bdlma_heapbypassallocator.h

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/// @file bdlma_heapbypassallocator.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlma_heapbypassallocator.h                                        -*-C++-*-
#ifndef INCLUDED_BDLMA_HEAPBYPASSALLOCATOR
#define INCLUDED_BDLMA_HEAPBYPASSALLOCATOR
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlma_heapbypassallocator bdlma_heapbypassallocator
/// @brief  Support memory allocation directly from virtual memory.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlma
/// @{
/// @addtogroup bdlma_heapbypassallocator
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlma_heapbypassallocator-purpose"> Purpose</a>
/// * <a href="#bdlma_heapbypassallocator-classes"> Classes </a>
/// * <a href="#bdlma_heapbypassallocator-description"> Description </a>
///   * <a href="#bdlma_heapbypassallocator-usage"> Usage </a>
///     * <a href="#bdlma_heapbypassallocator-example-1-basic-usage"> Example 1: Basic Usage </a>
///
/// # Purpose {#bdlma_heapbypassallocator-purpose}
/// Support memory allocation directly from virtual memory.
///
/// # Classes {#bdlma_heapbypassallocator-classes}
///
/// -   bdlma::HeapBypassAllocator: memory allocator directly from virtual memory
///
/// @see  bsls_alignmentutil
///
/// # Description {#bdlma_heapbypassallocator-description}
/// The `bdlma::HeapBypassAllocator` class provided by this
/// component implements a concrete allocator derived from the
/// `bslma::Allocator` interface that allocates memory directly from virtual
/// memory, bypassing the heap.  Reserves virtual memory in chunks so as to grow
/// infrequently.  The chunk size can optionally be chosen by providing an
/// argument for `replenishHint` in the constructor (see documentation for the
/// constructor for advice on how to choose a good value for `replenishHint`).
/// All memory allocated by this allocator is freed when the allocator's
/// destructor is called, but not before.  A natural use case is for backing
/// large, long-lived object pools, or cases where the heap may be corrupt.
/// @code
///                  ,---------------------------.
///                 (  bdlma::HeapBypassAllocator )
///                  `---------------------------'
///                                |         ctor
///                                V
///                       ,-----------------.
///                      (  bslma::Allocator )
///                       `-----------------'
///                                          dtor
///                                          allocate
///                                          deallocate      // no-op
/// @endcode
///
/// ## Usage {#bdlma_heapbypassallocator-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Usage {#bdlma_heapbypassallocator-example-1-basic-usage}
///
///
/// Here we allocate some memory using a heap bypass allocator, then write to
/// that memory, then read from it and verify the values written are preserved.
/// @code
/// {
///     enum {
///         k_LENGTH       = 10 * 1000,
///         k_NUM_SEGMENTS = 60
///     };
///
///     bdlma::HeapBypassAllocator hbpa;
/// @endcode
/// First, we allocate some segments:
/// @code
///     char *segments[k_NUM_SEGMENTS];
///     for (int i = 0; i < k_NUM_SEGMENTS; ++i) {
///         segments[i] = static_cast<char *>(hbpa.allocate(k_LENGTH));
///         BSLS_ASSERT(segments[i]);
///     }
/// @endcode
/// Next, we write to the segments:
/// @code
///     char c = 'a';
///     for (int i = 0; i < k_NUM_SEGMENTS; ++i) {
///         char *segment = segments[i];
///         for (int j = 0; j < k_LENGTH; ++j) {
///             c = (c + 1) & 0x7f;
///             segment[j] = c;
///         }
///     }
/// @endcode
/// Finally, we read from the segments and verify the written data is still
/// there:
/// @code
///     c = 'a';
///     for (int i = 0; i < k_NUM_SEGMENTS; ++i) {
///         char *segment = segments[i];
///         for (int j = 0; j < k_LENGTH; ++j) {
///             c = (c + 1) & 0x7f;
///             BSLS_ASSERT(segment[j] == c);  (void)segment;
///         }
///     }
/// @endcode
/// Memory is released upon destruction of object `hbpa` when it goes out of
/// scope.
/// @code
/// }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlma
 * @{
 */
/** @addtogroup bdlma_heapbypassallocator
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bslma_allocator.h>
#include <bslmt_mutex.h>
#include <bsls_alignmentutil.h>
#include <bsls_atomic.h>
#include <bsls_keyword.h>
#include <bsls_performancehint.h>
#include <bsls_types.h>
 
 
namespace bdlma {
 
                        // =========================
                        // class HeapBypassAllocator
                        // =========================
 
/// This class allows the caller to allocate memory directly from virtual
/// memory, without going through the heap like `malloc` or `new` would.
/// Note that the only way to free any memory allocated with this object is
/// to destroy the object, at which point all memory it has allocated is
/// freed.  Thread-safe.
///
/// See @ref bdlma_heapbypassallocator 
class HeapBypassAllocator : public bslma::Allocator {
 
