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 102Shutdown and Cleanup 103~~~~~~~~~~~~~~~~~~~~ 104 105During the initialization of EAL resources such as hugepage backed memory can be 106allocated by core components. The memory allocated during ``rte_eal_init()`` 107can be released by calling the ``rte_eal_cleanup()`` function. Refer to the 108API documentation for details. 109 110Multi-process Support 111~~~~~~~~~~~~~~~~~~~~~ 112 113The Linuxapp EAL allows a multi-process as well as a multi-threaded (pthread) deployment model. 114See chapter 115:ref:`Multi-process Support <Multi-process_Support>` for more details. 116 117Memory Mapping Discovery and Memory Reservation 118~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 119 120The allocation of large contiguous physical memory is done using the hugetlbfs kernel filesystem. 121The EAL provides an API to reserve named memory zones in this contiguous memory. 122The physical address of the reserved memory for that memory zone is also returned to the user by the memory zone reservation API. 123 124.. note:: 125 126 Memory reservations done using the APIs provided by rte_malloc are also backed by pages from the hugetlbfs filesystem. 127 128PCI Access 129~~~~~~~~~~ 130 131The EAL uses the /sys/bus/pci utilities provided by the kernel to scan the content on the PCI bus. 132To access PCI memory, a kernel module called uio_pci_generic provides a /dev/uioX device file 133and resource files in /sys 134that can be mmap'd to obtain access to PCI address space from the application. 135The DPDK-specific igb_uio module can also be used for this. Both drivers use the uio kernel feature (userland driver). 136 137Per-lcore and Shared Variables 138~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 139 140.. note:: 141 142 lcore refers to a logical execution unit of the processor, sometimes called a hardware *thread*. 143 144Shared variables are the default behavior. 145Per-lcore variables are implemented using *Thread Local Storage* (TLS) to provide per-thread local storage. 146 147Logs 148~~~~ 149 150A logging API is provided by EAL. 151By default, in a Linux application, logs are sent to syslog and also to the console. 152However, the log function can be overridden by the user to use a different logging mechanism. 153 154Trace and Debug Functions 155^^^^^^^^^^^^^^^^^^^^^^^^^ 156 157There are some debug functions to dump the stack in glibc. 158The rte_panic() function can voluntarily provoke a SIG_ABORT, 159which can trigger the generation of a core file, readable by gdb. 160 161CPU Feature Identification 162~~~~~~~~~~~~~~~~~~~~~~~~~~ 163 164The EAL can query the CPU at runtime (using the rte_cpu_get_features() function) to determine which CPU features are available. 165 166User Space Interrupt Event 167~~~~~~~~~~~~~~~~~~~~~~~~~~ 168 169+ User Space Interrupt and Alarm Handling in Host Thread 170 171The EAL creates a host thread to poll the UIO device file descriptors to detect the interrupts. 172Callbacks can be registered or unregistered by the EAL functions for a specific interrupt event 173and are called in the host thread asynchronously. 174The EAL also allows timed callbacks to be used in the same way as for NIC interrupts. 175 176.. note:: 177 178 In DPDK PMD, the only interrupts handled by the dedicated host thread are those for link status change 179 (link up and link down notification) and for sudden device removal. 180 181 182+ RX Interrupt Event 183 184The receive and transmit routines provided by each PMD don't limit themselves to execute in polling thread mode. 185To ease the idle polling with tiny throughput, it's useful to pause the polling and wait until the wake-up event happens. 186The RX interrupt is the first choice to be such kind of wake-up event, but probably won't be the only one. 187 188EAL provides the event APIs for this event-driven thread mode. 189Taking linuxapp as an example, the implementation relies on epoll. Each thread can monitor an epoll instance 190in which all the wake-up events' file descriptors are added. The event file descriptors are created and mapped to 191the interrupt vectors according to the UIO/VFIO spec. 192From bsdapp's perspective, kqueue is the alternative way, but not implemented yet. 193 194EAL initializes the mapping between event file descriptors and interrupt vectors, while each device initializes the mapping 195between interrupt vectors and queues. In this way, EAL actually is unaware of the interrupt cause on the specific vector. 196The eth_dev driver takes responsibility to program the latter mapping. 