On Tue, 15 Oct 2024 11:33:38 +0200
Mattias Rönnblom <mattias.ronnb...@ericsson.com> wrote:
> + * Lcore variables
> + *
> + * This API provides a mechanism to create and access per-lcore id
> + * variables in a space- and cycle-efficient manner.
> + *
> + * A per-lcore id variable (or lcore variable for short) holds a
> + * unique value for each EAL thread and registered non-EAL
> + * thread. There is one instance for each current and future lcore
> + * id-equipped thread, with a total of @c RTE_MAX_LCORE instances. The
> + * value of the lcore variable for one lcore id is independent from
> + * the values assigned to other lcore ids within the same variable.
> + *
> + * In order to access the values of an lcore variable, a handle is
> + * used. The type of the handle is a pointer to the value's type
> + * (e.g., for an @c uint32_t lcore variable, the handle is a
> + * <code>uint32_t *</code>). The handle type is used to inform the
> + * access macros of the type of the values. A handle may be passed
> + * between modules and threads just like any pointer, but its value
> + * must be treated as an opaque identifier. An allocated handle never
> + * has the value NULL.
> + *
> + * @b Creation
> + *
> + * An lcore variable is created in two steps:
> + * 1. Define an lcore variable handle by using @ref RTE_LCORE_VAR_HANDLE.
> + * 2. Allocate lcore variable storage and initialize the handle with
> + * a unique identifier by @ref RTE_LCORE_VAR_ALLOC or
> + * @ref RTE_LCORE_VAR_INIT. Allocation generally occurs at the time
> + * of module initialization, but may be done at any time.
> + *
> + * The lifetime of an lcore variable is not tied to the thread that
> + * created it. Its per lcore id values (up to @c RTE_MAX_LCORE) are
> + * available from the moment the lcore variable is created and
> + * continue to exist throughout the entire lifetime of the EAL,
> + * whether or not the lcore id is currently in use.
> + *
> + * Lcore variables cannot and need not be freed.
> + *
> + * @b Access
> + *
> + * The value of any lcore variable for any lcore id may be accessed
> + * from any thread (including unregistered threads), but it should
> + * only be *frequently* read from or written to by the owner.
> + *
> + * Values of the same lcore variable, associated with different lcore
> + * ids may be frequently read or written by their respective owners
> + * without risking false sharing.
> + *
> + * An appropriate synchronization mechanism (e.g., atomic loads and
> + * stores) should be employed to prevent data races between the owning
> + * thread and any other thread accessing the same value instance.
> + *
> + * The value of the lcore variable for a particular lcore id is
> + * accessed using @ref RTE_LCORE_VAR_LCORE.
> + *
> + * A common pattern is for an EAL thread or a registered non-EAL
> + * thread to access its own lcore variable value. For this purpose, a
> + * shorthand exists as @ref RTE_LCORE_VAR.
> + *
> + * Although the handle (as defined by @ref RTE_LCORE_VAR_HANDLE) is a
> + * pointer with the same type as the value, it may not be directly
> + * dereferenced and must be treated as an opaque identifier.
> + *
> + * Lcore variable handles and value pointers may be freely passed
> + * between different threads.
> + *
> + * @b Storage
> + *
> + * An lcore variable's values may be of a primitive type like @c int,
> + * but would more typically be a @c struct.
> + *
> + * The lcore variable handle introduces a per-variable (not
> + * per-value/per-lcore id) overhead of @c sizeof(void *) bytes, so
> + * there are some memory footprint gains to be made by organizing all
> + * per-lcore id data for a particular module as one lcore variable
> + * (e.g., as a struct).
> + *
> + * An application may define an lcore variable handle without ever
> + * allocating it.
> + *
> + * The size of an lcore variable's value must be less than the DPDK
> + * build-time constant @c RTE_MAX_LCORE_VAR.
> + *
> + * Lcore variables are stored in a series of lcore buffers, which are
> + * allocated from the libc heap. Heap allocation failures are treated
> + * as fatal.
> + *
> + * Lcore variables should generally *not* be @ref __rte_cache_aligned
> + * and need *not* include a @ref RTE_CACHE_GUARD field, since the use
> + * of these constructs are designed to avoid false sharing. In the
> + * case of an lcore variable instance, the thread most recently
> + * accessing nearby data structures should almost-always be the lcore
> + * variable's owner. Adding padding will increase the effective memory
> + * working set size, potentially reducing performance.
