gecko-dev/db/man/mancxx.roff/DbMpool.3

.ds TYPE CXX
.\"
.\" See the file LICENSE for redistribution information.
.\"
.\" Copyright (c) 1997, 1998
.\"	Sleepycat Software.  All rights reserved.
.\"
.\"	@(#)DbMpool.sox	10.22 (Sleepycat) 5/10/98
.\"
.\"
.\" See the file LICENSE for redistribution information.
.\"
.\" Copyright (c) 1997, 1998
.\"	Sleepycat Software.  All rights reserved.
.\"
.\"	@(#)macros.so	10.45 (Sleepycat) 5/4/98
.\"
.\" We don't want hyphenation for any HTML documents.
.ie '\*[HTML]'YES'\{\
.nh
\}
.el\{\
.ds Hy
.hy
..
.ds Nh
.nh
..
\}
.\" The alternative text macro
.\" This macro takes two arguments:
.\"	+ the text produced if this is a "C" manpage
.\"	+ the text produced if this is a "CXX" or "JAVA" manpage
.\"
.de Al
.ie '\*[TYPE]'C'\{\\$1
\}
.el\{\\$2
\}
..
.\" Scoped name macro.
.\" Produces a_b, a::b, a.b depending on language
.\" This macro takes two arguments:
.\"	+ the class or prefix (without underscore)
.\"	+ the name within the class or following the prefix
.de Sc
.ie '\*[TYPE]'C'\{\\$1_\\$2
\}
.el\{\
.ie '\*[TYPE]'CXX'\{\\$1::\\$2
\}
.el\{\\$1.\\$2
\}
\}
..
.\" Scoped name for Java.
.\" Produces Db.b, for Java, otherwise just b.  This macro is used for
.\" constants that must be scoped in Java, but are global otherwise.
.\" This macro takes two arguments:
.\"	+ the class
.\"	+ the name within the class or following the prefix
.de Sj
.ie '\*[TYPE]'JAVA'\{\
.TP 5
Db.\\$1\}
.el\{\
.TP 5
\\$1\}
..
.\" The general information text macro.
.de Gn
.ie '\*[TYPE]'C'\{The DB library is a family of groups of functions that provides a modular
programming interface to transactions and record-oriented file access.
The library includes support for transactions, locking, logging and file
page caching, as well as various indexed access methods.
Many of the functional groups (e.g., the file page caching functions)
are useful independent of the other DB functions,
although some functional groups are explicitly based on other functional
groups (e.g., transactions and logging).
\}
.el\{The DB library is a family of classes that provides a modular
programming interface to transactions and record-oriented file access.
The library includes support for transactions, locking, logging and file
page caching, as well as various indexed access methods.
Many of the classes (e.g., the file page caching class)
are useful independent of the other DB classes,
although some classes are explicitly based on other classes
(e.g., transactions and logging).
\}
For a general description of the DB package, see
.IR db_intro (3).
..
.\" The library error macro, the local error macro.
.\" These macros take one argument:
.\"	+ the function name.
.de Ee
The
.I \\$1
.ie '\*[TYPE]'C'\{function may fail and return
.I errno
\}
.el\{method may fail and throw a
.IR DbException (3)
.if '\*[TYPE]'CXX'\{
or return
.I errno
\}
\}
for any of the errors specified for the following DB and library functions:
..
.de Ec
In addition, the
.I \\$1
.ie '\*[TYPE]'C'\{function may fail and return
.I errno
\}
.el\{method may fail and throw a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or return
.I errno
\}
.el\{encapsulating an
.I errno
\}
\}
for the following conditions:
..
.de Ea
[EAGAIN]
A lock was unavailable.
..
.de Eb
[EBUSY]
The shared memory region was in use and the force flag was not set.
..
.de Em
[EAGAIN]
The shared memory region was locked and (repeatedly) unavailable.
..
.de Ei
[EINVAL]
An invalid flag value or parameter was specified.
..
.de Es
[EACCES]
An attempt was made to modify a read-only database.
..
.de Et
The DB_THREAD flag was specified and spinlocks are not implemented for
this architecture.
..
.de Ep
[EPERM]
Database corruption was detected.
All subsequent database calls (other than
.ie '\*[TYPE]'C'\{\
.IR DB->close )
\}
.el\{\
.IR Db::close )
\}
will return EPERM.
..
.de Ek
.if '\*[TYPE]'CXX'\{\
Methods marked as returning
.I errno
will, by default, throw an exception that encapsulates the error information.
