src/ipc/mem/PageStack.h

   1 /*
   2  * $Id$
   3  *
   4  */
   5
   6 #ifndef SQUID_IPC_MEM_PAGE_STACK_H
   7 #define SQUID_IPC_MEM_PAGE_STACK_H
   8
   9 #include "ipc/AtomicWord.h"
  10
  11 namespace Ipc {
  12
  13 namespace Mem {
  14
  15 class PageId;
  16
  17 /// Atomic container of "free" page numbers inside a single SharedMemory space.
  18 /// Assumptions: all page numbers are unique, positive, have an known maximum,
  19 /// and can be temporary unavailable as long as they are never trully lost.
  20 class PageStack {
  21 public:
  22     typedef uint32_t Value; ///< stack item type (a free page number)
  23
  24     PageStack(const uint32_t aPoolId, const unsigned int aCapacity, const size_t aPageSize);
  25
  26     unsigned int capacity() const { return theCapacity; }
  27     size_t pageSize() const { return thePageSize; }
  28     /// lower bound for the number of free pages
  29     unsigned int size() const { return max(0, theSize.get()); }
  30
  31     /// sets value and returns true unless no free page numbers are found
  32     bool pop(PageId &page);
  33     /// makes value available as a free page number to future pop() callers
  34     void push(PageId &page);
  35
  36     bool pageIdIsValid(const PageId &page) const;
  37
  38     /// total shared memory size required to share
  39     static size_t SharedMemorySize(const uint32_t aPoolId, const unsigned int capacity, const size_t pageSize);
  40     size_t sharedMemorySize() const;
  41
  42 private:
  43     /// stack index and size type (may temporary go negative)
  44     typedef int Offset;
  45
  46     // these help iterate the stack in search of a free spot or a page
  47     Offset next(const Offset idx) const { return (idx + 1) % theCapacity; }
  48     Offset prev(const Offset idx) const { return (theCapacity + idx - 1) % theCapacity; }
  49
  50     const uint32_t thePoolId; ///< pool ID
  51     const Offset theCapacity; ///< stack capacity, i.e. theItems size
  52     const size_t thePageSize; ///< page size, used to calculate shared memory size
  53     /// lower bound for the number of free pages (may get negative!)
  54     AtomicWordT<Offset> theSize;
  55
  56     /// last readable item index; just a hint, not a guarantee
  57     AtomicWordT<Offset> theLastReadable;
  58     /// first writable item index; just a hint, not a guarantee
  59     AtomicWordT<Offset> theFirstWritable;
  60
  61     typedef AtomicWordT<Value> Item;
  62     Item theItems[]; ///< page number storage
  63 };
  64
  65 } // namespace Mem
  66
  67 } // namespace Ipc
  68
  69 #endif // SQUID_IPC_MEM_PAGE_STACK_H