    // PRIVATE TYPES
 
    /// Represents a chunk of memory from which allocations are subsequently
    /// carved out.  Chunks are chained in order to allow iterating over
    /// them so they can be freed when the allocator is destroyed.
    struct Chunk {
 
        // PUBLIC DATA
        bsls::AtomicUint64  d_offset;  // byte offset from 'this' to free bytes
        size_type           d_size;    // total size, in bytes, of this chunk
        Chunk              *d_nextChunk_p;
    };
 
    // DATA
    bsls::AtomicPointer<Chunk> d_current;         // current chunk (list head)
    size_type                  d_chunkSize;       // minimum size of new chunks
    bslmt::Mutex               d_replenishMutex;  // acquired while allocating
    Chunk                      d_initialChunk;    // dummy chunk (at list tail)
 
  private:
    // NOT IMPLEMENTED
    HeapBypassAllocator(const HeapBypassAllocator&) BSLS_KEYWORD_DELETED;
    HeapBypassAllocator& operator=(
                              const HeapBypassAllocator&) BSLS_KEYWORD_DELETED;
 
  private:
    // PRIVATE CLASS METHODS
 
    /// Acquire a chunk of virtual memory of the specified `size` bytes from
    /// the operating system.  `size` must be a multiple of `d_chunkSize`
    /// and must be greater than 0.  Return the address of the allocated
    /// chunk, or null if allocation fails.  Each successful call to
    /// `systemAllocate` must eventually be paired with a matching call to
    /// `systemFree`.
    static void *systemAllocate(size_type size);
 
    /// Free the specified `chunk` of memory of the specified `size` bytes
    /// back to the operating system.  `chunk` must have been returned by a
    /// previous call to `systemAllocate` of the same size, and not
    /// previously freed.
    static void systemFree(void *chunk, size_type size);
 
    // PRIVATE MANIPULATORS
 
    /// Initialize the linked list of chunks and round up `d_chunkSize` to
    /// a multiple of the system's page size.
    void init();
 
    /// Allocate a new chunk of at least the specified `size` bytes and make
    /// it available for subsequent calls of `allocate` to carve out memory
    /// from.  Return a pointer to the chunk on success, null otherwise.
    Chunk *replenish(size_type size);
 
  public:
    // CREATORS
 
    HeapBypassAllocator();
    /// Create an empty allocator object.  Optionally specify
    /// `replenishHint` as the minimum number of bytes to allocate at a time
    /// when the allocator's memory needs to grow.  In a multi-threaded
    /// scenario, the programmer should ensure that `replenishHint` is large
    /// enough so that multiple threads will rarely need to replenish at the
    /// same time in order to minimize contention.
    explicit HeapBypassAllocator(size_type replenishHint);
 
    /// Destroy this object, releasing all managed buffers of memory that it
    /// has allocated.
    ~HeapBypassAllocator() BSLS_KEYWORD_OVERRIDE;
 
    // MANIPULATORS
 
    /// Allocate a buffer of memory having the specified `size` (in bytes),
    /// and alignment defined by `bsls::AlignmentUtil::BSLS_MAX_ALIGNMENT`,
    /// from virtual memory.  If `size` is 0, return a null pointer with no
    /// other effect.  If this allocator cannot return the requested number
    /// of bytes, then it will throw a `std::bad_alloc` exception in an
    /// exception-enabled build, or else will abort the program in a non-
    /// exception build.  The behavior is undefined unless `0 <= size`.
    void *allocate(bsls::Types::size_type size) BSLS_KEYWORD_OVERRIDE;
 
    /// This method has no effect for this heap bypass allocator.
    void deallocate(void *) BSLS_KEYWORD_OVERRIDE;
};
 
//=============================================================================
//                            INLINE DEFINITIONS
//=============================================================================
 
                         // -------------------------
                         // class HeapBypassAllocator
                         // -------------------------
// MANIPULATORS
inline
void *HeapBypassAllocator::allocate(size_type size)
{
    if (BSLS_PERFORMANCEHINT_PREDICT_UNLIKELY(size == 0)) {
        return 0;                                                     // RETURN
    }
    size = bsls::AlignmentUtil::roundUpToMaximalAlignment(size);
    Chunk *chunk = d_current.loadAcquire();
    do {
        bsls::Types::Uint64 offset = chunk->d_offset.addRelaxed(size);
        if (BSLS_PERFORMANCEHINT_PREDICT_LIKELY(offset <= chunk->d_size)) {
            return reinterpret_cast<char *>(chunk) + (offset - size); // RETURN
        }
        chunk = replenish(size);
    } while (chunk);
    return 0;
}
 
inline void HeapBypassAllocator::deallocate(void *)
{
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2016 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */