lcore_set>[@cpu_set][,<lcore_set>[@cpu_set],...]'
280 'lcore_set' and 'cpu_set' can be a single number, range or a group.
282 A number is a "digit([0-9]+)"; a range is "<number>-<number>"; a group is "(<number|range>[,<number|range>,...])".
284 If a '\@cpu_set' value is not supplied, the value of 'cpu_set' will default to the value of 'lcore_set'.
288 For example, "--lcores='1,2@(5-7),(3-5)@(0,2),(0,6),7-8'" which means start 9 EAL thread;
289 lcore 0 runs on cpuset 0x41 (cpu 0,6);
290 lcore 1 runs on cpuset 0x2 (cpu 1);
291 lcore 2 runs on cpuset 0xe0 (cpu 5,6,7);
292 lcore 3,4,5 runs on cpuset 0x5 (cpu 0,2);
293 lcore 6 runs on cpuset 0x41 (cpu 0,6);
294 lcore 7 runs on cpuset 0x80 (cpu 7);
295 lcore 8 runs on cpuset 0x100 (cpu 8).
297 Using this option, for each given lcore ID, the associated CPUs can be assigned.
298 It's also compatible with the pattern of corelist('-l') option.
300 non-EAL pthread support
301 ~~~~~~~~~~~~~~~~~~~~~~~
303 It is possible to use the DPDK execution context with any user pthread (aka. Non-EAL pthreads).
304 In a non-EAL pthread, the *_lcore_id* is always LCORE_ID_ANY which identifies that it is not an EAL thread with a valid, unique, *_lcore_id*.
305 Some libraries will use an alternative unique ID (e.g. TID), some will not be impacted at all, and some will work but with limitations (e.g. timer and mempool libraries).
307 All these impacts are mentioned in :ref:`known_issue_label` section.
312 There are two public APIs ``rte_thread_set_affinity()`` and ``rte_thread_get_affinity()`` introduced for threads.
313 When they're used in any pthread context, the Thread Local Storage(TLS) will be set/get.
315 Those TLS include *_cpuset* and *_socket_id*:
317 * *_cpuset* stores the CPUs bitmap to which the pthread is affinitized.
319 * *_socket_id* stores the NUMA node of the CPU set. If the CPUs in CPU set belong to different NUMA node, the *_socket_id* will be set to SOCKET_ID_ANY.
322 .. _known_issue_label:
329 The rte_mempool uses a per-lcore cache inside the mempool.
330 For non-EAL pthreads, ``rte_lcore_id()`` will not return a valid number.
331 So for now, when rte_mempool is used with non-EAL pthreads, the put/get operations will bypass the default mempool cache and there is a performance penalty because of this bypass.
332 Only user-owned external caches can be used in a non-EAL context in conjunction with ``rte_mempool_generic_put()`` and ``rte_mempool_generic_get()`` that accept an explicit cache parameter.
336 rte_ring supports multi-producer enqueue and multi-consumer dequeue.
337 However, it is non-preemptive, this has a knock on effect of making rte_mempool non-preemptable.
341 The "non-preemptive" constraint means:
343 - a pthread doing multi-producers enqueues on a given ring must not
344 be preempted by another pthread doing a multi-producer enqueue on
346 - a pthread doing multi-consumers dequeues on a given ring must not
347 be preempted by another pthread doing a multi-consumer dequeue on
350 Bypassing this constraint may cause the 2nd pthread to spin until the 1st one is scheduled again.
351 Moreover, if the 1st pthread is preempted by a context that has an higher priority, it may even cause a dead lock.
353 This does not mean it cannot be used, simply, there is a need to narrow down the situation when it is used by multi-pthread on the same core.
355 1. It CAN be used for any single-producer or single-consumer situation.
357 2. It MAY be used by multi-producer/consumer pthread whose scheduling policy are all SCHED_OTHER(cfs). User SHOULD be aware of the performance penalty before using it.
359 3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR.
363 Running ``rte_timer_manager()`` on a non-EAL pthread is not allowed. However, resetting/stopping the timer from a non-EAL pthread is allowed.
367 In non-EAL pthreads, there is no per thread loglevel and logtype, global loglevels are used.
371 The debug statistics of rte_ring, rte_mempool and rte_timer are not supported in a non-EAL pthread.
376 The following is a simple example of cgroup control usage, there are two pthreads(t0 and t1) doing packet I/O on the same core ($CPU).
