1.. BSD LICENSE 2 Copyright(c) 2010-2014 Intel Corporation. All rights reserved. 3 All rights reserved. 4 5 Redistribution and use in source and binary forms, with or without 6 modification, are permitted provided that the following conditions 7 are met: 8 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 14 distribution. 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. 18 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. 30 31.. _Environment_Abstraction_Layer: 32 33Environment Abstraction Layer 34============================= 35 36The Environment Abstraction Layer (EAL) is responsible for gaining access to low-level resources such as hardware and memory space. 37It provides a generic interface that hides the environment specifics from the applications and libraries. 38It 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). 40 41Typical services expected from the EAL are: 42 43* DPDK Loading and Launching: 44 The DPDK and its application are linked as a single application and must be loaded by some means. 45 46* Core Affinity/Assignment Procedures: 47 The EAL provides mechanisms for assigning execution units to specific cores as well as creating execution instances. 48 49* System Memory Reservation: 50 The EAL facilitates the reservation of different memory zones, for example, physical memory areas for device interactions. 51 52* PCI Address Abstraction: The EAL provides an interface to access PCI address space. 53 54* Trace and Debug Functions: Logs, dump_stack, panic and so on. 55 56* Utility Functions: Spinlocks and atomic counters that are not provided in libc. 57 58* CPU Feature Identification: Determine at runtime if a particular feature, for example, Intel® AVX is supported. 59 Determine if the current CPU supports the feature set that the binary was compiled for. 60 61* Interrupt Handling: Interfaces to register/unregister callbacks to specific interrupt sources. 62 63* Alarm Functions: Interfaces to set/remove callbacks to be run at a specific time. 64 65EAL in a Linux-userland Execution Environment 66--------------------------------------------- 67 68In a Linux user space environment, the DPDK application runs as a user-space application using the pthread library. 69PCI 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. 70Refer to the UIO: User-space drivers documentation in the Linux kernel. This memory is mmap'd in the application. 71 72The EAL performs physical memory allocation using mmap() in hugetlbfs (using huge page sizes to increase performance). 73This memory is exposed to DPDK service layers such as the :ref:`Mempool Library <Mempool_Library>`. 74 75At this point, the DPDK services layer will be initialized, then through pthread setaffinity calls, 76each execution unit will be assigned to a specific logical core to run as a user-level thread. 77 78The time reference is provided by the CPU Time-Stamp Counter (TSC) or by the HPET kernel API through a mmap() call. 79 80Initialization and Core Launching 81~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 82 83Part of the initialization is done by the start function of glibc. 84A check is also performed at initialization time to ensure that the micro architecture type chosen in the config file is supported by the CPU. 85Then, the main() function is called. The core initialization and launch is done in rte_eal_init() (see the API documentation). 86It consist of calls to the pthread library (more specifically, pthread_self(), pthread_create(), and pthread_setaffinity_np()). 87 88.. _figure_linuxapp_launch: 89 90.. figure:: img/linuxapp_launch.* 91 92 EAL Initialization in a Linux Application Environment 93 94 95.. note:: 96 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. 101 102Multi-process Support 103~~~~~~~~~~~~~~~~~~~~~ 104 105The Linuxapp EAL allows a multi-process as well as a multi-threaded (pthread) deployment model. 106See chapter 2.20 107:ref:`Multi-process Support <Multi-process_Support>` for more details. 108 109Memory Mapping Discovery and Memory Reservation 110~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 111 112The allocation of large contiguous physical memory is done using the hugetlbfs kernel filesystem. 113The EAL provides an API to reserve named memory zones in this contiguous memory. 114The physical address of the reserved memory for that memory zone is also returned to the user by the memory zone reservation API. 115 116.. note:: 117 118 Memory reservations done using the APIs provided by the rte_malloc library are also backed by pages from the hugetlbfs filesystem. 119 120Xen Dom0 support without hugetbls 121~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 122 123The existing memory management implementation is based on the Linux kernel hugepage mechanism. 124However, Xen Dom0 does not support hugepages, so a new Linux kernel module rte_dom0_mm is added to workaround this limitation. 125 126The EAL uses IOCTL interface to notify the Linux kernel module rte_dom0_mm to allocate memory of specified size, 127and get all memory segments information from the module, 128and the EAL uses MMAP interface to map the allocated memory. 