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 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 rte_malloc 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 Event 170~~~~~~~~~~~~~~~~~~~~~~~~~~ 171 172+ User Space Interrupt and Alarm Handling in Host Thread 173 174The EAL creates a host thread to poll the UIO device file descriptors to detect the interrupts. 175Callbacks can be registered or unregistered by the EAL functions for a specific interrupt event 176and are called in the host thread asynchronously. 177The EAL also allows timed callbacks to be used in the same way as for NIC interrupts. 178 179.. note:: 180 181 In DPDK PMD, the only interrupts handled by the dedicated host thread are those for link status change, 182 i.e. link up and link down notification. 183 184 185+ RX Interrupt Event 186 187The receive and transmit routines provided by each PMD don't limit themselves to execute in polling thread mode. 188To ease the idle polling with tiny throughput, it's useful to pause the polling and wait until the wake-up event happens. 189The RX interrupt is the first choice to be such kind of wake-up event, but probably won't be the only one. 190 191EAL provides the event APIs for this event-driven thread mode. 192Taking linuxapp as an example, the implementation relies on epoll. Each thread can monitor an epoll instance 193in which all the wake-up events' file descriptors are added. The event file descriptors are created and mapped to 194the interrupt vectors according to the UIO/VFIO spec. 195From bsdapp's perspective, kqueue is the alternative way, but not implemented yet. 196 197EAL initializes the mapping between event file descriptors and interrupt vectors, while each device initializes the mapping 198between interrupt vectors and queues. In this way, EAL actually is unaware of the interrupt cause on the specific vector. 199The eth_dev driver takes responsibility to program the latter mapping. 200 201.. note:: 202 203 Per queue RX interrupt event is only allowed in VFIO which supports multiple MSI-X vector. In UIO, the RX interrupt 204 together with other interrupt causes shares the same vector. In this case, when RX interrupt and LSC(link status change) 205 interrupt are both enabled(intr_conf.lsc == 1 && intr_conf.rxq == 1), only the former is capable. 206 207The RX interrupt are controlled/enabled/disabled by ethdev APIs - 'rte_eth_dev_rx_intr_*'. They return failure if the PMD 208hasn't support them yet. The intr_conf.rxq flag is used to turn on the capability of RX interrupt per device. 209 210Blacklisting 211~~~~~~~~~~~~ 212 213The EAL PCI device blacklist functionality can be used to mark certain NIC ports as blacklisted, 214so they are ignored by the DPDK. 215The ports to be blacklisted are identified using the PCIe* description (Domain:Bus:Device.Function). 216 217Misc Functions 218~~~~~~~~~~~~~~ 219 220Locks and atomic operations are per-architecture (i686 and x86_64). 221 222Memory Segments and Memory Zones (memzone) 223------------------------------------------ 224 225The mapping of physical memory is provided by this feature in the EAL. 226As physical memory can have gaps, the memory is described in a table of descriptors, 227and each descriptor (called rte_memseg ) describes a contiguous portion of memory. 228 229On top of this, the memzone allocator's role is to reserve contiguous portions of physical memory. 230These zones are identified by a unique name when the memory is reserved. 231 232The rte_memzone descriptors are also located in the configuration structure. 233This structure is accessed using rte_eal_get_configuration(). 234The lookup (by name) of a memory zone returns a descriptor containing the physical address of the memory zone. 235 236Memory zones can be reserved with specific start address alignment by supplying the align parameter 237(by default, they are aligned to cache line size). 238The alignment value should be a power of two and not less than the cache line size (64 bytes). 239Memory zones can also be reserved from either 2 MB or 1 GB hugepages, provided that both are available on the system. 240 241 242Multiple pthread 243---------------- 244 245DPDK usually pins one pthread per core to avoid the overhead of task switching. 246This allows for significant performance gains, but lacks flexibility and is not always efficient. 247 248Power management helps to improve the CPU efficiency by limiting the CPU runtime frequency. 249However, alternately it is possible to utilize the idle cycles available to take advantage of 250the full capability of the CPU. 251 252By taking advantage of cgroup, the CPU utilization quota can be simply assigned. 253This gives another way to improve the CPU efficiency, however, there is a prerequisite; 254DPDK must handle the context switching between multiple pthreads per core. 255 256For further flexibility, it is useful to set pthread affinity not only to a CPU but to a CPU set. 257 258EAL pthread and lcore Affinity 259~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 260 261The term "lcore" refers to an EAL thread, which is really a Linux/FreeBSD pthread. 