Buffered files (for faster disk access)
I am working with large files and writing directly to disk is slow. Because the file is large I cannot load it in a TMemoryStream.
TFileStream is not buffered so I want to know if there is a custom library that can offer buffered streams or should I rely only on the buffering offered by OS. Is the OS buffering reliable? I mean if the cache is full an old file (mine) might be flushed from cache in order to make room for a new file.
My file is in the GB range. It contains millions of records. Unfortunately, the records are not of fix size. So, I have to do millions of readings (between 4 and 500 bytes). The reading (and the writing) is sequential. I don't jump up and down into the file (which I think is ideal for buffering).
In the end, I have to write such file back to disk (again millions of small writes).
A word of praise for David Heffernan!
David provided a GREAT piece of code that provides buffered disk access.
PEOPLE YOU HAVE TO HAVE HIS BufferedFileStream! It is gold. And don't forget to upvote.
Thanks David.
Speed tests:
Input file: 317MB.SFF
Delphi stream: 9.84sec
David's stream: 2.05sec
______________________________________
More tests:
Input file: input2_700MB.txt
Lines: 19 millions
Compiler optimization: ON
I/O check: On
FastMM: release mode
**HDD**
Reading: **linear** (ReadLine) (PS: multiply time with 10)
We see clear performance drop at 8KB. Recommended 16 or 32KB
Time: 618 ms Cache size: 64KB.
Time: 622 ms Cache size: 128KB.
Time: 622 ms Cache size: 24KB.
Time: 622 ms Cache size: 32KB.
Time: 622 ms Cache size: 64KB.
Time: 624 ms Cache size: 256KB.
Time: 625 ms Cache size: 18KB.
Time: 626 ms Cache size: 26KB.
Time: 626 ms Cache size: 1024KB.
Time: 626 ms Cache size: 16KB.
Time: 628 ms Cache size: 42KB.
Time: 644 ms Cache size: 8KB. <--- no difference until 8K
Time: 664 ms Cache size: 4KB.
Time: 705 ms Cache size: 2KB.
Time: 791 ms Cache size: 1KB.
Time: 795 ms Cache size: 1KB.
**SSD**
We see a small improvement as we go towards higher buffers. Recommended 16 or 32KB
Time: 610 ms Cache size: 128KB.
Time: 611 ms Cache size: 256KB.
Time: 614 ms Cache size: 32KB.
Time: 623 ms Cache size: 16KB.
Time: 625 ms Cache size: 66KB.
Time: 639 ms Cache size: 8KB. <--- definitively not good with 8K
Time: 660 ms Cache size: 4KB.
______
Reading: **Random** (ReadInteger) (100000 reads)
SSD
Time: 064 ms. Cache size: 1KB. Count: 100000. RAM: 13.27 MB <-- probably the best buffer size for ReadInteger is 4bytes!
Time: 067 ms. Cache size: 2KB. Count: 100000. RAM: 13.27 MB
Time: 080 ms. Cache size: 4KB. Count: 100000. RAM: 13.27 MB
Time: 098 ms. Cache size: 8KB. Count: 100000. RAM: 13.27 MB
Time: 140 ms. Cache size: 16KB. Count: 100000. RAM: 13.27 MB
Time: 213 ms. Cache size: 32KB. Count: 100000. RAM: 13.27 MB
Time: 360 ms. Cache size: 64KB. Count: 100000. RAM: 13.27 MB
Conclusion: don't use it for "random" reading
Answer
Windows file caching is very effective, especially if you are using Vista or later. TFileStream is a loose wrapper around the Windows ReadFile() and WriteFile() API functions and for many use cases the only thing faster is a memory mapped file.
However, there is one common scenario where TFileStream becomes a performance bottleneck. That is if you read or write small amounts of data with each call to the stream read or write functions. For example if you read an array of integers one item at a time then you incur a significant overhead by reading 4 bytes at a time in the calls to ReadFile().
Again, memory mapped files are an excellent way to solve this bottleneck, but the other commonly used approach is to read a much larger buffer, many kilobytes say, and then resolve future reads of the stream from this in memory cache rather than further calls to ReadFile(). This approach only really works for sequential access.