197 198.. note:: 199 200 Per queue RX interrupt event is only allowed in VFIO which supports multiple MSI-X vector. In UIO, the RX interrupt 201 together with other interrupt causes shares the same vector. In this case, when RX interrupt and LSC(link status change) 202 interrupt are both enabled(intr_conf.lsc == 1 && intr_conf.rxq == 1), only the former is capable. 203 204The RX interrupt are controlled/enabled/disabled by ethdev APIs - 'rte_eth_dev_rx_intr_*'. They return failure if the PMD 205hasn't support them yet. The intr_conf.rxq flag is used to turn on the capability of RX interrupt per device. 206 207+ Device Removal Event 208 209This event is triggered by a device being removed at a bus level. Its 210underlying resources may have been made unavailable (i.e. PCI mappings 211unmapped). The PMD must make sure that on such occurrence, the application can 212still safely use its callbacks. 213 214This event can be subscribed to in the same way one would subscribe to a link 215status change event. The execution context is thus the same, i.e. it is the 216dedicated interrupt host thread. 217 218Considering this, it is likely that an application would want to close a 219device having emitted a Device Removal Event. In such case, calling 220``rte_eth_dev_close()`` can trigger it to unregister its own Device Removal Event 221callback. Care must be taken not to close the device from the interrupt handler 222context. It is necessary to reschedule such closing operation. 223 224Blacklisting 225~~~~~~~~~~~~ 226 227The EAL PCI device blacklist functionality can be used to mark certain NIC ports as blacklisted, 228so they are ignored by the DPDK. 229The ports to be blacklisted are identified using the PCIe* description (Domain:Bus:Device.Function). 230 231Misc Functions 232~~~~~~~~~~~~~~ 233 234Locks and atomic operations are per-architecture (i686 and x86_64). 235 236Memory Segments and Memory Zones (memzone) 237------------------------------------------ 238 239The mapping of physical memory is provided by this feature in the EAL. 240As physical memory can have gaps, the memory is described in a table of descriptors, 241and each descriptor (called rte_memseg ) describes a contiguous portion of memory. 242 243On top of this, the memzone allocator's role is to reserve contiguous portions of physical memory. 244These zones are identified by a unique name when the memory is reserved. 245 246The rte_memzone descriptors are also located in the configuration structure. 247This structure is accessed using rte_eal_get_configuration(). 248The lookup (by name) of a memory zone returns a descriptor containing the physical address of the memory zone. 249 250Memory zones can be reserved with specific start address alignment by supplying the align parameter 251(by default, they are aligned to cache line size). 252The alignment value should be a power of two and not less than the cache line size (64 bytes). 253Memory zones can also be reserved from either 2 MB or 1 GB hugepages, provided that both are available on the system. 254 255 256Multiple pthread 257---------------- 258 259DPDK usually pins one pthread per core to avoid the overhead of task switching. 260This allows for significant performance gains, but lacks flexibility and is not always efficient. 261 262Power management helps to improve the CPU efficiency by limiting the CPU runtime frequency. 263However, alternately it is possible to utilize the idle cycles available to take advantage of 264the full capability of the CPU. 265 266By taking advantage of cgroup, the CPU utilization quota can be simply assigned. 267This gives another way to improve the CPU efficiency, however, there is a prerequisite; 268DPDK must handle the context switching between multiple pthreads per core. 269 270For further flexibility, it is useful to set pthread affinity not only to a CPU but to a CPU set. 271 272EAL pthread and lcore Affinity 273~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 274 275The term "lcore" refers to an EAL thread, which is really a Linux/FreeBSD pthread. 276"EAL pthreads" are created and managed by EAL and execute the tasks issued by *remote_launch*. 277In each EAL pthread, there is a TLS (Thread Local Storage) called *_lcore_id* for unique identification. 278As EAL pthreads usually bind 1:1 to the physical CPU, the *_lcore_id* is typically equal to the CPU ID. 279 280When using multiple pthreads, however, the binding is no longer always 1:1 between an EAL pthread and a specified physical CPU. 281The EAL pthread may have affinity to a CPU set, and as such the *_lcore_id* will not be the same as the CPU ID. 282For this reason, there is an EAL long option '--lcores' defined to assign the CPU affinity of lcores. 283For a specified lcore ID or ID group, the option allows setting the CPU set for that EAL pthread. 284 285The format pattern: 286 --lcores='<lcore_set>[@cpu_set][,<lcore_set>[@cpu_set],...]' 