262"EAL pthreads" are created and managed by EAL and execute the tasks issued by *remote_launch*. 263In each EAL pthread, there is a TLS (Thread Local Storage) called *_lcore_id* for unique identification. 264As EAL pthreads usually bind 1:1 to the physical CPU, the *_lcore_id* is typically equal to the CPU ID. 265 266When using multiple pthreads, however, the binding is no longer always 1:1 between an EAL pthread and a specified physical CPU. 267The EAL pthread may have affinity to a CPU set, and as such the *_lcore_id* will not be the same as the CPU ID. 268For this reason, there is an EAL long option '--lcores' defined to assign the CPU affinity of lcores. 269For a specified lcore ID or ID group, the option allows setting the CPU set for that EAL pthread. 270 271The format pattern: 272 --lcores='<lcore_set>[@cpu_set][,<lcore_set>[@cpu_set],...]' 273 274'lcore_set' and 'cpu_set' can be a single number, range or a group. 275 276A number is a "digit([0-9]+)"; a range is "<number>-<number>"; a group is "(<number|range>[,<number|range>,...])". 277 278If a '\@cpu_set' value is not supplied, the value of 'cpu_set' will default to the value of 'lcore_set'. 279 280 :: 281 282 For example, "--lcores='1,2@(5-7),(3-5)@(0,2),(0,6),7-8'" which means start 9 EAL thread; 283 lcore 0 runs on cpuset 0x41 (cpu 0,6); 284 lcore 1 runs on cpuset 0x2 (cpu 1); 285 lcore 2 runs on cpuset 0xe0 (cpu 5,6,7); 286 lcore 3,4,5 runs on cpuset 0x5 (cpu 0,2); 287 lcore 6 runs on cpuset 0x41 (cpu 0,6); 288 lcore 7 runs on cpuset 0x80 (cpu 7); 289 lcore 8 runs on cpuset 0x100 (cpu 8). 290 291Using this option, for each given lcore ID, the associated CPUs can be assigned. 292It's also compatible with the pattern of corelist('-l') option. 293 294non-EAL pthread support 295~~~~~~~~~~~~~~~~~~~~~~~ 296 297It is possible to use the DPDK execution context with any user pthread (aka. Non-EAL pthreads). 298In 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*. 299Some 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). 300 301All these impacts are mentioned in :ref:`known_issue_label` section. 302 303Public Thread API 304~~~~~~~~~~~~~~~~~ 305 306There are two public APIs ``rte_thread_set_affinity()`` and ``rte_pthread_get_affinity()`` introduced for threads. 307When they're used in any pthread context, the Thread Local Storage(TLS) will be set/get. 308 309Those TLS include *_cpuset* and *_socket_id*: 310 311* *_cpuset* stores the CPUs bitmap to which the pthread is affinitized. 312 313* *_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. 314 315 316.. _known_issue_label: 317 318Known Issues 319~~~~~~~~~~~~ 320 321+ rte_mempool 322 323 The rte_mempool uses a per-lcore cache inside the mempool. 324 For non-EAL pthreads, ``rte_lcore_id()`` will not return a valid number. 325 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. 326 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. 327 328+ rte_ring 329 330 rte_ring supports multi-producer enqueue and multi-consumer dequeue. 331 However, it is non-preemptive, this has a knock on effect of making rte_mempool non-preemptable. 332 333 .. note:: 334 335 The "non-preemptive" constraint means: 336 337 - a pthread doing multi-producers enqueues on a given ring must not 338 be preempted by another pthread doing a multi-producer enqueue on 339 the same ring. 340 - a pthread doing multi-consumers dequeues on a given ring must not 341 be preempted by another pthread doing a multi-consumer dequeue on 342 the same ring. 343 344 Bypassing this constraint may cause the 2nd pthread to spin until the 1st one is scheduled again. 345 Moreover, if the 1st pthread is preempted by a context that has an higher priority, it may even cause a dead lock. 346 347 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. 348 349 1. It CAN be used for any single-producer or single-consumer situation. 350 351 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. 352 353 3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR. 354 355+ rte_timer 356 357 Running ``rte_timer_manager()`` on a non-EAL pthread is not allowed. However, resetting/stopping the timer from a non-EAL pthread is allowed. 358 359+ rte_log 360 361 In non-EAL pthreads, there is no per thread loglevel and logtype, global loglevels are used. 362 363+ misc 364 365 The debug statistics of rte_ring, rte_mempool and rte_timer are not supported in a non-EAL pthread. 366 367cgroup control 368~~~~~~~~~~~~~~ 369 370The 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). 371We expect only 50% of CPU spend on packet IO. 372 373 .. code-block:: console 374 375 mkdir /sys/fs/cgroup/cpu/pkt_io 376 mkdir /sys/fs/cgroup/cpuset/pkt_io 377 378 echo $cpu > /sys/fs/cgroup/cpuset/cpuset.cpus 379 380 echo $t0 > /sys/fs/cgroup/cpu/pkt_io/tasks 381 echo $t0 > /sys/fs/cgroup/cpuset/pkt_io/tasks 382 383 echo $t1 > /sys/fs/cgroup/cpu/pkt_io/tasks 384 echo $t1 > /sys/fs/cgroup/cpuset/pkt_io/tasks 385 386 cd /sys/fs/cgroup/cpu/pkt_io 387 echo 100000 > pkt_io/cpu.cfs_period_us 388 echo 50000 > pkt_io/cpu.cfs_quota_us 389 390 391Malloc 392------ 393 394The EAL provides a malloc API to allocate any-sized memory. 