129For each memory segment, the physical addresses are contiguous within it but actual hardware addresses are contiguous within 2MB. 130 131PCI Access 132~~~~~~~~~~ 133 134The EAL uses the /sys/bus/pci utilities provided by the kernel to scan the content on the PCI bus. 135To access PCI memory, a kernel module called uio_pci_generic provides a /dev/uioX device file 136and resource files in /sys 137that can be mmap'd to obtain access to PCI address space from the application. 138The DPDK-specific igb_uio module can also be used for this. Both drivers use the uio kernel feature (userland driver). 139 140Per-lcore and Shared Variables 141~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 142 143.. note:: 144 145 lcore refers to a logical execution unit of the processor, sometimes called a hardware *thread*. 146 147Shared variables are the default behavior. 148Per-lcore variables are implemented using *Thread Local Storage* (TLS) to provide per-thread local storage. 149 150Logs 151~~~~ 152 153A logging API is provided by EAL. 154By default, in a Linux application, logs are sent to syslog and also to the console. 155However, the log function can be overridden by the user to use a different logging mechanism. 156 157Trace and Debug Functions 158^^^^^^^^^^^^^^^^^^^^^^^^^ 159 160There are some debug functions to dump the stack in glibc. 161The rte_panic() function can voluntarily provoke a SIG_ABORT, 162which can trigger the generation of a core file, readable by gdb. 163 164CPU Feature Identification 165~~~~~~~~~~~~~~~~~~~~~~~~~~ 166 167The EAL can query the CPU at runtime (using the rte_cpu_get_feature() function) to determine which CPU features are available. 168 169User Space Interrupt and Alarm Handling 170~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 171 172The EAL creates a host thread to poll the UIO device file descriptors to detect the interrupts. 173Callbacks can be registered or unregistered by the EAL functions for a specific interrupt event 174and are called in the host thread asynchronously. 175The EAL also allows timed callbacks to be used in the same way as for NIC interrupts. 176 177.. note:: 178 179 The only interrupts supported by the DPDK Poll-Mode Drivers are those for link status change, 180 i.e. link up and link down notification. 181 182Blacklisting 183~~~~~~~~~~~~ 184 185The EAL PCI device blacklist functionality can be used to mark certain NIC ports as blacklisted, 186so they are ignored by the DPDK. 187The ports to be blacklisted are identified using the PCIe* description (Domain:Bus:Device.Function). 188 189Misc Functions 190~~~~~~~~~~~~~~ 191 192Locks and atomic operations are per-architecture (i686 and x86_64). 193 194Memory Segments and Memory Zones (memzone) 195------------------------------------------ 196 197The mapping of physical memory is provided by this feature in the EAL. 198As physical memory can have gaps, the memory is described in a table of descriptors, 199and each descriptor (called rte_memseg ) describes a contiguous portion of memory. 200 201On top of this, the memzone allocator's role is to reserve contiguous portions of physical memory. 202These zones are identified by a unique name when the memory is reserved. 203 204The rte_memzone descriptors are also located in the configuration structure. 205This structure is accessed using rte_eal_get_configuration(). 206The lookup (by name) of a memory zone returns a descriptor containing the physical address of the memory zone. 207 208Memory zones can be reserved with specific start address alignment by supplying the align parameter 209(by default, they are aligned to cache line size). 210The alignment value should be a power of two and not less than the cache line size (64 bytes). 211Memory zones can also be reserved from either 2 MB or 1 GB hugepages, provided that both are available on the system. 212 213 214Multiple pthread 215---------------- 216 217DPDK usually pins one pthread per core to avoid the overhead of task switching. 218This allows for significant performance gains, but lacks flexibility and is not always efficient. 219 220Power management helps to improve the CPU efficiency by limiting the CPU runtime frequency. 221However, alternately it is possible to utilize the idle cycles available to take advantage of 222the full capability of the CPU. 223 224By taking advantage of cgroup, the CPU utilization quota can be simply assigned. 225This gives another way to improve the CPU efficiency, however, there is a prerequisite; 226DPDK must handle the context switching between multiple pthreads per core. 227 228For further flexibility, it is useful to set pthread affinity not only to a CPU but to a CPU set. 229 230EAL pthread and lcore Affinity 231~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 232 233The term "lcore" refers to an EAL thread, which is really a Linux/FreeBSD pthread. 234"EAL pthreads" are created and managed by EAL and execute the tasks issued by *remote_launch*. 235In each EAL pthread, there is a TLS (Thread Local Storage) called *_lcore_id* for unique identification. 