From the use pattern described in your updated question, I think you may find the following classes would improve performance for you:
unit BufferedFileStream;
interface
uses
SysUtils, Math, Classes, Windows;
type
TBaseCachedFileStream = class(TStream)
private
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
function _AddRef: Integer; stdcall;
function _Release: Integer; stdcall;
protected
FHandle: THandle;
FOwnsHandle: Boolean;
FCache: PByte;
FCacheSize: Integer;
FPosition: Int64;//the current position in the file (relative to the beginning of the file)
FCacheStart: Int64;//the postion in the file of the start of the cache (relative to the beginning of the file)
FCacheEnd: Int64;//the postion in the file of the end of the cache (relative to the beginning of the file)
FFileName: string;
FLastError: DWORD;
procedure HandleError(const Msg: string);
procedure RaiseSystemError(const Msg: string; LastError: DWORD); overload;
procedure RaiseSystemError(const Msg: string); overload;
procedure RaiseSystemErrorFmt(const Msg: string; const Args: array of const);
function CreateHandle(FlagsAndAttributes: DWORD): THandle; virtual; abstract;
function GetFileSize: Int64; virtual;
procedure SetSize(NewSize: Longint); override;
procedure SetSize(const NewSize: Int64); override;
function FileRead(var Buffer; Count: Longword): Integer;
function FileWrite(const Buffer; Count: Longword): Integer;
function FileSeek(const Offset: Int64; Origin: TSeekOrigin): Int64;
public
constructor Create(const FileName: string); overload;
constructor Create(const FileName: string; CacheSize: Integer); overload;
constructor Create(const FileName: string; CacheSize: Integer; Handle: THandle); overload; virtual;
destructor Destroy; override;
property CacheSize: Integer read FCacheSize;
function Read(var Buffer; Count: Longint): Longint; override;
function Write(const Buffer; Count: Longint): Longint; override;
function Seek(const Offset: Int64; Origin: TSeekOrigin): Int64; override;
end;
TBaseCachedFileStreamClass = class of TBaseCachedFileStream;
IDisableStreamReadCache = interface
['{0B6D0004-88D1-42D5-BC0F-447911C0FC21}']
procedure DisableStreamReadCache;
procedure EnableStreamReadCache;
end;
TReadOnlyCachedFileStream = class(TBaseCachedFileStream, IDisableStreamReadCache)
(* This class works by filling the cache each time a call to Read is made and
FPosition is outside the existing cache. By filling the cache we mean
reading from the file into the temporary cache. Calls to Read when
FPosition is in the existing cache are then dealt with by filling the
buffer with bytes from the cache.
*)
private
FUseAlignedCache: Boolean;
FViewStart: Int64;
FViewLength: Int64;
FDisableStreamReadCacheRefCount: Integer;
procedure DisableStreamReadCache;
procedure EnableStreamReadCache;
procedure FlushCache;
protected
function CreateHandle(FlagsAndAttributes: DWORD): THandle; override;
function GetFileSize: Int64; override;
public
constructor Create(const FileName: string; CacheSize: Integer; Handle: THandle); overload; override;
property UseAlignedCache: Boolean read FUseAlignedCache write FUseAlignedCache;
function Read(var Buffer; Count: Longint): Longint; override;
procedure SetViewWindow(const ViewStart, ViewLength: Int64);
end;
TWriteCachedFileStream = class(TBaseCachedFileStream, IDisableStreamReadCache)
(* This class works by caching calls to Write. By this we mean temporarily
storing the bytes to be written in the cache. As each call to Write is
processed the cache grows. The cache is written to file when:
1. A call to Write is made when the cache is full.
2. A call to Write is made and FPosition is outside the cache (this
must be as a result of a call to Seek).
3. The class is destroyed.
Note that data can be read from these streams but the reading is not
cached and in fact a read operation will flush the cache before
attempting to read the data.