The default error behavior can be changed, see
.IR DbException (3).
\}
..
.\" The SEE ALSO text macro
.de Sa
.\" make the line long for nroff.
.if n .ll 72
.nh
.na
.IR db_archive (1),
.IR db_checkpoint (1),
.IR db_deadlock (1),
.IR db_dump (1),
.IR db_load (1),
.IR db_recover (1),
.IR db_stat (1),
.IR db_intro (3),
.ie '\*[TYPE]'C'\{\
.IR db_appinit (3),
.IR db_cursor (3),
.IR db_dbm (3),
.IR db_internal (3),
.IR db_lock (3),
.IR db_log (3),
.IR db_mpool (3),
.IR db_open (3),
.IR db_thread (3),
.IR db_txn (3)
\}
.el\{\
.IR db_internal (3),
.IR db_thread (3),
.IR Db (3),
.IR Dbc (3),
.IR DbEnv (3),
.IR DbException (3),
.IR DbInfo (3),
.IR DbLock (3),
.IR DbLockTab (3),
.IR DbLog (3),
.IR DbLsn (3),
.IR DbMpool (3),
.if !'\*[TYPE]'JAVA'\{\
.IR DbMpoolFile (3),
\}
.IR Dbt (3),
.IR DbTxn (3),
.IR DbTxnMgr (3)
\}
.ad
.Hy
..
.\" The function header macro.
.\" This macro takes one argument:
.\"	+ the function name.
.de Fn
.in 2
.I \\$1
.in
..
.\" The XXX_open function text macro, for merged create/open calls.
.\" This macro takes two arguments:
.\"	+ the interface, e.g., "transaction region"
.\"	+ the prefix, e.g., "txn" (or the class name for C++, e.g., "DbTxn")
.de Co
.ie '\*[TYPE]'C'\{\
.Fn \\$2_open
The
.I \\$2_open
function copies a pointer, to the \\$1 identified by the
.B directory
.IR dir ,
into the memory location referenced by
.IR regionp .
.PP
If the
.I dbenv
argument to
.I \\$2_open
was initialized using
.IR db_appinit ,
.I dir
is interpreted as described by
.IR db_appinit (3).
\}
.el\{\
.Fn \\$2::open
The
.I \\$2::open
.ie '\*[TYPE]'CXX'\{\
method copies a pointer, to the \\$1 identified by the
.B directory
.IR dir ,
into the memory location referenced by
.IR regionp .
\}
.el\{\
method returns a \\$1 identified by the
.B directory
.IR dir .
\}
.PP
If the
.I dbenv
argument to
.I \\$2::open
was initialized using
.IR DbEnv::appinit ,
.I dir
is interpreted as described by
.IR DbEnv (3).
\}
.PP
Otherwise,
if
.I dir
is not NULL,
it is interpreted relative to the current working directory of the process.
If
.I dir
is NULL,
the following environment variables are checked in order:
``TMPDIR'', ``TEMP'', and ``TMP''.
If one of them is set,
\\$1 files are created relative to the directory it specifies.
If none of them are set, the first possible one of the following
directories is used:
.IR /var/tmp ,
.IR /usr/tmp ,
.IR /temp ,
.IR /tmp ,
.I C:/temp
and
.IR C:/tmp .
.PP
All files associated with the \\$1 are created in this directory.
This directory must already exist when
.ie '\*[TYPE]'C'\{
\\$1_open
\}
.el\{\
\\$2::open
\}
is called.
If the \\$1 already exists,
the process must have permission to read and write the existing files.
If the \\$1 does not already exist,
it is optionally created and initialized.
..
.\" The common close language macro, for discarding created regions
.\" This macro takes one argument:
.\"	+ the function prefix, e.g., txn (the class name for C++, e.g., DbTxn)
.de Cc
In addition, if the
.I dir
argument to
.ie '\*[TYPE]'C'\{\
.ds Va db_appinit
.ds Vo \\$1_open
.ds Vu \\$1_unlink
\}
.el\{\
.ds Va DbEnv::appinit
.ds Vo \\$1::open
.ds Vu \\$1::unlink
\}
.I \\*(Vo
was NULL
and
.I dbenv
was not initialized using
.IR \\*(Va ,
.if '\\$1'memp'\{\
or the DB_MPOOL_PRIVATE flag was set,
\}
all files created for this shared region will be removed,
as if
.I \\*(Vu
were called.
.rm Va
.rm Vo
.rm Vu
..