236As EAL pthreads usually bind 1:1 to the physical CPU, the *_lcore_id* is typically equal to the CPU ID. 237 238When using multiple pthreads, however, the binding is no longer always 1:1 between an EAL pthread and a specified physical CPU. 239The EAL pthread may have affinity to a CPU set, and as such the *_lcore_id* will not be the same as the CPU ID. 240For this reason, there is an EAL long option '--lcores' defined to assign the CPU affinity of lcores. 241For a specified lcore ID or ID group, the option allows setting the CPU set for that EAL pthread. 242 243The format pattern: 244 --lcores='<lcore_set>[@cpu_set][,<lcore_set>[@cpu_set],...]' 245 246'lcore_set' and 'cpu_set' can be a single number, range or a group. 247 248A number is a "digit([0-9]+)"; a range is "<number>-<number>"; a group is "(<number|range>[,<number|range>,...])". 249 250If a '\@cpu_set' value is not supplied, the value of 'cpu_set' will default to the value of 'lcore_set'. 251 252 :: 253 254 For example, "--lcores='1,2@(5-7),(3-5)@(0,2),(0,6),7-8'" which means start 9 EAL thread; 255 lcore 0 runs on cpuset 0x41 (cpu 0,6); 256 lcore 1 runs on cpuset 0x2 (cpu 1); 257 lcore 2 runs on cpuset 0xe0 (cpu 5,6,7); 258 lcore 3,4,5 runs on cpuset 0x5 (cpu 0,2); 259 lcore 6 runs on cpuset 0x41 (cpu 0,6); 260 lcore 7 runs on cpuset 0x80 (cpu 7); 261 lcore 8 runs on cpuset 0x100 (cpu 8). 262 263Using this option, for each given lcore ID, the associated CPUs can be assigned. 264It's also compatible with the pattern of corelist('-l') option. 265 266non-EAL pthread support 267~~~~~~~~~~~~~~~~~~~~~~~ 268 269It is possible to use the DPDK execution context with any user pthread (aka. Non-EAL pthreads). 270In 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*. 271Some 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). 272 273All these impacts are mentioned in :ref:`known_issue_label` section. 274 275Public Thread API 276~~~~~~~~~~~~~~~~~ 277 278There are two public APIs ``rte_thread_set_affinity()`` and ``rte_pthread_get_affinity()`` introduced for threads. 279When they're used in any pthread context, the Thread Local Storage(TLS) will be set/get. 280 281Those TLS include *_cpuset* and *_socket_id*: 282 283* *_cpuset* stores the CPUs bitmap to which the pthread is affinitized. 284 285* *_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. 286 287 288.. _known_issue_label: 289 290Known Issues 291~~~~~~~~~~~~ 292 293+ rte_mempool 294 295 The rte_mempool uses a per-lcore cache inside the mempool. 296 For non-EAL pthreads, ``rte_lcore_id()`` will not return a valid number. 297 So for now, when rte_mempool is used with non-EAL pthreads, the put/get operations will bypass the mempool cache and there is a performance penalty because of this bypass. 298 Support for non-EAL mempool cache is currently being enabled. 299 300+ rte_ring 301 302 rte_ring supports multi-producer enqueue and multi-consumer dequeue. 303 However, it is non-preemptive, this has a knock on effect of making rte_mempool non-preemptable. 304 305 .. note:: 306 307 The "non-preemptive" constraint means: 308 309 - a pthread doing multi-producers enqueues on a given ring must not 310 be preempted by another pthread doing a multi-producer enqueue on 311 the same ring. 312 - a pthread doing multi-consumers dequeues on a given ring must not 313 be preempted by another pthread doing a multi-consumer dequeue on 314 the same ring. 315 316 Bypassing this constraint it may cause the 2nd pthread to spin until the 1st one is scheduled again. 317 Moreover, if the 1st pthread is preempted by a context that has an higher priority, it may even cause a dead lock. 318 319 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. 320 321 1. It CAN be used for any single-producer or single-consumer situation. 322 323 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. 324 325 3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR. 326 327 ``RTE_RING_PAUSE_REP_COUNT`` is defined for rte_ring to reduce contention. It's mainly for case 2, a yield is issued after number of times pause repeat. 328 329 It adds a sched_yield() syscall if the thread spins for too long while waiting on the other thread to finish its operations on the ring. 330 This gives the preempted thread a chance to proceed and finish with the ring enqueue/dequeue operation. 331 332+ rte_timer 333 334 Running ``rte_timer_manager()`` on a non-EAL pthread is not allowed. However, resetting/stopping the timer from a non-EAL pthread is allowed. 335 336+ rte_log 337 338 In non-EAL pthreads, there is no per thread loglevel and logtype, global loglevels are used. 339 340+ misc 341 342 The debug statistics of rte_ring, rte_mempool and rte_timer are not supported in a non-EAL pthread. 343 344cgroup control 345~~~~~~~~~~~~~~ 346 347The 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). 348We expect only 50% of CPU spend on packet IO. 349 350 .. code-block:: console 351 352 mkdir /sys/fs/cgroup/cpu/pkt_io 353 mkdir /sys/fs/cgroup/cpuset/pkt_io 354 355 echo $cpu > /sys/fs/cgroup/cpuset/cpuset.cpus 356 357 echo $t0 > /sys/fs/cgroup/cpu/pkt_io/tasks 358 echo $t0 > /sys/fs/cgroup/cpuset/pkt_io/tasks 359 360 echo $t1 > /sys/fs/cgroup/cpu/pkt_io/tasks 361 echo $t1 > /sys/fs/cgroup/cpuset/pkt_io/tasks 362 363 cd /sys/fs/cgroup/cpu/pkt_io 364 echo 100000 > pkt_io/cpu.cfs_period_us 365 echo 50000 > pkt_io/cpu.cfs_quota_us 366 367 368Malloc 369------ 370 371The EAL provides a malloc API to allocate any-sized memory. 