*)
private
FFileSize: Int64;
FReadStream: TReadOnlyCachedFileStream;
FReadStreamCacheSize: Integer;
FReadStreamUseAlignedCache: Boolean;
procedure DisableStreamReadCache;
procedure EnableStreamReadCache;
procedure CreateReadStream;
procedure FlushCache;
protected
function CreateHandle(FlagsAndAttributes: DWORD): THandle; override;
function GetFileSize: Int64; override;
public
constructor Create(const FileName: string; CacheSize, ReadStreamCacheSize: Integer; ReadStreamUseAlignedCache: Boolean); overload;
destructor Destroy; override;
function Read(var Buffer; Count: Longint): Longint; override;
function Write(const Buffer; Count: Longint): Longint; override;
end;
implementation
function GetFileSizeEx(hFile: THandle; var FileSize: Int64): BOOL; stdcall; external kernel32;
function SetFilePointerEx(hFile: THandle; DistanceToMove: Int64; lpNewFilePointer: PInt64; dwMoveMethod: DWORD): BOOL; stdcall; external kernel32;
{ TBaseCachedFileStream }
constructor TBaseCachedFileStream.Create(const FileName: string);
begin
Create(FileName, 0);
end;
constructor TBaseCachedFileStream.Create(const FileName: string; CacheSize: Integer);
begin
Create(FileName, CacheSize, 0);
end;
constructor TBaseCachedFileStream.Create(const FileName: string; CacheSize: Integer; Handle: THandle);
const
DefaultCacheSize = 16*1024;
//16kb - this was chosen empirically - don't make it too large otherwise the progress report is 'jerky'
begin
inherited Create;
FFileName := FileName;
FOwnsHandle := Handle=0;
if FOwnsHandle then begin
FHandle := CreateHandle(FILE_ATTRIBUTE_NORMAL);
end else begin
FHandle := Handle;
end;
FCacheSize := CacheSize;
if FCacheSize<=0 then begin
FCacheSize := DefaultCacheSize;
end;
GetMem(FCache, FCacheSize);
end;
destructor TBaseCachedFileStream.Destroy;
begin
FreeMem(FCache);
if FOwnsHandle and (FHandle<>0) then begin
CloseHandle(FHandle);
end;
inherited;
end;
function TBaseCachedFileStream.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
if GetInterface(IID, Obj) then begin
Result := S_OK;
end else begin
Result := E_NOINTERFACE;
end;
end;
function TBaseCachedFileStream._AddRef: Integer;
begin
Result := -1;
end;
function TBaseCachedFileStream._Release: Integer;
begin
Result := -1;
end;
procedure TBaseCachedFileStream.HandleError(const Msg: string);
begin
if FLastError<>0 then begin
RaiseSystemError(Msg, FLastError);
end;
end;
procedure TBaseCachedFileStream.RaiseSystemError(const Msg: string; LastError: DWORD);
begin
raise EStreamError.Create(Trim(Msg+' ')+SysErrorMessage(LastError));
end;
procedure TBaseCachedFileStream.RaiseSystemError(const Msg: string);
begin
RaiseSystemError(Msg, GetLastError);
end;
procedure TBaseCachedFileStream.RaiseSystemErrorFmt(const Msg: string; const Args: array of const);
var
LastError: DWORD;
begin
LastError := GetLastError; // must call GetLastError before Format
RaiseSystemError(Format(Msg, Args), LastError);
end;
function TBaseCachedFileStream.GetFileSize: Int64;
begin
if not GetFileSizeEx(FHandle, Result) then begin
RaiseSystemErrorFmt('GetFileSizeEx failed for %s.', [FFileName]);
end;
end;
procedure TBaseCachedFileStream.SetSize(NewSize: Longint);
begin
SetSize(Int64(NewSize));
end;
procedure TBaseCachedFileStream.SetSize(const NewSize: Int64);
begin
Seek(NewSize, soBeginning);
if not Windows.SetEndOfFile(FHandle) then begin
RaiseSystemErrorFmt('SetEndOfFile for %s.', [FFileName]);
end;
end;
function TBaseCachedFileStream.FileRead(var Buffer; Count: Longword): Integer;
begin
if Windows.ReadFile(FHandle, Buffer, Count, LongWord(Result), nil) then begin
FLastError := 0;
end else begin
FLastError := GetLastError;
Result := -1;
end;
end;
function TBaseCachedFileStream.FileWrite(const Buffer; Count: Longword): Integer;
begin
if Windows.WriteFile(FHandle, Buffer, Count, LongWord(Result), nil) then begin
FLastError := 0;
end else begin
FLastError := GetLastError;
Result := -1;
end;
end;
function TBaseCachedFileStream.FileSeek(const Offset: Int64; Origin: TSeekOrigin): Int64;
begin
if not SetFilePointerEx(FHandle, Offset, @Result, ord(Origin)) then begin
RaiseSystemErrorFmt('SetFilePointerEx failed for %s.', [FFileName]);
end;
end;
function TBaseCachedFileStream.Read(var Buffer; Count: Integer): Longint;
begin
raise EAssertionFailed.Create('Cannot read from this stream');
end;
function TBaseCachedFileStream.Write(const Buffer; Count: Integer): Longint;
begin
raise EAssertionFailed.Create('Cannot write to this stream');
end;
function TBaseCachedFileStream.Seek(const Offset: Int64; Origin: TSeekOrigin): Int64;
//Set FPosition to the value specified - if this has implications for the
//cache then overriden Write and Read methods must deal with those.
