Fair Scheduling and File Caching

Enabling +fair scheduling

The MExtendedFileInterface API enables fair scheduling. The FAT, FAT32 and LFFS file systems +support MExtendedFileInterface, this API allows requests +to be split into one or more segments with the aOffset parameter. Support has been built in for large files by changing +the file’s position parameter (aPos) from a TInt to a TInt64.

To enable +fair scheduling, the file system mounted on a particular drive must +support CFileCB::MExtendedFileInterface. The object +returned by CFileSystem::NewFileL() must be derived +from CFileCB and CFileCB::MExtendedFileInterface. The file server determines whether this is the case by calling CFileCB::GetInterface.

The CFileCB::MExtendedFileInterface interface is as follows:

class MExtendedFileInterface +{ +public: + virtual void ReadL(TInt64 aPos, TInt& aLength, TDes8* aDes, const RMessagePtr2& aMessage, TInt aOffset) = 0; + virtual void WriteL(TInt64 aPos, TInt& aLength, const TDesC8* aDes, const RMessagePtr2& aMessage, TInt aOffset) = 0; + virtual void SetSizeL(TInt64 aSize) = 0; +};

The file system indicates its support for MExtendedFileInterface by intercepting the CFileCB::EExtendedFileInterface enumeration in the CFileCB::GetInterface() method +as in the following example:

TInt CFatFileCB::GetInterface(TInt aInterfaceId, TAny*& aInterface, TAny* aInput) + { + switch(aInterfaceId) + { + case EExtendedFileInterface: + ((CFileCB::MExtendedFileInterface*&) aInterface) = this; + return KErrNone; + + //etc. + + default: + return CFileCB::GetInterface(aInterfaceId, aInterface, aInput); + } + }

Enabling +read and write caching

To enable caching, the file system +must support reads and writes to local buffers, which are buffers +created and owned by the file server itself rather by a client of +the file server.

The local media subsystem already supports +local messages, but the file system and any file system extensions +need to be examined carefully to ensure that they do not call any RMessagePtr2 methods, otherwise a panic results.

The file server calls CMountCB::LocalBufferSupport() to find out whether the file system and all file system extensions +mounted on a particular drive fully support local buffers before it +enables caching. The file system handles the CMountCB::ELocalBufferSupport enumeration in its CMountCB::GetInterface() method +and passes the request on to the first proxy drive, as in the following +example:

TInt CFatMountCB::GetInterface(TInt aInterfaceId, TAny*& aInterface, TAny* /*aInput*/) + { + TInt r= KErrNone; + switch(aInterfaceId) + { + // file caching supported, so pass query on to first proxy drive + case CMountCB::ELocalBufferSupport: + r = LocalDrive()->LocalBufferSupport(); + break; + default: + r=KErrNotSupported; + } + return r; + }

If no file system extensions are loaded, then +the query is eventually handled by CLocalProxyDrive::GetInterface(), which returns KErrNone to indicate that TBusLocalDrive and the local media sub-system support local +buffers.

If there are any file system extensions, they must +handle the CProxyDrive::ELocalBufferSupport enumeration, +as in the following example:

TInt CBitExtProxyDrive::GetInterface(TInt aInterfaceId, TAny*& aInterface, TAny* aInput) + { + switch(aInterfaceId) + { + // file caching supported, so pass query on to next proxy drive + case ELocalBufferSupport: + return CBaseExtProxyDrive::LocalBufferSupport(); + + default: + return CBaseExtProxyDrive::GetInterface(aInterfaceId, aInterface, aInput); + } + }

Configuring +caching

The global and drive-specific cache property defaults +are defined in f32\sfile\sf_file_cache_defs.h. They may be overridden for a particular drive by appropriate entries +in the Base Starter component's ESTART.TXT configuration +file. The format of this file has been enhanced to support the .ini file style syntax:

C: 0 ELOCAL FAT 0 FS_FORMAT_COLD,FS_SYNC_DRIVE # IRAM +D: 1 ELOCAL FAT 0 FS_SCANDRIVE # MMC (textshell) +I: 2 ELOCAL FAT 0 FS_FORMAT_CORRUPT # NAND - USER DATA +K: 8 ELFFS LFFS 0 FS_FORMAT_CORRUPT # LFFS + +[DriveD] +FileCacheSize 256 +FairSchedulingLen 128 +FileCacheRead ON +FileCacheReadAhead ON +FileCacheWrite ON +ClosedFileKeepAliveTime 3000 +DirtyDataFlushTime 3000 +FileCacheReadAsync ON + +[DriveI] +FileCacheWrite ENABLED + +[DriveK] +FileCacheWrite ENABLED + +[FileCache] +GlobalCacheEnabled ON +GlobalCacheSize 32768 +GlobalCacheMaxLockedSize 1024 +LowMemoryThreshold 10

In this example, write caching +is enabled on drives I and K and has been turned on by default on +drive D.

Most of the properties in the example are set to +their default values: the definitions are redundant but are shown +for illustrative purposes.

For details, see Base Starter Technology.

Global +cache settings

The following table holds the global caching +properties:

+ + + + +

Property

Type

Comments

Default value

+ + +

GlobalCacheEnabled

boolean

on or off

+ + +

GlobalCacheSize

integer

Size of disconnected chunk shared by all files in kilobytes.

32768K (32MB)

+ + +

GlobalCacheMaxLockedSize

integer

Maximum amount of locked RAM for all files in kilobytes.

1024K (1 MB)

+ + +

LowMemoryThreshold

integer

Low memory threshold expressed as a percentage of total +RAM. If the amount of RAM drops below this value, attempts to allocate +memory for file caching fails.

10%

+ + + +

File +cache settings

The properties FileCacheRead, FileCacheReadAhead and FileCacheWrite may each be in one of 3 states:

OFF - file caching is permanently off and cannot be turned on.
ENABLED - file caching is off by default but may be turned on using an appropriate +file open mode, such as EFileReadBuffered
ON - file caching is on by default but may be turned off using an appropriate +file open mode, such as EFileReadDirectIO.

See Run time cache settings.

If FileCacheReadAsync is set, media driver reads on this drive are asynchronous. This +results in read ahead requests being issued before completing a client +read request. However, if FileCacheReadAsync is OFF, +then media driver reads on this drive are synchronous. So, issuing +a read ahead is likely to block any client thread that is normally +running at lower priority than the media driver's thread. In this +case read ahead requests only happen during periods of inactivity +to improve latency.

+ + + + +

Property

Type

Comments

Default value

+ + +

FileCacheSize

integer

Maximum amount of locked or unlocked RAM per file.

128K

+ + +

FairSchedulingLen

integer

Read & write requests greater than this length are split +up into 2 or more requests.

128K

+ + +

FileCacheRead

string

OFF, ENABLED or ON.

ENABLED

+ + +

FileCacheReadAhead

string

OFF, ENABLED or ON.

+ + +

FileCacheWrite

string

OFF, ENABLED or ON.

ENABLED

+ + +

FileCacheReadAsync

boolean

ON or OFF. If set, media driver reads on this drive are +asynchronous.

OFF

+ + +

ClosedFileKeepAliveTime

integer

Time after which a file is removed from the closed file +queue, in milliseconds.

3000ms (3 secs)

+ + +

DirtyDataFlushTime

integer

Time after which a file containing dirty data is flushed, +in milliseconds.

3000ms (3 secs)

+ + + +