377 We expect only 50% of CPU spend on packet IO.
379 .. code-block:: console
381 mkdir /sys/fs/cgroup/cpu/pkt_io
382 mkdir /sys/fs/cgroup/cpuset/pkt_io
384 echo $cpu > /sys/fs/cgroup/cpuset/cpuset.cpus
386 echo $t0 > /sys/fs/cgroup/cpu/pkt_io/tasks
387 echo $t0 > /sys/fs/cgroup/cpuset/pkt_io/tasks
389 echo $t1 > /sys/fs/cgroup/cpu/pkt_io/tasks
390 echo $t1 > /sys/fs/cgroup/cpuset/pkt_io/tasks
392 cd /sys/fs/cgroup/cpu/pkt_io
393 echo 100000 > pkt_io/cpu.cfs_period_us
394 echo 50000 > pkt_io/cpu.cfs_quota_us
400 The EAL provides a malloc API to allocate any-sized memory.
402 The objective of this API is to provide malloc-like functions to allow
403 allocation from hugepage memory and to facilitate application porting.
404 The *DPDK API Reference* manual describes the available functions.
406 Typically, these kinds of allocations should not be done in data plane
407 processing because they are slower than pool-based allocation and make
408 use of locks within the allocation and free paths.
409 However, they can be used in configuration code.
411 Refer to the rte_malloc() function description in the *DPDK API Reference*
412 manual for more information.
417 When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains
418 overwrite protection fields to help identify buffer overflows.
420 Alignment and NUMA Constraints
421 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
423 The rte_malloc() takes an align argument that can be used to request a memory
424 area that is aligned on a multiple of this value (which must be a power of two).
426 On systems with NUMA support, a call to the rte_malloc() function will return
427 memory that has been allocated on the NUMA socket of the core which made the call.
428 A set of APIs is also provided, to allow memory to be explicitly allocated on a
429 NUMA socket directly, or by allocated on the NUMA socket where another core is
430 located, in the case where the memory is to be used by a logical core other than
431 on the one doing the memory allocation.
436 This API is meant to be used by an application that requires malloc-like
437 functions at initialization time.
439 For allocating/freeing data at runtime, in the fast-path of an application,
440 the memory pool library should be used instead.
442 Internal Implementation
443 ~~~~~~~~~~~~~~~~~~~~~~~
448 There are two data structure types used internally in the malloc library:
450 * struct malloc_heap - used to track free space on a per-socket basis
452 * struct malloc_elem - the basic element of allocation and free-space
453 tracking inside the library.
455 Structure: malloc_heap
456 """"""""""""""""""""""
458 The malloc_heap structure is used to manage free space on a per-socket basis.
459 Internally, there is one heap structure per NUMA node, which allows us to
460 allocate memory to a thread based on the NUMA node on which this thread runs.
461 While this does not guarantee that the memory will be used on that NUMA node,
462 it is no worse than a scheme where the memory is always allocated on a fixed
465 The key fields of the heap structure and their function are described below
466 (see also diagram above):
468 * lock - the lock field is needed to synchronize access to the heap.
469 Given that the free space in the heap is tracked using a linked list,
470 we need a lock to prevent two threads manipulating the list at the same time.
472 * free_head - this points to the first element in the list of free nodes for
477 The malloc_heap structure does not keep track of in-use blocks of memory,
478 since these are never touched except when they are to be freed again -
479 at which point the pointer to the block is an input to the free() function.
481 .. _figure_malloc_heap:
483 .. figure:: img/malloc_heap.*
485 Example of a malloc heap and malloc elements within the malloc library
490 Structure: malloc_elem
491 """"""""""""""""""""""
493 The malloc_elem structure is used as a generic header structure for various
495 It is used in three different ways - all shown in the diagram above:
497 #. As a header on a block of free or allocated memory - normal case
499 #. As a padding header inside a block of memory
501 #. As an end-of-memseg marker
503 The most important fields in the structure and how they are used are described below.
507 If the usage of a particular field in one of the above three usages is not
508 described, the field can be assumed to have an undefined value in that
509 situation, for example, for padding headers only the "state" and "pad"
510 fields have valid values.
512 * heap - this pointer is a reference back to the heap structure from which
513 this block was allocated.