.\" The DB_ENV information macro.
.\" This macro takes two arguments:
.\"	+ the function called to open, e.g., "txn_open"
.\"	+ the function called to close, e.g., "txn_close"
.de En
.ie '\*[TYPE]'C'\{\
based on the
.I dbenv
argument to
.IR \\$1 ,
which is a pointer to a structure of type DB_ENV (typedef'd in <db.h>).
Applications will normally use the same DB_ENV structure (initialized
by
.IR db_appinit (3)),
as an argument to all of the subsystems in the DB package.
.PP
References to the DB_ENV structure are maintained by DB,
so it may not be discarded until the last close function,
corresponding to an open function for which it was an argument,
has returned.
In order to ensure compatibility with future releases of DB, all fields of
the DB_ENV structure that are not explicitly set should be initialized to 0
before the first time the structure is used.
Do this by declaring the structure external or static, or by calling the C
library routine
.IR bzero (3)
or
.IR memset (3).
.PP
The fields of the DB_ENV structure used by
.I \\$1
are described below.
.if '\*[TYPE]'CXX'\{\
As references to the DB_ENV structure may be maintained by
.IR \\$1 ,
it is necessary that the DB_ENV structure and memory it references be valid
until the
.I \\$2
function is called.
\}
.ie '\\$1'db_appinit'\{The
.I dbenv
argument may not be NULL.
If any of the fields of the
.I dbenv
are set to 0,
defaults appropriate for the system are used where possible.
\}
.el\{If
.I dbenv
is NULL
or any of its fields are set to 0,
defaults appropriate for the system are used where possible.
\}
.PP
The following fields in the DB_ENV structure may be initialized before calling
.IR \\$1 :
\}
.el\{\
based on which set methods have been used.
It is expected that applications will use a single DbEnv object as the
argument to all of the subsystems in the DB package.
The fields of the DbEnv object used by
.I \\$1
are described below.
As references to the DbEnv object may be maintained by
.IR \\$1 ,
it is necessary that the DbEnv object and memory it references be valid
until the object is destroyed.
.ie '\\$1'appinit'\{\
The
.I dbenv
argument may not be NULL.
If any of the fields of the
.I dbenv
are set to 0,
defaults appropriate for the system are used where possible.
\}
.el\{\
Any of the DbEnv fields that are not explicitly set will default to
appropriate values.
\}
.PP
The following fields in the DbEnv object may be initialized, using the
appropriate set method, before calling
.IR \\$1 :
\}
..
.\" The DB_ENV common fields macros.
.de Se
.if '\*[TYPE]'JAVA'\{\
.TP 5
DbErrcall db_errcall;
.ns
.TP 5
String db_errpfx;
.ns
.TP 5
int db_verbose;
The error fields of the DbEnv behave as described for
.IR DbEnv (3).
\}
.ie '\*[TYPE]'CXX'\{\
.TP 5
void *(*db_errcall)(char *db_errpfx, char *buffer);
.ns
.TP 5
FILE *db_errfile;
.ns
.TP 5
const char *db_errpfx;
.ns
.TP 5
class ostream *db_error_stream;
.ns
.TP 5
int db_verbose;
The error fields of the DbEnv behave as described for
.IR DbEnv (3).
\}
.el\{\
void *(*db_errcall)(char *db_errpfx, char *buffer);
.ns
.TP 5
FILE *db_errfile;
.ns
.TP 5
const char *db_errpfx;
.ns
.TP 5
int db_verbose;
The error fields of the DB_ENV behave as described for
.IR db_appinit (3).
.sp
\}
..
.\" The open flags.
.de Fm
The
.I flags
and
.I mode
arguments specify how files will be opened and/or created when they
don't already exist.
The flags value is specified by
.BR or 'ing
together one or more of the following values:
.Sj DB_CREATE
Create any underlying files, as necessary.
If the files do not already exist and the DB_CREATE flag is not specified,
the call will fail.
..
.\" DB_THREAD open flag macro.
.\" This macro takes two arguments:
.\"	+ the open function name
.\"	+ the object it returns.
.de Ft
.TP 5
.Sj DB_THREAD
Cause the \\$2 handle returned by the
.I \\$1
.Al function method
to be useable by multiple threads within a single address space,
i.e., to be ``free-threaded''.
.if '\*[TYPE]'JAVA'\{\
Threading is assumed in the Java API,
so no special flags are required,
and DB functions will always behave as if the DB_THREAD flag was specified.
\}
..