395 396The objective of this API is to provide malloc-like functions to allow 397allocation from hugepage memory and to facilitate application porting. 398The *DPDK API Reference* manual describes the available functions. 399 400Typically, these kinds of allocations should not be done in data plane 401processing because they are slower than pool-based allocation and make 402use of locks within the allocation and free paths. 403However, they can be used in configuration code. 404 405Refer to the rte_malloc() function description in the *DPDK API Reference* 406manual for more information. 407 408Cookies 409~~~~~~~ 410 411When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains 412overwrite protection fields to help identify buffer overflows. 413 414Alignment and NUMA Constraints 415~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 416 417The rte_malloc() takes an align argument that can be used to request a memory 418area that is aligned on a multiple of this value (which must be a power of two). 419 420On systems with NUMA support, a call to the rte_malloc() function will return 421memory that has been allocated on the NUMA socket of the core which made the call. 422A set of APIs is also provided, to allow memory to be explicitly allocated on a 423NUMA socket directly, or by allocated on the NUMA socket where another core is 424located, in the case where the memory is to be used by a logical core other than 425on the one doing the memory allocation. 426 427Use Cases 428~~~~~~~~~ 429 430This API is meant to be used by an application that requires malloc-like 431functions at initialization time. 432 433For allocating/freeing data at runtime, in the fast-path of an application, 434the memory pool library should be used instead. 435 436Internal Implementation 437~~~~~~~~~~~~~~~~~~~~~~~ 438 439Data Structures 440^^^^^^^^^^^^^^^ 441 442There are two data structure types used internally in the malloc library: 443 444* struct malloc_heap - used to track free space on a per-socket basis 445 446* struct malloc_elem - the basic element of allocation and free-space 447 tracking inside the library. 448 449Structure: malloc_heap 450"""""""""""""""""""""" 451 452The malloc_heap structure is used to manage free space on a per-socket basis. 453Internally, there is one heap structure per NUMA node, which allows us to 454allocate memory to a thread based on the NUMA node on which this thread runs. 455While this does not guarantee that the memory will be used on that NUMA node, 456it is no worse than a scheme where the memory is always allocated on a fixed 457or random node. 458 459The key fields of the heap structure and their function are described below 460(see also diagram above): 461 462* lock - the lock field is needed to synchronize access to the heap. 463 Given that the free space in the heap is tracked using a linked list, 464 we need a lock to prevent two threads manipulating the list at the same time. 465 466* free_head - this points to the first element in the list of free nodes for 467 this malloc heap. 468 469.. note:: 470 471 The malloc_heap structure does not keep track of in-use blocks of memory, 472 since these are never touched except when they are to be freed again - 473 at which point the pointer to the block is an input to the free() function. 474 475.. _figure_malloc_heap: 476 477.. figure:: img/malloc_heap.* 478 479 Example of a malloc heap and malloc elements within the malloc library 480 481 482.. _malloc_elem: 483 484Structure: malloc_elem 485"""""""""""""""""""""" 486 487The malloc_elem structure is used as a generic header structure for various 488blocks of memory. 489It is used in three different ways - all shown in the diagram above: 490 491#. As a header on a block of free or allocated memory - normal case 492 493#. As a padding header inside a block of memory 494 495#. As an end-of-memseg marker 496 497The most important fields in the structure and how they are used are described below. 498 499.. note:: 500 501 If the usage of a particular field in one of the above three usages is not 502 described, the field can be assumed to have an undefined value in that 503 situation, for example, for padding headers only the "state" and "pad" 504 fields have valid values. 505 506* heap - this pointer is a reference back to the heap structure from which 507 this block was allocated. 508 It is used for normal memory blocks when they are being freed, to add the 509 newly-freed block to the heap's free-list. 510 511* prev - this pointer points to the header element/block in the memseg 512 immediately behind the current one. When freeing a block, this pointer is 513 used to reference the previous block to check if that block is also free. 514 If so, then the two free blocks are merged to form a single larger block. 515 516* next_free - this pointer is used to chain the free-list of unallocated 517 memory blocks together. 