begin
case Origin of
soBeginning:
FPosition := Offset;
soEnd:
FPosition := GetFileSize+Offset;
soCurrent:
inc(FPosition, Offset);
end;
Result := FPosition;
end;
{ TReadOnlyCachedFileStream }
constructor TReadOnlyCachedFileStream.Create(const FileName: string; CacheSize: Integer; Handle: THandle);
begin
inherited;
SetViewWindow(0, inherited GetFileSize);
end;
function TReadOnlyCachedFileStream.CreateHandle(FlagsAndAttributes: DWORD): THandle;
begin
Result := Windows.CreateFile(
PChar(FFileName),
GENERIC_READ,
FILE_SHARE_READ,
nil,
OPEN_EXISTING,
FlagsAndAttributes,
0
);
if Result=INVALID_HANDLE_VALUE then begin
RaiseSystemErrorFmt('Cannot open %s.', [FFileName]);
end;
end;
procedure TReadOnlyCachedFileStream.DisableStreamReadCache;
begin
inc(FDisableStreamReadCacheRefCount);
end;
procedure TReadOnlyCachedFileStream.EnableStreamReadCache;
begin
dec(FDisableStreamReadCacheRefCount);
end;
procedure TReadOnlyCachedFileStream.FlushCache;
begin
FCacheStart := 0;
FCacheEnd := 0;
end;
function TReadOnlyCachedFileStream.GetFileSize: Int64;
begin
Result := FViewLength;
end;
procedure TReadOnlyCachedFileStream.SetViewWindow(const ViewStart, ViewLength: Int64);
begin
if ViewStart<0 then begin
raise EAssertionFailed.Create('Invalid view window');
end;
if (ViewStart+ViewLength)>inherited GetFileSize then begin
raise EAssertionFailed.Create('Invalid view window');
end;
FViewStart := ViewStart;
FViewLength := ViewLength;
FPosition := 0;
FCacheStart := 0;
FCacheEnd := 0;
end;
function TReadOnlyCachedFileStream.Read(var Buffer; Count: Longint): Longint;
var
NumOfBytesToCopy, NumOfBytesLeft, NumOfBytesRead: Longint;
CachePtr, BufferPtr: PByte;
begin
if FDisableStreamReadCacheRefCount>0 then begin
FileSeek(FPosition+FViewStart, soBeginning);
Result := FileRead(Buffer, Count);
if Result=-1 then begin
Result := 0;//contract is to return number of bytes that were read
end;
inc(FPosition, Result);
end else begin
Result := 0;
NumOfBytesLeft := Count;
BufferPtr := @Buffer;
while NumOfBytesLeft>0 do begin
if (FPosition<FCacheStart) or (FPosition>=FCacheEnd) then begin
//the current position is not available in the cache so we need to re-fill the cache
FCacheStart := FPosition;
if UseAlignedCache then begin
FCacheStart := FCacheStart - (FCacheStart mod CacheSize);
end;
FileSeek(FCacheStart+FViewStart, soBeginning);
NumOfBytesRead := FileRead(FCache^, CacheSize);
if NumOfBytesRead=-1 then begin
exit;
end;
Assert(NumOfBytesRead>=0);
FCacheEnd := FCacheStart+NumOfBytesRead;
if NumOfBytesRead=0 then begin
FLastError := ERROR_HANDLE_EOF;//must be at the end of the file
break;
end;
end;
//read from cache to Buffer
NumOfBytesToCopy := Min(FCacheEnd-FPosition, NumOfBytesLeft);
CachePtr := FCache;
inc(CachePtr, FPosition-FCacheStart);
Move(CachePtr^, BufferPtr^, NumOfBytesToCopy);
inc(Result, NumOfBytesToCopy);
inc(FPosition, NumOfBytesToCopy);
inc(BufferPtr, NumOfBytesToCopy);
dec(NumOfBytesLeft, NumOfBytesToCopy);
end;
end;
end;
{ TWriteCachedFileStream }
constructor TWriteCachedFileStream.Create(const FileName: string; CacheSize, ReadStreamCacheSize: Integer; ReadStreamUseAlignedCache: Boolean);