.\" The mode macro.
.\" This macro takes one argument:
.\"	+ the subsystem name.
.de Mo
All files created by the \\$1 are created with mode
.I mode
(as described in
.IR chmod (2))
and modified by the process' umask value at the time of creation (see
.IR umask (2)).
The group ownership of created files is based on the system and directory
defaults, and is not further specified by DB.
..
.\" The application exits macro.
.\" This macro takes one argument:
.\"	+ the application name.
.de Ex
The
.I \\$1
utility exits 0 on success, and >0 if an error occurs.
..
.\" The application -h section.
.\" This macro takes one argument:
.\"	+ the application name
.de Dh
DB_HOME
If the
.B \-h
option is not specified and the environment variable
.I DB_HOME
is set, it is used as the path of the database home, as described in
.IR db_appinit (3).
..
.\" The function DB_HOME ENVIRONMENT VARIABLES section.
.\" This macro takes one argument:
.\"	+ the open function name
.de Eh
DB_HOME
If the
.I dbenv
argument to
.I \\$1
was initialized using
.IR db_appinit ,
the environment variable DB_HOME may be used as the path of the database
home for the interpretation of the
.I dir
argument to
.IR \\$1 ,
as described in
.IR db_appinit (3).
.if \\n(.$>1 \{Specifically,
.I \\$1
is affected by the configuration string value of \\$2.\}
..
.\" The function TMPDIR ENVIRONMENT VARIABLES section.
.\" This macro takes two arguments:
.\"	+ the interface, e.g., "transaction region"
.\"	+ the prefix, e.g., "txn" (or the class name for C++, e.g., "DbTxn")
.de Ev
TMPDIR
If the
.I dbenv
argument to
.ie '\*[TYPE]'C'\{\
.ds Vo \\$2_open
\}
.el\{\
.ds Vo \\$2::open
\}
.I \\*(Vo
was NULL or not initialized using
.IR db_appinit ,
the environment variable TMPDIR may be used as the directory in which to
create the \\$1,
as described in the
.I \\*(Vo
section above.
.rm Vo
..
.\" The unused flags macro.
.de Fl
The
.I flags
parameter is currently unused, and must be set to 0.
..
.\" The no-space TP macro.
.de Nt
.br
.ns
.TP 5
..
.\" The return values of the functions macros.
.\" Rc is the standard two-value return with a suffix for more values.
.\" Ro is the standard two-value return but there were previous values.
.\" Rt is the standard two-value return, returning errno, 0, or < 0.
.\" These macros take one argument:
.\"	+ the routine name
.de Rc
The
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure,
0 on success,
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure,
0 on success,
\}
.el\{that encapsulates an
.I errno
on failure,
\}
\}
..
.de Ro
Otherwise, the
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure and 0 on success.
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure and 0 on success.
\}
.el\{that encapsulates an
.I errno
on failure,
\}
\}
..
.de Rt
The
.I \\$1
.ie '\*[TYPE]'C'\{function returns the value of
.I errno
on failure and 0 on success.
\}
.el\{method throws a
.IR DbException (3)
.ie '\*[TYPE]'CXX'\{or returns the value of
.I errno
on failure and 0 on success.
\}
.el\{that encapsulates an
.I errno
on failure.
\}
\}
..
.\" The TXN id macro.
.de Tx
.IP
If the file is being accessed under transaction protection,
the
.I txnid
parameter is a transaction ID returned from
.IR txn_begin ,
otherwise, NULL.
..
.\" The XXX_unlink function text macro.
.\" This macro takes two arguments:
.\"	+ the interface, e.g., "transaction region"
.\"	+ the prefix (for C++, this is the class name)
.de Un
.ie '\*[TYPE]'C'\{\
.ds Va db_appinit
.ds Vc \\$2_close
.ds Vo \\$2_open
.ds Vu \\$2_unlink
\}
.el\{\
.ds Va DbEnv::appinit
.ds Vc \\$2::close
.ds Vo \\$2::open
.ds Vu \\$2::unlink
\}
.Fn \\*(Vu
The
.I \\*(Vu
.Al function method
destroys the \\$1 identified by the directory
.IR dir ,
removing all files used to implement the \\$1.
.ie '\\$2'log' \{(The log files themselves and the directory
.I dir
are not removed.)\}
.el \{(The directory
.I dir
is not removed.)\}
If there are processes that have called
.I \\*(Vo
without calling
.I \\*(Vc
(i.e., there are processes currently using the \\$1),
.I \\*(Vu
will fail without further action,
unless the force flag is set,
in which case
.I \\*(Vu
will attempt to remove the \\$1 files regardless of any processes
still using the \\$1.