287 288'lcore_set' and 'cpu_set' can be a single number, range or a group. 289 290A number is a "digit([0-9]+)"; a range is "<number>-<number>"; a group is "(<number|range>[,<number|range>,...])". 291 292If a '\@cpu_set' value is not supplied, the value of 'cpu_set' will default to the value of 'lcore_set'. 293 294 :: 295 296 For example, "--lcores='1,2@(5-7),(3-5)@(0,2),(0,6),7-8'" which means start 9 EAL thread; 297 lcore 0 runs on cpuset 0x41 (cpu 0,6); 298 lcore 1 runs on cpuset 0x2 (cpu 1); 299 lcore 2 runs on cpuset 0xe0 (cpu 5,6,7); 300 lcore 3,4,5 runs on cpuset 0x5 (cpu 0,2); 301 lcore 6 runs on cpuset 0x41 (cpu 0,6); 302 lcore 7 runs on cpuset 0x80 (cpu 7); 303 lcore 8 runs on cpuset 0x100 (cpu 8). 304 305Using this option, for each given lcore ID, the associated CPUs can be assigned. 306It's also compatible with the pattern of corelist('-l') option. 307 308non-EAL pthread support 309~~~~~~~~~~~~~~~~~~~~~~~ 310 311It is possible to use the DPDK execution context with any user pthread (aka. Non-EAL pthreads). 312In 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*. 313Some 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). 314 315All these impacts are mentioned in :ref:`known_issue_label` section. 316 317Public Thread API 318~~~~~~~~~~~~~~~~~ 319 320There are two public APIs ``rte_thread_set_affinity()`` and ``rte_thread_get_affinity()`` introduced for threads. 321When they're used in any pthread context, the Thread Local Storage(TLS) will be set/get. 322 323Those TLS include *_cpuset* and *_socket_id*: 324 325* *_cpuset* stores the CPUs bitmap to which the pthread is affinitized. 326 327* *_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. 328 329 330.. _known_issue_label: 331 332Known Issues 333~~~~~~~~~~~~ 334 335+ rte_mempool 336 337 The rte_mempool uses a per-lcore cache inside the mempool. 338 For non-EAL pthreads, ``rte_lcore_id()`` will not return a valid number. 339 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. 340 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. 341 342+ rte_ring 343 344 rte_ring supports multi-producer enqueue and multi-consumer dequeue. 345 However, it is non-preemptive, this has a knock on effect of making rte_mempool non-preemptable. 346 347 .. note:: 348 349 The "non-preemptive" constraint means: 350 351 - a pthread doing multi-producers enqueues on a given ring must not 352 be preempted by another pthread doing a multi-producer enqueue on 353 the same ring. 354 - a pthread doing multi-consumers dequeues on a given ring must not 355 be preempted by another pthread doing a multi-consumer dequeue on 356 the same ring. 357 358 Bypassing this constraint may cause the 2nd pthread to spin until the 1st one is scheduled again. 359 Moreover, if the 1st pthread is preempted by a context that has an higher priority, it may even cause a dead lock. 360 361 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. 362 363 1. It CAN be used for any single-producer or single-consumer situation. 364 365 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. 366 367 3. It MUST not be used by multi-producer/consumer pthreads, whose scheduling policies are SCHED_FIFO or SCHED_RR. 368 369+ rte_timer 370 371 Running ``rte_timer_manager()`` on a non-EAL pthread is not allowed. However, resetting/stopping the timer from a non-EAL pthread is allowed. 372 373+ rte_log 374 375 In non-EAL pthreads, there is no per thread loglevel and logtype, global loglevels are used. 376 377+ misc 378 379 The debug statistics of rte_ring, rte_mempool and rte_timer are not supported in a non-EAL pthread. 380 381cgroup control 382~~~~~~~~~~~~~~ 383 384The 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). 385We expect only 50% of CPU spend on packet IO. 386 387 .. code-block:: console 388 389 mkdir /sys/fs/cgroup/cpu/pkt_io 390 mkdir /sys/fs/cgroup/cpuset/pkt_io 391 392 echo $cpu > /sys/fs/cgroup/cpuset/cpuset.cpus 393 394 echo $t0 > /sys/fs/cgroup/cpu/pkt_io/tasks 395 echo $t0 > /sys/fs/cgroup/cpuset/pkt_io/tasks 396 397 echo $t1 > /sys/fs/cgroup/cpu/pkt_io/tasks 398 echo $t1 > /sys/fs/cgroup/cpuset/pkt_io/tasks 399 400 cd /sys/fs/cgroup/cpu/pkt_io 401 echo 100000 > pkt_io/cpu.cfs_period_us 402 echo 50000 > pkt_io/cpu.cfs_quota_us 403 404 405Malloc 406------ 407 408The EAL provides a malloc API to allocate any-sized memory. 409 410The objective of this API is to provide malloc-like functions to allow 411allocation from hugepage memory and to facilitate application porting. 412The *DPDK API Reference* manual describes the available functions. 413 414Typically, these kinds of allocations should not be done in data plane 415processing because they are slower than pool-based allocation and make 416use of locks within the allocation and free paths. 