> + *
> + * Lcore variable values are initialized to zero by default.
> + *
> + * Lcore variables are not stored in huge page memory.
> + *
> + * @b Example
> + *
> + * Below is an example of the use of an lcore variable:
> + *
> + * @code{.c}
> + * struct foo_lcore_state {
> + * int a;
> + * long b;
> + * };
> + *
> + * static RTE_LCORE_VAR_HANDLE(struct foo_lcore_state, lcore_states);
> + *
> + * long foo_get_a_plus_b(void)
> + * {
> + * struct foo_lcore_state *state = RTE_LCORE_VAR(lcore_states);
> + *
> + * return state->a + state->b;
> + * }
> + *
> + * RTE_INIT(rte_foo_init)
> + * {
> + * RTE_LCORE_VAR_ALLOC(lcore_states);
> + *
> + * unsigned int lcore_id;
> + * struct foo_lcore_state *state;
> + * RTE_LCORE_VAR_FOREACH(lcore_id, state, lcore_states) {
> + * (initialize 'state')
> + * }
> + *
> + * (other initialization)
> + * }
> + * @endcode
> + *
> + *
> + * @b Alternatives
> + *
> + * Lcore variables are designed to replace a pattern exemplified below:
> + * @code{.c}
> + * struct __rte_cache_aligned foo_lcore_state {
> + * int a;
> + * long b;
> + * RTE_CACHE_GUARD;
> + * };
> + *
> + * static struct foo_lcore_state lcore_states[RTE_MAX_LCORE];
> + * @endcode
> + *
> + * This scheme is simple and effective, but has one drawback: the data
> + * is organized so that objects related to all lcores for a particular
> + * module are kept close in memory. At a bare minimum, this requires
> + * sizing data structures (e.g., using `__rte_cache_aligned`) to an
> + * even number of cache lines to avoid false sharing. With CPU
> + * hardware prefetching and memory loads resulting from speculative
> + * execution (functions which seemingly are getting more eager faster
> + * than they are getting more intelligent), one or more "guard" cache
> + * lines may be required to separate one lcore's data from another's
> + * and prevent false sharing.
> + *
> + * Lcore variables offer the advantage of working with, rather than
> + * against, the CPU's assumptions. A next-line hardware prefetcher,
> + * for example, may function as intended (i.e., to the benefit, not
> + * detriment, of system performance).
> + *
> + * Another alternative to @ref rte_lcore_var.h is the @ref
> + * rte_per_lcore.h API, which makes use of thread-local storage (TLS,
> + * e.g., GCC __thread or C11 _Thread_local). The main differences
> + * between by using the various forms of TLS (e.g., @ref
> + * RTE_DEFINE_PER_LCORE or _Thread_local) and the use of lcore
> + * variables are:
> + *
> + * * The lifecycle of a thread-local variable instance is tied to
> + * that of the thread. The data cannot be accessed before the
> + * thread has been created, nor after it has exited. As a result,
> + * thread-local variables must be initialized in a "lazy" manner
> + * (e.g., at the point of thread creation). Lcore variables may be
> + * accessed immediately after having been allocated (which may occur
> + * before any thread beyond the main thread is running).
> + * * A thread-local variable is duplicated across all threads in the
> + * process, including unregistered non-EAL threads (i.e.,
> + * "regular" threads). For DPDK applications heavily relying on
> + * multi-threading (in conjunction to DPDK's "one thread per core"
> + * pattern), either by having many concurrent threads or
> + * creating/destroying threads at a high rate, an excessive use of
> + * thread-local variables may cause inefficiencies (e.g.,
> + * increased thread creation overhead due to thread-local storage
> + * initialization or increased total RAM footprint usage). Lcore
> + * variables *only* exist for threads with an lcore id.
> + * * If data in thread-local storage may be shared between threads
> + * (i.e., can a pointer to a thread-local variable be passed to
> + * and successfully dereferenced by non-owning thread) depends on
> + * the specifics of the TLS implementation. With GCC __thread and
> + * GCC _Thread_local, data sharing between threads is supported.
> + * In the C11 standard, accessing another thread's _Thread_local
> + * object is implementation-defined. Lcore variable instances may
> + * be accessed reliably by any thread.
> + */
For me this comment too wordy for code and belongs in the documentation instead.
Could also be reduced to more precise succinct language.
