UT_ThreadSafeCache.h

Go to the documentation of this file.

/*
 * PROPRIETARY INFORMATION.  This software is proprietary to
 * Side Effects Software Inc., and is not to be reproduced,
 * transmitted, or disclosed in any way without written permission.
 *
 * NAME:        UT_ThreadSafeCache.h ( UT Library, C++ )
 *
 * COMMENTS:    A fast, thread-safe cache to be used for high-performance
 *              applications.
 */

#ifndef __UT_ThreadSafeCache__
#define __UT_ThreadSafeCache__

#include "UT_API.h"
#include <map>
#include <SYS/SYS_AtomicInt.h>
#include <SYS/SYS_MemoryOrder.h>
#include <SYS/SYS_Types.h>
#include "UT_LockUtil.h"
#include "UT_Lock.h"

//#define ITEM_TIMING
#ifdef ITEM_TIMING
    #include "UT_StopWatch.h"
#endif

// Explicit instantiations are provided for:
// - UT_Lock
// - UT_RecursiveSpinLock
template <class Lock>
class UT_API UT_ThreadSafeCache
{
public:
    class unsafe_traverser;
    
    // A cache item stub.  You must implement a subclass to provide
    // allocation and deallocation routines.
    class UT_CacheItem
    {
    public:
                 UT_CacheItem()
                     : myPrev(0), myNext(0), myMemory(0)
                     , myRefCount(0), myTimeStamp(0)
                     , myAllocated(false) {}
        virtual ~UT_CacheItem() { UT_ASSERT(!getRefCount()); }

        // Return a string to be used in diagnostic messages about
        // different cache types.
        virtual const char      *getItemType() const = 0;

        // Retrieve the amount of memory consumed by the cache item
        int64            getMemoryUsage() const { return myMemory; }

    protected:
        // Allocate an item and return the total amount of memory consumed
        // by the cache item in bytes.
        virtual int64    allocate(void *parms) = 0;

        // Deallocate the item and free associated memory.
        virtual void     deallocate(void *user_data) = 0;

    private:
        int              getRefCount() const    { return myRefCount.load(SYS_MEMORY_ORDER_SEQ_CST); }
        int              incRefCount()          { return myRefCount.add(1); }
        int              decRefCount()          { return myRefCount.add(-1); }

        bool             isInCache() const      { return myNext; }

        // Unlink the item, leaving the existing next/prev unchanged (clear
        // them with clearLinks()).
        void             unlink()
                         {
                             myPrev->myNext = myNext;
                             myNext->myPrev = myPrev;
                         }
        // Set links to 0
        void             clearLinks()
                         {
                             myPrev = 0;
                             myNext = 0;
                         }
        // Insert the item before the given item
        void             link(UT_CacheItem *before)
                         {
                             // NOTE: Although isInCache() is called from
                             // unlocked code, and it only checks whether
                             // myNext is non-NULL, nothing after that depends
                             // on the other values set in this function, so
                             // the other values here don't need to be set
                             // first.

                             myNext = before;
                             myPrev = before->myPrev;

                             before->myPrev->myNext = this;
                             before->myPrev = this;
                         }
        // Link to ourselves - for a circular list of size 1
        void             selfLink()
                         {
                             myNext = this;
                             myPrev = this;
                         }

    private:
        // Force the compiler to read/write myNext and myPrev when
        // indicated, instead of saving temporary values or reordering
        // the access.  myNext is used to determine if this is cached,
        // which could be changed by another thread, so is important
        // to re-read each time.
        UT_CacheItem *volatile myNext;
        UT_CacheItem *volatile myPrev;

        // NOTE: Even myMemory and myTimeStamp must be volatile, because
        //       they get read before locking, then read again after locking,
        //       possibly having been written by another thread in between.
        volatile int64   myMemory;
        volatile uint64  myTimeStamp;
#ifdef ITEM_TIMING
        double           myAllocationTime;
#endif
        SYS_AtomicInt32  myRefCount;

        // Force the compiler to read/write myAllocated when
        // indicated, instead of saving temporary values or reordering
        // the access.  myAllocated could be changed by another thread,
        // upon allocating or deallocating, so is important to re-read
        // each time.
        volatile bool    myAllocated;

        friend class     UT_ThreadSafeCache;
        friend class     UT_ThreadSafeCache::unsafe_traverser;
    };

public:
    // The destroy flag controls whether the cache will clean up its
    // allocated items on destruction.
     UT_ThreadSafeCache(bool destroy = true);
    ~UT_ThreadSafeCache();

    // Set cache size in megabytes
    void                setMaxMemory(int64 size_mb, bool prune = false);
    // Set cache size in bytes
    void                setMaxMemoryBytes(int64 size_bytes, bool prune = false);
    int64               getMaxMemoryBytes() const
                            { return myMemoryLimit; }

    // Get total cache memory usage in bytes, including all allocated item
    // memory.
    int64               getMemoryUsage() const  { return myMemoryUsed; }