417However, they can be used in configuration code. 418 419Refer to the rte_malloc() function description in the *DPDK API Reference* 420manual for more information. 421 422Cookies 423~~~~~~~ 424 425When CONFIG_RTE_MALLOC_DEBUG is enabled, the allocated memory contains 426overwrite protection fields to help identify buffer overflows. 427 428Alignment and NUMA Constraints 429~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 430 431The rte_malloc() takes an align argument that can be used to request a memory 432area that is aligned on a multiple of this value (which must be a power of two). 433 434On systems with NUMA support, a call to the rte_malloc() function will return 435memory that has been allocated on the NUMA socket of the core which made the call. 436A set of APIs is also provided, to allow memory to be explicitly allocated on a 437NUMA socket directly, or by allocated on the NUMA socket where another core is 438located, in the case where the memory is to be used by a logical core other than 439on the one doing the memory allocation. 440 441Use Cases 442~~~~~~~~~ 443 444This API is meant to be used by an application that requires malloc-like 445functions at initialization time. 446 447For allocating/freeing data at runtime, in the fast-path of an application, 448the memory pool library should be used instead. 449 450Internal Implementation 451~~~~~~~~~~~~~~~~~~~~~~~ 452 453Data Structures 454^^^^^^^^^^^^^^^ 455 456There are two data structure types used internally in the malloc library: 457 458* struct malloc_heap - used to track free space on a per-socket basis 459 460* struct malloc_elem - the basic element of allocation and free-space 461 tracking inside the library. 462 463Structure: malloc_heap 464"""""""""""""""""""""" 465 466The malloc_heap structure is used to manage free space on a per-socket basis. 467Internally, there is one heap structure per NUMA node, which allows us to 468allocate memory to a thread based on the NUMA node on which this thread runs. 469While this does not guarantee that the memory will be used on that NUMA node, 470it is no worse than a scheme where the memory is always allocated on a fixed 471or random node. 472 473The key fields of the heap structure and their function are described below 474(see also diagram above): 475 476* lock - the lock field is needed to synchronize access to the heap. 477 Given that the free space in the heap is tracked using a linked list, 478 we need a lock to prevent two threads manipulating the list at the same time. 479 480* free_head - this points to the first element in the list of free nodes for 481 this malloc heap. 482 483.. note:: 484 485 The malloc_heap structure does not keep track of in-use blocks of memory, 486 since these are never touched except when they are to be freed again - 487 at which point the pointer to the block is an input to the free() function. 488 489.. _figure_malloc_heap: 490 491.. figure:: img/malloc_heap.* 492 493 Example of a malloc heap and malloc elements within the malloc library 494 495 496.. _malloc_elem: 497 498Structure: malloc_elem 499"""""""""""""""""""""" 500 501The malloc_elem structure is used as a generic header structure for various 502blocks of memory. 503It is used in three different ways - all shown in the diagram above: 504 505#. As a header on a block of free or allocated memory - normal case 506 507#. As a padding header inside a block of memory 508 509#. As an end-of-memseg marker 510 511The most important fields in the structure and how they are used are described below. 512 513.. note:: 514 515 If the usage of a particular field in one of the above three usages is not 516 described, the field can be assumed to have an undefined value in that 517 situation, for example, for padding headers only the "state" and "pad" 518 fields have valid values. 519 520* heap - this pointer is a reference back to the heap structure from which 521 this block was allocated. 522 It is used for normal memory blocks when they are being freed, to add the 523 newly-freed block to the heap's free-list. 524 525* prev - this pointer points to the header element/block in the memseg 526 immediately behind the current one. When freeing a block, this pointer is 527 used to reference the previous block to check if that block is also free. 528 If so, then the two free blocks are merged to form a single larger block. 529 530* next_free - this pointer is used to chain the free-list of unallocated 531 memory blocks together. 532 It is only used in normal memory blocks; on ``malloc()`` to find a suitable 533 free block to allocate and on ``free()`` to add the newly freed element to 534 the free-list. 535 536* state - This field can have one of three values: ``FREE``, ``BUSY`` or 537 ``PAD``. 