begin
inherited Create(FileName, CacheSize);
FReadStreamCacheSize := ReadStreamCacheSize;
FReadStreamUseAlignedCache := ReadStreamUseAlignedCache;
end;
destructor TWriteCachedFileStream.Destroy;
begin
FlushCache;//make sure that the final calls to Write get recorded in the file
FreeAndNil(FReadStream);
inherited;
end;
function TWriteCachedFileStream.CreateHandle(FlagsAndAttributes: DWORD): THandle;
begin
Result := Windows.CreateFile(
PChar(FFileName),
GENERIC_READ or GENERIC_WRITE,
0,
nil,
CREATE_ALWAYS,
FlagsAndAttributes,
0
);
if Result=INVALID_HANDLE_VALUE then begin
RaiseSystemErrorFmt('Cannot create %s.', [FFileName]);
end;
end;
procedure TWriteCachedFileStream.DisableStreamReadCache;
begin
CreateReadStream;
FReadStream.DisableStreamReadCache;
end;
procedure TWriteCachedFileStream.EnableStreamReadCache;
begin
Assert(Assigned(FReadStream));
FReadStream.EnableStreamReadCache;
end;
function TWriteCachedFileStream.GetFileSize: Int64;
begin
Result := FFileSize;
end;
procedure TWriteCachedFileStream.CreateReadStream;
begin
if not Assigned(FReadStream) then begin
FReadStream := TReadOnlyCachedFileStream.Create(FFileName, FReadStreamCacheSize, FHandle);
FReadStream.UseAlignedCache := FReadStreamUseAlignedCache;
end;
end;
procedure TWriteCachedFileStream.FlushCache;
var
NumOfBytesToWrite: Longint;
begin
if Assigned(FCache) then begin
NumOfBytesToWrite := FCacheEnd-FCacheStart;
if NumOfBytesToWrite>0 then begin
FileSeek(FCacheStart, soBeginning);
if FileWrite(FCache^, NumOfBytesToWrite)<>NumOfBytesToWrite then begin
RaiseSystemErrorFmt('FileWrite failed for %s.', [FFileName]);
end;
if Assigned(FReadStream) then begin
FReadStream.FlushCache;
end;
end;
FCacheStart := FPosition;
FCacheEnd := FPosition;
end;
end;
function TWriteCachedFileStream.Read(var Buffer; Count: Integer): Longint;
begin
FlushCache;
CreateReadStream;
Assert(FReadStream.FViewStart=0);
if FReadStream.FViewLength<>FFileSize then begin
FReadStream.SetViewWindow(0, FFileSize);
end;
FReadStream.Position := FPosition;
Result := FReadStream.Read(Buffer, Count);
inc(FPosition, Result);
end;
function TWriteCachedFileStream.Write(const Buffer; Count: Longint): Longint;
var
NumOfBytesToCopy, NumOfBytesLeft: Longint;
CachePtr, BufferPtr: PByte;
begin
Result := 0;
NumOfBytesLeft := Count;
BufferPtr := @Buffer;
while NumOfBytesLeft>0 do begin
if ((FPosition<FCacheStart) or (FPosition>FCacheEnd))//the current position is outside the cache
or (FPosition-FCacheStart=FCacheSize)//the cache is full
then begin
FlushCache;
Assert(FCacheStart=FPosition);
end;
//write from Buffer to the cache
NumOfBytesToCopy := Min(FCacheSize-(FPosition-FCacheStart), NumOfBytesLeft);
CachePtr := FCache;
inc(CachePtr, FPosition-FCacheStart);
Move(BufferPtr^, CachePtr^, NumOfBytesToCopy);
inc(Result, NumOfBytesToCopy);
inc(FPosition, NumOfBytesToCopy);
FCacheEnd := Max(FCacheEnd, FPosition);
inc(BufferPtr, NumOfBytesToCopy);
dec(NumOfBytesLeft, NumOfBytesToCopy);
end;
FFileSize := Max(FFileSize, FPosition);
end;
end.