    // Get number of cached items
    int64               entries() const         { return myEntries; }

    // Set user data that is passed to allocate()/deallocate()
    void                setUserData(void *data) { myUserData = data; }

    // Control whether we evict items when memory limit is used. Default is
    // true.
    void                setCheckMemoryLimit(bool check)
                            { myCheckMemoryLimit = check; }
    bool                getCheckMemoryLimit() const
                            { return myCheckMemoryLimit; }

    // Access an item in the cache.  If the item is not already in the
    // cache, it will be inserted at the head.  Calling this operation
    // acquires an access lock that must be released with release().  parms
    // will be passed directly to the allocate() operation.
    void                access(UT_CacheItem *item, void *parms = 0)
    {
        item->incRefCount();

        // This prevents the memory load in item->isInCache() from occurring
        // before the store in item->incRefCount(), which is necessary for
        // the reasons outlined in freeItem().  Basically, item could be
        // deallocated after calling item->incRefCount(), but only if
        // item->incRefCount() occurs after item->isInCache() becomes false.
        // Thus, so long as item->isInCache() is read after
        // item->incRefCount(), it won't get a stale true value, which would
        // have falsely indicated that allocation isn't necessary.
        SYSmemoryFence();

        // Before moving items in the cache, check
        // whether the head has been moved a
        // sufficient number of times to make it
        // useful to move the item back to the head
        // of the cache.  This way, if the same set
        // of items are always referenced we never
        // need to modify the cache.
        if (item->isInCache() && isNearHead(item))
        {
            return;
        }

        // Now, reorder the item in the cache
        accessLockedReorder(item, parms);
    }

    // Release the access lock on an item.  For every call to access()
    // there must be a corresponding call to release().
    void                release(UT_CacheItem *item)
                        {
                            if (!item->decRefCount() && !item->isInCache())
                                freeItem(item);
                        }

    // Release the access lock and remove an item from the cache, if there
    // are no more references.
    void                releaseAndDeallocate(UT_CacheItem *item)
                        {
                            if (!item->decRefCount())
                                freeItem(item);
                        }

    // Deallocates and removes an item from the cache.  This is not
    // thread-safe while anything has a reference to the item.
    void                freeItem(UT_CacheItem *item);

    // Clear all entries from the cache.
    void                freeAllItems();

    // Prune entries from the cache until we hit the given memory usage
    void                pruneItems(int64 memory_limit);

    struct StatItem
    {
        const char      *myType;
        int64            myInCache;
        int64            myFaults;
        int64            myGenerated;
        int64            myItemSize;
    };

    void                getStats(int64 &mem_used,
                                int64 &mem_limit,
                                UT_Array<StatItem> &breakdown) const;

    // Prints fault statistics categorized by item type
    void                dumpStats(const char *name) const;

    class UT_API unsafe_traverser
    {
        public:
            unsafe_traverser()
                : myCurr(NULL)
                , myHead(NULL)
            {}
            unsafe_traverser(const unsafe_traverser &src)
                : myCurr(src.myCurr)
                , myHead(src.myHead)
            {}
            ~unsafe_traverser() {}

            const UT_CacheItem  *get() const    { return myCurr; }
            int         operator==(const unsafe_traverser &cmp) const
                            { return myCurr == cmp.myCurr; }
            int         operator!=(const unsafe_traverser &cmp) const
                            { return myCurr != cmp.myCurr; }
            bool        atEnd() const
                            { return !myCurr; }
            void        rewind()
                            { myCurr = myHead; }
            unsafe_traverser    &operator++()
                            {
                                if (myCurr && myCurr->myNext != myHead)
                                    myCurr = myCurr->myNext;
                                else
                                    myCurr = NULL;
                                return *this;
                            }
            const unsafe_traverser      &operator=(const unsafe_traverser &src)
                            {
                                myCurr = src.myCurr;
                                myHead = src.myHead;
                                return *this;
                            }
        private:
            unsafe_traverser(UT_CacheItem *head)
                : myCurr(head)
                , myHead(head)
                {}
            const UT_CacheItem  *myCurr, *myHead;
            friend class UT_ThreadSafeCache;
    };

    unsafe_traverser    unsafe_begin() const
                            { return unsafe_traverser(myHead); }
    unsafe_traverser    unsafe_end() const
                            { return unsafe_traverser(); }

private:
    /// Reorder the list due to access - requires locking
    void                accessLockedReorder(UT_CacheItem *item, void *parms);

    void                pruneItemsNoLock(int64 memory_limit);
    void                addToCache(UT_CacheItem *item);
    void                removeFromCache(UT_CacheItem *item);
    void                allocateItem(UT_CacheItem *item, void *parms)
    {
        if (!item->myAllocated)
        {
            UT_AutoObjLockType<Lock>
                lock(myObjLock, item);
            if (!item->myAllocated)
            {
#ifdef ITEM_TIMING
                UT_StopWatch    timer;
                timer.start();
#endif
                item->myMemory = item->allocate(
                        parms ? parms : myUserData);

                // Ensure that the data allocated are written out
                // to main memory before setting myAllocated to true,
                // else something could read myAllocated, see that it's
                // true, read an out-of-date data pointer, probably still
                // NULL, and crash dereferencing it.
                SYSstoreFence();

                item->myAllocated = true;

                // NOTE: The automatic unlock will do a second SYSstoreFence,
                //       ensuring that myAllocated will be true before the
                //       lock is released.