372 373The objective of this API is to provide malloc-like functions to allow 374allocation from hugepage memory and to facilitate application porting. 375The *DPDK API Reference* manual describes the available functions. 376 377Typically, these kinds of allocations should not be done in data plane 378processing because they are slower than pool-based allocation and make 379use of locks within the allocation and free paths. 380However, they can be used in configuration code. 381 382Refer to the rte_malloc() function description in the *DPDK API Reference* 383manual for more information. 384 385Cookies 386~~~~~~~ 387 388When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains 389overwrite protection fields to help identify buffer overflows. 390 391Alignment and NUMA Constraints 392~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 393 394The rte_malloc() takes an align argument that can be used to request a memory 395area that is aligned on a multiple of this value (which must be a power of two). 396 397On systems with NUMA support, a call to the rte_malloc() function will return 398memory that has been allocated on the NUMA socket of the core which made the call. 399A set of APIs is also provided, to allow memory to be explicitly allocated on a 400NUMA socket directly, or by allocated on the NUMA socket where another core is 401located, in the case where the memory is to be used by a logical core other than 402on the one doing the memory allocation. 403 404Use Cases 405~~~~~~~~~ 406 407This API is meant to be used by an application that requires malloc-like 408functions at initialization time. 409 410For allocating/freeing data at runtime, in the fast-path of an application, 411the memory pool library should be used instead. 412 413Internal Implementation 414~~~~~~~~~~~~~~~~~~~~~~~ 415 416Data Structures 417^^^^^^^^^^^^^^^ 418 419There are two data structure types used internally in the malloc library: 420 421* struct malloc_heap - used to track free space on a per-socket basis 422 423* struct malloc_elem - the basic element of allocation and free-space 424 tracking inside the library. 425 426Structure: malloc_heap 427"""""""""""""""""""""" 428 429The malloc_heap structure is used to manage free space on a per-socket basis. 430Internally, there is one heap structure per NUMA node, which allows us to 431allocate memory to a thread based on the NUMA node on which this thread runs. 432While this does not guarantee that the memory will be used on that NUMA node, 433it is no worse than a scheme where the memory is always allocated on a fixed 434or random node. 435 436The key fields of the heap structure and their function are described below 437(see also diagram above): 438 439* lock - the lock field is needed to synchronize access to the heap. 440 Given that the free space in the heap is tracked using a linked list, 441 we need a lock to prevent two threads manipulating the list at the same time. 442 443* free_head - this points to the first element in the list of free nodes for 444 this malloc heap. 445 446.. note:: 447 448 The malloc_heap structure does not keep track of in-use blocks of memory, 449 since these are never touched except when they are to be freed again - 450 at which point the pointer to the block is an input to the free() function. 451 452.. _figure_malloc_heap: 453 454.. figure:: img/malloc_heap.* 455 456 Example of a malloc heap and malloc elements within the malloc library 457 458 459.. _malloc_elem: 460 461Structure: malloc_elem 462"""""""""""""""""""""" 463 464The malloc_elem structure is used as a generic header structure for various 465blocks of memory. 466It is used in three different ways - all shown in the diagram above: 467 468#. As a header on a block of free or allocated memory - normal case 469 470#. As a padding header inside a block of memory 471 472#. As an end-of-memseg marker 473 474The most important fields in the structure and how they are used are described below. 475 476.. note:: 477 478 If the usage of a particular field in one of the above three usages is not 479 described, the field can be assumed to have an undefined value in that 480 situation, for example, for padding headers only the "state" and "pad" 481 fields have valid values. 482 483* heap - this pointer is a reference back to the heap structure from which 484 this block was allocated. 485 It is used for normal memory blocks when they are being freed, to add the 486 newly-freed block to the heap's free-list. 487 488* prev - this pointer points to the header element/block in the memseg 489 immediately behind the current one. When freeing a block, this pointer is 490 used to reference the previous block to check if that block is also free. 491 If so, then the two free blocks are merged to form a single larger block. 