518 It is only used in normal memory blocks; on ``malloc()`` to find a suitable 519 free block to allocate and on ``free()`` to add the newly freed element to 520 the free-list. 521 522* state - This field can have one of three values: ``FREE``, ``BUSY`` or 523 ``PAD``. 524 The former two are to indicate the allocation state of a normal memory block 525 and the latter is to indicate that the element structure is a dummy structure 526 at the end of the start-of-block padding, i.e. where the start of the data 527 within a block is not at the start of the block itself, due to alignment 528 constraints. 529 In that case, the pad header is used to locate the actual malloc element 530 header for the block. 531 For the end-of-memseg structure, this is always a ``BUSY`` value, which 532 ensures that no element, on being freed, searches beyond the end of the 533 memseg for other blocks to merge with into a larger free area. 534 535* pad - this holds the length of the padding present at the start of the block. 536 In the case of a normal block header, it is added to the address of the end 537 of the header to give the address of the start of the data area, i.e. the 538 value passed back to the application on a malloc. 539 Within a dummy header inside the padding, this same value is stored, and is 540 subtracted from the address of the dummy header to yield the address of the 541 actual block header. 542 543* size - the size of the data block, including the header itself. 544 For end-of-memseg structures, this size is given as zero, though it is never 545 actually checked. 546 For normal blocks which are being freed, this size value is used in place of 547 a "next" pointer to identify the location of the next block of memory that 548 in the case of being ``FREE``, the two free blocks can be merged into one. 549 550Memory Allocation 551^^^^^^^^^^^^^^^^^ 552 553On EAL initialization, all memsegs are setup as part of the malloc heap. 554This setup involves placing a dummy structure at the end with ``BUSY`` state, 555which may contain a sentinel value if ``CONFIG_RTE_MALLOC_DEBUG`` is enabled, 556and a proper :ref:`element header<malloc_elem>` with ``FREE`` at the start 557for each memseg. 558The ``FREE`` element is then added to the ``free_list`` for the malloc heap. 559 560When an application makes a call to a malloc-like function, the malloc function 561will first index the ``lcore_config`` structure for the calling thread, and 562determine the NUMA node of that thread. 563The NUMA node is used to index the array of ``malloc_heap`` structures which is 564passed as a parameter to the ``heap_alloc()`` function, along with the 565requested size, type, alignment and boundary parameters. 566 567The ``heap_alloc()`` function will scan the free_list of the heap, and attempt 568to find a free block suitable for storing data of the requested size, with the 569requested alignment and boundary constraints. 570 571When a suitable free element has been identified, the pointer to be returned 572to the user is calculated. 573The cache-line of memory immediately preceding this pointer is filled with a 574struct malloc_elem header. 575Because of alignment and boundary constraints, there could be free space at 576the start and/or end of the element, resulting in the following behavior: 577 578#. Check for trailing space. 579 If the trailing space is big enough, i.e. > 128 bytes, then the free element 580 is split. 581 If it is not, then we just ignore it (wasted space). 582 583#. Check for space at the start of the element. 584 If the space at the start is small, i.e. <=128 bytes, then a pad header is 585 used, and the remaining space is wasted. 586 If, however, the remaining space is greater, then the free element is split. 587 588The advantage of allocating the memory from the end of the existing element is 589that no adjustment of the free list needs to take place - the existing element 590on the free list just has its size pointer adjusted, and the following element 591has its "prev" pointer redirected to the newly created element. 592 593Freeing Memory 594^^^^^^^^^^^^^^ 595 596To free an area of memory, the pointer to the start of the data area is passed 597to the free function. 598The size of the ``malloc_elem`` structure is subtracted from this pointer to get 599the element header for the block. 600If this header is of type ``PAD`` then the pad length is further subtracted from 601the pointer to get the proper element header for the entire block. 602 603From this element header, we get pointers to the heap from which the block was 604allocated and to where it must be freed, as well as the pointer to the previous 605element, and via the size field, we can calculate the pointer to the next element. 606These next and previous elements are then checked to see if they are also 607``FREE``, and if so, they are merged with the current element. 608This means that we can never have two ``FREE`` memory blocks adjacent to one 609another, as they are always merged into a single block. 610