#ifdef ITEM_TIMING
                item->myAllocationTime = timer.lap();
#endif
            }
        }

        // Ensure that code in the calling function doesn't
        // speculatively read data from item before the read of
        // item->myAllocated above.  Without it, what could happen is:
        // 0) item->myAllocated starts false, and some subclass member,
        //    p, starts NULL.
        // 1) This thread (in the calling function after this call)
        //    speculatively reads p, getting NULL.
        // 2) Another thread sets p to a valid address, and does a store
        //    fence, to ensure it's written out.
        // 3) The other thread sets myAllocated to true.
        // 4) The other thread unlocks, doing a store fence just before
        //    unlocking, to ensure myAllocated is written out.
        // 5) This thread (at the top of this function) reads myAllocated,
        //    getting true.
        // 6) The calling function already has a value for p, and myAllocated
        //    is true, so it dereferences p, and crashes.
        SYSloadFence();
    }

    // Heuristic to decide when an item is close to the head.  Items can
    // only be moved when this method returns false.
    bool                isNearHead(const UT_CacheItem *item) const
                        { return myHeadTime - item->myTimeStamp <=
                            myQuarterEntries; }

private:

    typedef typename Lock::Scope    LockScope;

    // This class keeps track of fault statistics for a given type of item
    // in the cache.
    class FaultStats
    {
    public:
        FaultStats()
            : myFaults(0)
            , myTotalMemory(0)
            , myTotalTime(0) {}

    public:
        int64   myFaults;
        int64   myTotalMemory;
        double  myTotalTime;
    };

    typedef std::map<const char *, FaultStats>  FaultMap;

    UT_ObjLockType<Lock>         myObjLock;
    Lock                         myThreadLock;
    UT_CacheItem                *myHead;
    int64                        myMemoryLimit;
    int64                        myMemoryUsed;
    int64                        myEntries;
    int64                        myQuarterEntries;
    FaultMap                     myFaultMap;
    uint64                       myHeadTime;
    void                        *myUserData;
    bool                         myDestroy;
    bool                         myCheckMemoryLimit;
};

/// This can be used as a unified cache for a process.  The cache will not
/// destroy any items left in the cache when the process exits.
class UT_API UT_UnifiedCache 
{
public:
    using cache_type = UT_ThreadSafeCache<UT_Lock>;

    static cache_type &get() { return theCache; }

    /// Set the amount of memory to be used by the cache, in MB
    static void         setMaxMemory(int size_mb)
                        {
                            theCache.setMaxMemory(size_mb);
                            theMemorySet = true;
                        }

    /// Check whether the memory limit has been set
    static bool         isMaxMemorySet()
                        { return theMemorySet; }

    static int64        getMemoryUsage()
                        { return theCache.getMemoryUsage(); }

private:
    static UT_ThreadSafeCache<UT_Lock>  theCache;
    static bool         theMemorySet;
};

/// Scoped accessor object for accessing unified cache items. 
template<typename T>
class UT_UnifiedCacheAccessor
{
public:
    /// Default initializer. Can be used to initialize the accessor to a cache
    /// item right away. Otherwise call @c reset to set the item to access.
    UT_UnifiedCacheAccessor(T *item = 0, void *parm = 0)
    {
        access(item, parm);
    }

    /// Release the currently accessed cache item, if any.
    ~UT_UnifiedCacheAccessor()
    {
        release();
    }

    /// Returns @c true if no item is being accessed.
    bool empty() const { return myItem == 0; }
    
    /// Returns the current item being accessed
    T *item() const { return myItem; }
    
    /// Indirection operator for the current item being accessed. It is the
    /// caller's responsibility to ensure that the item is valid, e.g. by
    /// calling @c empty on it first.
    T *operator->() const { return myItem; }

    /// Reset the currently accessed cached item to a new item. If the item
    /// given is the same as the one currently being accessed, this call is a
    /// no-op, even if a new @c param value is given. Reset the item to @c
    /// nullptr and back to the wanted item if a change in @c parm value is
    /// required.
    void reset(T *item, void *parm = 0)
    {
        if (myItem != item)
        {
            release();
            access(item, parm);
        }
    }

private:
    void access(T *item, void *parm)
    {
        myItem = item;
        if (myItem)
            UT_UnifiedCache::get().access(item, parm);
    }
    void release()
    {
        if (myItem)
        {
            UT_UnifiedCache::get().release(myItem);
            myItem = 0;
        }
    }

    T   *myItem;
};

#endif