492 493* next_free - this pointer is used to chain the free-list of unallocated 494 memory blocks together. 495 It is only used in normal memory blocks; on ``malloc()`` to find a suitable 496 free block to allocate and on ``free()`` to add the newly freed element to 497 the free-list. 498 499* state - This field can have one of three values: ``FREE``, ``BUSY`` or 500 ``PAD``. 501 The former two are to indicate the allocation state of a normal memory block 502 and the latter is to indicate that the element structure is a dummy structure 503 at the end of the start-of-block padding, i.e. where the start of the data 504 within a block is not at the start of the block itself, due to alignment 505 constraints. 506 In that case, the pad header is used to locate the actual malloc element 507 header for the block. 508 For the end-of-memseg structure, this is always a ``BUSY`` value, which 509 ensures that no element, on being freed, searches beyond the end of the 510 memseg for other blocks to merge with into a larger free area. 511 512* pad - this holds the length of the padding present at the start of the block. 513 In the case of a normal block header, it is added to the address of the end 514 of the header to give the address of the start of the data area, i.e. the 515 value passed back to the application on a malloc. 516 Within a dummy header inside the padding, this same value is stored, and is 517 subtracted from the address of the dummy header to yield the address of the 518 actual block header. 519 520* size - the size of the data block, including the header itself. 521 For end-of-memseg structures, this size is given as zero, though it is never 522 actually checked. 523 For normal blocks which are being freed, this size value is used in place of 524 a "next" pointer to identify the location of the next block of memory that 525 in the case of being ``FREE``, the two free blocks can be merged into one. 526 527Memory Allocation 528^^^^^^^^^^^^^^^^^ 529 530On EAL initialisation, all memsegs are setup as part of the malloc heap. 531This setup involves placing a dummy structure at the end with ``BUSY`` state, 532which may contain a sentinel value if ``CONFIG_RTE_MALLOC_DEBUG`` is enabled, 533and a proper :ref:`element header<malloc_elem>` with ``FREE`` at the start 534for each memseg. 535The ``FREE`` element is then added to the ``free_list`` for the malloc heap. 536 537When an application makes a call to a malloc-like function, the malloc function 538will first index the ``lcore_config`` structure for the calling thread, and 539determine the NUMA node of that thread. 540The NUMA node is used to index the array of ``malloc_heap`` structures which is 541passed as a parameter to the ``heap_alloc()`` function, along with the 542requested size, type, alignment and boundary parameters. 543 544The ``heap_alloc()`` function will scan the free_list of the heap, and attempt 545to find a free block suitable for storing data of the requested size, with the 546requested alignment and boundary constraints. 547 548When a suitable free element has been identified, the pointer to be returned 549to the user is calculated. 550The cache-line of memory immediately preceding this pointer is filled with a 551struct malloc_elem header. 552Because of alignment and boundary constraints, there could be free space at 553the start and/or end of the element, resulting in the following behavior: 554 555#. Check for trailing space. 556 If the trailing space is big enough, i.e. > 128 bytes, then the free element 557 is split. 558 If it is not, then we just ignore it (wasted space). 559 560#. Check for space at the start of the element. 561 If the space at the start is small, i.e. <=128 bytes, then a pad header is 562 used, and the remaining space is wasted. 563 If, however, the remaining space is greater, then the free element is split. 564 565The advantage of allocating the memory from the end of the existing element is 566that no adjustment of the free list needs to take place - the existing element 567on the free list just has its size pointer adjusted, and the following element 568has its "prev" pointer redirected to the newly created element. 569 570Freeing Memory 571^^^^^^^^^^^^^^ 572 573To free an area of memory, the pointer to the start of the data area is passed 574to the free function. 575The size of the ``malloc_elem`` structure is subtracted from this pointer to get 576the element header for the block. 577If this header is of type ``PAD`` then the pad length is further subtracted from 578the pointer to get the proper element header for the entire block. 579 580From this element header, we get pointers to the heap from which the block was 581allocated and to where it must be freed, as well as the pointer to the previous 582element, and via the size field, we can calculate the pointer to the next element. 583These next and previous elements are then checked to see if they are also 584``FREE``, and if so, they are merged with the current element. 585This means that we can never have two ``FREE`` memory blocks adjacent to one 586another, as they are always merged into a single block. 587