TCache


class description - source file - inheritance tree

class TCache : public TObject

    private:
Int_t FlushList(TList* list) Int_t FlushNew() Int_t Free(ULong_t upto) TCache::TPage* ReadPage(Seek_t offset) void SetPageSize(Int_t size) Int_t WritePage(TCache::TPage* page) protected:
public:
TCache TCache(Int_t maxCacheSize, TFile* file, Int_t pageSize = kDfltPageSize) TCache TCache(TCache&) virtual void ~TCache() static TClass* Class() Int_t Flush() Int_t GetActiveCacheSize() const Int_t GetLowLevel() const Int_t GetMaxCacheSize() const Int_t GetPageSize() const virtual TClass* IsA() const Int_t ReadBuffer(Seek_t offset, char* buf, Int_t len) Int_t Resize(Int_t maxCacheSize) void SetLowLevel(Int_t percentOfHigh) virtual void ShowMembers(TMemberInspector& insp, char* parent) virtual void Streamer(TBuffer& b) void StreamerNVirtual(TBuffer& b) Int_t WriteBuffer(Seek_t offset, const char* buf, Int_t len)

Data Members

    private:
TCache::TCacheList* fCache hash list containing cached pages TSortedList* fNew list constaining new pages that have to be written to disk TList* fFree list containing unused pages TFile* fFile file for which pages are being cached Seek_t fEOF end of file ULong_t fHighWater high water mark (i.e. maximum cache size in bytes) ULong_t fLowWater low water mark (free pages till low water mark is reached) Int_t fPageSize size of cached pages Int_t fLowLevel low water mark is at low level percent of high Int_t fDiv page size divider Bool_t fRecursive true to prevent recusively calling ReadBuffer() protected:
public:
static const enum TObject:: kDfltPageSize static const enum TObject:: kDfltLowLevel

Class Description

                                                                      
 TCache                                                               
                                                                      
 A caching system to speed up network I/O, i.e. when there is         
 no operating system caching support (like the buffer cache for       
 local disk I/O). The cache makes sure that every I/O is done with    
 a (large) fixed length buffer thereby avoiding many small I/O's.     
 The default page size is 512KB. The cache size is not very important 
 when writing sequentially a file, since the pages will not be        
 reused. In that case use a small cache containing 10 to 20 pages.    
 In case a file is used for random-access the cache size should be    
 taken much larger to avoid re-reading pages over the network.        
 Notice that the TTree's have their own caching mechanism (see        
 TTree::SetMaxVirtualSize()), so when using mainly TTree's with large 
 basket buffers the cache can be kept quite small.                    
 Currently the TCache system is used by the classes TNetFile,         
 TRFIOFile and TWebFile.                                              
                                                                      
 Extra improvement would be to run the Free() process in a separate   
 thread. Possible flush parameters:                                   
 nfract  25   fraction of dirty buffers above which the flush process 
              is activated                                            
 ndirty  500  maximum number of buffer block which may be written     
              during a flush                                          
                                                                      


TCache(Int_t maxCacheSize, TFile *file, Int_t pageSize)
 Create a file cache. Mainly useful for remote files like TNetFile,
 TWebFile and TRFIOFile where the local OS is not doing any caching.
 The maxCacheSize is in MBytes and the pageSize is in bytes (default
 being kDfltPageSize).

~TCache()
 Clean up cache.

void SetPageSize(Int_t size)
 Make sure the page size is a power of two, with a minimum of 4096.

Int_t ReadBuffer(Seek_t offset, char *buf, Int_t len)
 Return in buf len bytes starting at offset. Returns < 0 in
 case of error, 0 in case ReadBuffer() was recursively called
 via ReadPage() and 1 in case of success.

Int_t WritePage(TPage *page)
 Write dirty page to file. Returns -1 in case of error.

Int_t WriteBuffer(Seek_t offset, const char *buf, Int_t len)
 Write a buffer to the cache. Returns < 0 in case of error, 0 in
 case WriteBuffer() was recursively called via WritePage() and 1
 in case of success.

Int_t Free(ULong_t upto)
 Free pages so that specified number of bytes remains in the cache.
 Dirty pages are written to file. Returns < 0 in case of error
 (typically when there was an error writing a dirty page).

Int_t FlushList(TList *list)
 Flush all dirty pages in the specified list to file. Return < 0 in
 case of error (typically when there was an error writing a dirty page).

Int_t Flush()
 Flush all dirty pages to file. Return < 0 in case of error
 (typically when there was an error writing a dirty page).

Int_t FlushNew()
 Flush all new pages to file. New pages are pages in the fNew list.
 When a page is in this list it means that this page extends the file
 (i.e. is adds new pages to the file). The issue with new pages is that
 they need to be flushed in the right order. One can not write at an
 offset past the EOF. Therefore the new pages are put in a sorted
 list and then written in ascending fOffset order.

Int_t Resize(Int_t maxCacheSize)
 Resize the cache (either increase or decrease the size). Returns the
 previous cache size or -1 in case of error. Input and output is in MB's.
 Minimum cache size is 1 MB.

Int_t GetActiveCacheSize() const
 Get size of all allocated cache pages (in use and free pages).

void SetLowLevel(Int_t percentOfHigh)
 Set the low water level. This is in percent of the maxCacheSize.
 When the cache is full it will be freed up to percentOfHigh. The
 default is kDfltLowLevel=70%.



Inline Functions


        TCache::TPage* ReadPage(Seek_t offset)
                 Int_t GetMaxCacheSize() const
                 Int_t GetPageSize() const
                 Int_t GetLowLevel() const
               TClass* Class()
               TClass* IsA() const
                  void ShowMembers(TMemberInspector& insp, char* parent)
                  void Streamer(TBuffer& b)
                  void StreamerNVirtual(TBuffer& b)
                TCache TCache(TCache&)


Author: Fons Rademakers 13/01/2001
Last update: root/net:$Name: $:$Id: TCache.cxx,v 1.5 2001/02/09 14:04:26 rdm Exp $
Copyright (C) 1995-2001, Rene Brun and Fons Rademakers. *


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