514 It is used for normal memory blocks when they are being freed, to add the
515 newly-freed block to the heap's free-list.
517 * prev - this pointer points to the header element/block in the memseg
518 immediately behind the current one. When freeing a block, this pointer is
519 used to reference the previous block to check if that block is also free.
520 If so, then the two free blocks are merged to form a single larger block.
522 * next_free - this pointer is used to chain the free-list of unallocated
523 memory blocks together.
524 It is only used in normal memory blocks; on ``malloc()`` to find a suitable
525 free block to allocate and on ``free()`` to add the newly freed element to
528 * state - This field can have one of three values: ``FREE``, ``BUSY`` or
530 The former two are to indicate the allocation state of a normal memory block
531 and the latter is to indicate that the element structure is a dummy structure
532 at the end of the start-of-block padding, i.e. where the start of the data
533 within a block is not at the start of the block itself, due to alignment
535 In that case, the pad header is used to locate the actual malloc element
536 header for the block.
537 For the end-of-memseg structure, this is always a ``BUSY`` value, which
538 ensures that no element, on being freed, searches beyond the end of the
539 memseg for other blocks to merge with into a larger free area.
541 * pad - this holds the length of the padding present at the start of the block.
542 In the case of a normal block header, it is added to the address of the end
543 of the header to give the address of the start of the data area, i.e. the
544 value passed back to the application on a malloc.
545 Within a dummy header inside the padding, this same value is stored, and is
546 subtracted from the address of the dummy header to yield the address of the
549 * size - the size of the data block, including the header itself.
550 For end-of-memseg structures, this size is given as zero, though it is never
552 For normal blocks which are being freed, this size value is used in place of
553 a "next" pointer to identify the location of the next block of memory that
554 in the case of being ``FREE``, the two free blocks can be merged into one.
559 On EAL initialization, all memsegs are setup as part of the malloc heap.
560 This setup involves placing a dummy structure at the end with ``BUSY`` state,
561 which may contain a sentinel value if ``CONFIG_RTE_MALLOC_DEBUG`` is enabled,
562 and a proper :ref:`element header<malloc_elem>` with ``FREE`` at the start
564 The ``FREE`` element is then added to the ``free_list`` for the malloc heap.
566 When an application makes a call to a malloc-like function, the malloc function
567 will first index the ``lcore_config`` structure for the calling thread, and
568 determine the NUMA node of that thread.
569 The NUMA node is used to index the array of ``malloc_heap`` structures which is
570 passed as a parameter to the ``heap_alloc()`` function, along with the
571 requested size, type, alignment and boundary parameters.
573 The ``heap_alloc()`` function will scan the free_list of the heap, and attempt
574 to find a free block suitable for storing data of the requested size, with the
575 requested alignment and boundary constraints.
577 When a suitable free element has been identified, the pointer to be returned
578 to the user is calculated.
579 The cache-line of memory immediately preceding this pointer is filled with a
580 struct malloc_elem header.
581 Because of alignment and boundary constraints, there could be free space at
582 the start and/or end of the element, resulting in the following behavior:
584 #. Check for trailing space.
585 If the trailing space is big enough, i.e. > 128 bytes, then the free element
587 If it is not, then we just ignore it (wasted space).
589 #. Check for space at the start of the element.
590 If the space at the start is small, i.e. <=128 bytes, then a pad header is
591 used, and the remaining space is wasted.
592 If, however, the remaining space is greater, then the free element is split.
594 The advantage of allocating the memory from the end of the existing element is
595 that no adjustment of the free list needs to take place - the existing element
596 on the free list just has its size pointer adjusted, and the following element
597 has its "prev" pointer redirected to the newly created element.
602 To free an area of memory, the pointer to the start of the data area is passed
603 to the free function.
604 The size of the ``malloc_elem`` structure is subtracted from this pointer to get
605 the element header for the block.
606 If this header is of type ``PAD`` then the pad length is further subtracted from
607 the pointer to get the proper element header for the entire block.
609 From this element header, we get pointers to the heap from which the block was
610 allocated and to where it must be freed, as well as the pointer to the previous
611 element, and via the size field, we can calculate the pointer to the next element.
612 These next and previous elements are then checked to see if they are also
613 ``FREE``, and if so, they are merged with the current element.
614 This means that we can never have two ``FREE`` memory blocks adjacent to one
615 another, as they are always merged into a single block.