gecko-dev/xpcom/ds/nsRecyclingAllocator.h

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 * Contributor(s):
 *	Suresh Duddi <dp@netscape.com>
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/*
 * nsRecyclingAllocator
 *
 * This allocator is useful when we cycle through a small set of allocations
 * repeatedly with minimal overlap. For eg. something we do for every gif
 * file read (or) buffers required for decompression of every file from jar.
 *
 * What this does is keeps around the first set of memory allocated and
 * reuses it subsequently. If all buckets are full, this falls back to
 * malloc/free
 *
 * Uses a timer to release all memory allocated if not used for more than
 * 10 secs automatically.
 *
 * Also there is a 4 byte maintenance overhead on every allocation.
 *
 * This allocator is thread safe.
 *
 * CAVEATS: As the number of buckets increases, this allocators performance
 *          will drop. As a general guideline, dont use this for more
 *          than NS_MAX_BLOCKS
 */

#ifndef nsRecyclingAllocator_h__
#define nsRecyclingAllocator_h__

#include "nscore.h"
#include "pratom.h"
#include "prlock.h"
#include "nsIRecyclingAllocator.h"
#include "nsIGenericFactory.h"

#define NS_DEFAULT_RECYCLE_TIMEOUT 10  // secs
#define NS_MAX_BLOCKS              24
#define NS_ALLOCATOR_OVERHEAD_BYTES (sizeof(Block)) // bytes

class nsITimer;
class nsIMemory;

class NS_COM nsRecyclingAllocator {
 protected:
    struct Block {
      PRSize bytes;
    };

    // Make |BlockStoreNode| a |friend| so it can access |Block|.
    struct BlockStoreNode;
    friend struct BlockStoreNode;

    struct BlockStoreNode {
      BlockStoreNode() : bytes(0), block(nsnull), next(nsnull) {};
      PRSize bytes;
      Block *block;
      BlockStoreNode *next;
    };

#define DATA(block) ((void *)(((char *)block) + NS_ALLOCATOR_OVERHEAD_BYTES))
#define DATA_TO_BLOCK(data) ((Block *)((char *)(data) - NS_ALLOCATOR_OVERHEAD_BYTES))

    // mMaxBlocks: Maximum number of blocks that can be allocated
    PRUint32 mMaxBlocks;

    // mBlocks:
    //  All blocks used or not.
    BlockStoreNode *mBlocks;

    // mFreeList
    //  A linked list of free blocks sorted by increasing order of size
    BlockStoreNode* mFreeList;

    // mNotUsedList
    //  A linked list of BlockStoreNodes that are not used to store
    //  any block information. When we add blocks into mFreeList, we
    //  take BlockStoreNode from here.
    BlockStoreNode* mNotUsedList;

    // mLock: Thread safety of mFreeList and mNotUsedList
    PRLock *mLock;

    // Timer for freeing unused memory
    nsITimer *mRecycleTimer;

    // mRecycleAfter:
    //  Allocator should be untouched for this many seconds for freeing
    //  unused Blocks.
    PRUint32 mRecycleAfter;

    // mTouched:
    //  says if the allocator touched any bucket. If allocator didn't touch
    //  any bucket over a time time interval, timer will call FreeUnusedBuckets()
    PRInt32 mTouched;

    // mId:
    //  a string for identifying the user of nsRecyclingAllocator
    //  User mainly for debug prints
    const char *mId;

#ifdef DEBUG
    // mNAllocated: Number of blocks allocated
    PRInt32 mNAllocated;
#endif

 public:

    // nbucket : number of buckets to hold. Capped at NS_MAX_BUCKET
    // recycleAfter : Try recycling allocated buckets after this many seconds
    // id : a string used to identify debug prints. Will not be released.
    nsRecyclingAllocator(PRUint32 nbucket = 0, PRUint32 recycleAfter = NS_DEFAULT_RECYCLE_TIMEOUT,
                         const char *id = NULL);
    ~nsRecyclingAllocator();

    nsresult Init(PRUint32 nbucket, PRUint32 recycleAfter, const char *id);

    // Allocation and free routines
    void* Malloc(PRSize size, PRBool zeroit = PR_FALSE);
    void  Free(void *ptr);

    void* Calloc(PRUint32 items, PRSize size)
    {
        return Malloc(items * size, PR_TRUE);
    }

    // FreeUnusedBuckets - Frees any bucket memory that isn't in use
    void FreeUnusedBuckets();

 protected:

    // Timer callback to trigger unused memory
    static void nsRecycleTimerCallback(nsITimer *aTimer, void *aClosure);

    // Freelist management
    // FindFreeBlock: return a free block that can hold bytes (best fit)
    Block* FindFreeBlock(PRSize bytes);
    // AddToFreeList: adds block into our freelist for future retrieval.
    //  Returns PR_TRUE is addition was successful. PR_FALSE otherewise.
    PRBool AddToFreeList(Block* block);

    // Touch will mark that someone used this allocator
    // Timer based release will free unused memory only if allocator
    // was not touched for mRecycleAfter seconds.
    void Touch() {
        if (!mTouched)
            PR_AtomicSet(&mTouched, 1);
    }
    void Untouch() {
        PR_AtomicSet(&mTouched, 0);
    }

    friend void nsRecycleTimerCallback(nsITimer *aTimer, void *aClosure);
};

// ----------------------------------------------------------------------
// Wrapping the recyling allocator with nsIMemory
// ----------------------------------------------------------------------

// Wrapping the nsRecyclingAllocator with nsIMemory
class nsRecyclingAllocatorImpl : public nsRecyclingAllocator, public nsIRecyclingAllocator {
public:
    NS_DECL_ISUPPORTS
    NS_DECL_NSIMEMORY
    NS_DECL_NSIRECYCLINGALLOCATOR

    nsRecyclingAllocatorImpl()
    {
    }

private:
    ~nsRecyclingAllocatorImpl() {}
};
#endif // nsRecyclingAllocator_h__