538 The former two are to indicate the allocation state of a normal memory block 539 and the latter is to indicate that the element structure is a dummy structure 540 at the end of the start-of-block padding, i.e. where the start of the data 541 within a block is not at the start of the block itself, due to alignment 542 constraints. 543 In that case, the pad header is used to locate the actual malloc element 544 header for the block. 545 For the end-of-memseg structure, this is always a ``BUSY`` value, which 546 ensures that no element, on being freed, searches beyond the end of the 547 memseg for other blocks to merge with into a larger free area. 548 549* pad - this holds the length of the padding present at the start of the block. 550 In the case of a normal block header, it is added to the address of the end 551 of the header to give the address of the start of the data area, i.e. the 552 value passed back to the application on a malloc. 553 Within a dummy header inside the padding, this same value is stored, and is 554 subtracted from the address of the dummy header to yield the address of the 555 actual block header. 556 557* size - the size of the data block, including the header itself. 558 For end-of-memseg structures, this size is given as zero, though it is never 559 actually checked. 560 For normal blocks which are being freed, this size value is used in place of 561 a "next" pointer to identify the location of the next block of memory that 562 in the case of being ``FREE``, the two free blocks can be merged into one. 563 564Memory Allocation 565^^^^^^^^^^^^^^^^^ 566 567On EAL initialization, all memsegs are setup as part of the malloc heap. 568This setup involves placing a dummy structure at the end with ``BUSY`` state, 569which may contain a sentinel value if ``CONFIG_RTE_MALLOC_DEBUG`` is enabled, 570and a proper :ref:`element header<malloc_elem>` with ``FREE`` at the start 571for each memseg. 572The ``FREE`` element is then added to the ``free_list`` for the malloc heap. 573 574When an application makes a call to a malloc-like function, the malloc function 575will first index the ``lcore_config`` structure for the calling thread, and 576determine the NUMA node of that thread. 577The NUMA node is used to index the array of ``malloc_heap`` structures which is 578passed as a parameter to the ``heap_alloc()`` function, along with the 579requested size, type, alignment and boundary parameters. 580 581The ``heap_alloc()`` function will scan the free_list of the heap, and attempt 582to find a free block suitable for storing data of the requested size, with the 583requested alignment and boundary constraints. 584 585When a suitable free element has been identified, the pointer to be returned 586to the user is calculated. 587The cache-line of memory immediately preceding this pointer is filled with a 588struct malloc_elem header. 589Because of alignment and boundary constraints, there could be free space at 590the start and/or end of the element, resulting in the following behavior: 591 592#. Check for trailing space. 593 If the trailing space is big enough, i.e. > 128 bytes, then the free element 594 is split. 595 If it is not, then we just ignore it (wasted space). 596 597#. Check for space at the start of the element. 598 If the space at the start is small, i.e. <=128 bytes, then a pad header is 599 used, and the remaining space is wasted. 600 If, however, the remaining space is greater, then the free element is split. 601 602The advantage of allocating the memory from the end of the existing element is 603that no adjustment of the free list needs to take place - the existing element 604on the free list just has its size pointer adjusted, and the following element 605has its "prev" pointer redirected to the newly created element. 606 607Freeing Memory 608^^^^^^^^^^^^^^ 609 610To free an area of memory, the pointer to the start of the data area is passed 611to the free function. 612The size of the ``malloc_elem`` structure is subtracted from this pointer to get 613the element header for the block. 614If this header is of type ``PAD`` then the pad length is further subtracted from 615the pointer to get the proper element header for the entire block. 616 617From this element header, we get pointers to the heap from which the block was 618allocated and to where it must be freed, as well as the pointer to the previous 619element, and via the size field, we can calculate the pointer to the next element. 620These next and previous elements are then checked to see if they are also 621``FREE``, and if so, they are merged with the current element. 622This means that we can never have two ``FREE`` memory blocks adjacent to one 623another, as they are always merged into a single block. 624