112 lines
2.5 KiB
ReStructuredText
112 lines
2.5 KiB
ReStructuredText
.. _examples:
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.. toctree
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:maxdepth: 2
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Examples
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========
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Arena allocation
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-----------------
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.. code-block:: cpp
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#include <xo/alloc2/arena/DArena.hpp>
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using namespace xo::mm;
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using namespace std;
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Create an arena:
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.. code-block:: cpp
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// create arena, size 64k
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DArena arena = DArena::map(ArenaConfig { .size_ = 64*1024; });
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cout << arena.lo() << ".." << arena.hi();
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This determines a VM memory address range.
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Actually address range is rounded up to a whole number of VM pages.
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Size here is a hard maximum. It cannot be changed for this arena instance.
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.. code-block:: cpp
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arena.reserved(); // 64k
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arena.committed(); // 0k
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arena.allocated(); // ok
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arena.available(); // 0k
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Although we know the address range for arena, it doesn't own any physical
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memory yet. Two ways to commit memory:
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1. Attempt allocation:
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.. code-block:: cpp
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std::byte * mem = arena.alloc(5*1024);
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if (!mem)
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throw std::runtime_error("alloc failed");
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arena.reserved(); // 64k
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arena.committed(); // 8k - 2 pages
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arena.allocateed(); // 5k
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arena.available(); // 3k
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2. Expand committed memory explicitly:
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.. code-block:: cpp
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bool ok = arena.expand(5*1024);
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assert(ok);
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arena.reserved(); // 64k
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arena.committed(); // 8k - 2 pages
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arena.allocated(); // 0k
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arena.available(); // 8k
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Examining alloc metadata
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------------------------
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Given a successful allocation:
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.. code-block:: cpp
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std::size_t req_z = 5*1024;
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std::byte * mem = arena.alloc(req_z);
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if (!mem)
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throw std::runtime_error("alloc failed");
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AllocInfo info = arena.alloc_info(mem);
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info.payload(); // [mem, mem + req_z (+ up to 7 bytes padding)]
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info.is_valid(); // true
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info.guard_lo(); // guard bytes preceding alloc
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info.guard_hi(); // guard bytes following alloc
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Recycling memory
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----------------
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.. code-block:: cpp
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// arena in non-empty state
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arena.reserved(); // 64k
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arena.committed(); // 8k - 2 pages
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arena.allocateed(); // 5k
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arena.available(); // 3k
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arena.clear();
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arena.reserved() // 64k
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arena.committed(); // 8k - 2 pages
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arena.allocated(); // 0k
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arena.available(); // 8k
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Memory released by @ref Darena::clear is still committed.
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It's in use as far as operating system is concerned.
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To release memory to the operating system, destroy arena:
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.. code-block:: cpp
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arena.~DArena(); // or just let arena go out of scope
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