.PP
The result of attempting to forcibly destroy the region when a process
has the region open is unspecified.
Processes using a shared memory region maintain an open file descriptor
for it.
On UNIX systems, the region removal should succeed
and processes that have already joined the region should continue to
run in the region without change,
however processes attempting to join the \\$1 will either fail or
attempt to create a new region.
On other systems, e.g., WNT, where the
.IR unlink (2)
system call will fail if any process has an open file descriptor
for the file,
the region removal will fail.
.PP
In the case of catastrophic or system failure,
database recovery must be performed (see
.IR db_recover (1)
or the DB_RECOVER and DB_RECOVER_FATAL flags to
.IR \\*(Va (3)).
Alternatively, if recovery is not required because no database state is
maintained across failures,
it is possible to clean up a \\$1 by removing all of the
files in the directory specified to the
.I \\*(Vo
.Al function, method,
as \\$1 files are never created in any directory other than the one
specified to
.IR \\*(Vo .
Note, however,
that this has the potential to remove files created by the other DB
subsystems in this database environment.
.PP
.Rt \\*(Vu
.rm Va
.rm Vo
.rm Vu
.rm Vc
..
.\" Signal paragraph for standard utilities.
.\" This macro takes one argument:
.\"	+ the utility name.
.de Si
The
.I \\$1
utility attaches to DB shared memory regions.
In order to avoid region corruption,
it should always be given the chance to detach and exit gracefully.
To cause
.I \\$1
to clean up after itself and exit,
send it an interrupt signal (SIGINT).
..
.\" Logging paragraph for standard utilities.
.\" This macro takes one argument:
.\"	+ the utility name.
.de Pi
.B \-L
Log the execution of the \\$1 utility to the specified file in the
following format, where ``###'' is the process ID, and the date is
the time the utility starting running.
.sp
\\$1: ### Wed Jun 15 01:23:45 EDT 1995
.sp
This file will be removed if the \\$1 utility exits gracefully.
..
.\" Malloc paragraph.
.\" This macro takes one argument:
.\"	+ the allocated object
.de Ma
.if !'\*[TYPE]'JAVA'\{\
\\$1 are created in allocated memory.
If
.I db_malloc
is non-NULL,
it is called to allocate the memory,
otherwise,
the library function
.IR malloc (3)
is used.
The function
.I db_malloc
must match the calling conventions of the
.IR malloc (3)
library routine.
Regardless,
the caller is responsible for deallocating the returned memory.
To deallocate the returned memory,
free each returned memory pointer;
pointers inside the memory do not need to be individually freed.
\}
..
.\" Underlying function paragraph.
.\" This macro takes two arguments:
.\"	+ the function name
.\"	+ the utility name
.de Uf
The
.I \\$1
.Al function method
is the underlying function used by the
.IR \\$2 (1)
utility.
See the source code for the
.I \\$2
utility for an example of using
.I \\$1
in a UNIX environment.
..
.\" Underlying function paragraph, for C++.
.\" This macro takes three arguments:
.\"	+ the C++ method name
.\"	+ the function name for C
.\"	+ the utility name
.de Ux
The
.I \\$1
method is based on the C
.I \\$2
function, which
is the underlying function used by the
.IR \\$3 (1)
utility.
See the source code for the
.I \\$3
utility for an example of using
.I \\$2
in a UNIX environment.
..
.\" Stat field macro.
.de Sf
.ie '\*[TYPE]'CXX'\{\
u_int32_t \\$1;\}
.el\{\
int get_\\$1();\}
..
.TH DbMpool 3 "May 10, 1998"
.UC 7
.SH NAME
DbMpool \- shared memory buffer pool
.SH SYNOPSIS
.nf
.ft B
.ie '\*[TYPE]'CXX'\{
#include <db_cxx.h>

static int
DbMpool::open(char *dir,
.ti +5
u_int32_t flags, int mode, DbEnv *dbenv, DbMpool **regionp);

int
DbMpool::close();

static int
DbMpool::unlink(const char *dir, int force, DbEnv *);

int
DbMpool::db_register(int ftype,
.ti +5
int (*pgin)(db_pgno_t pgno, void *pgaddr, Dbt *pgcookie),
.ti +5
int (*pgout)(db_pgno_t pgno, void *pgaddr, Dbt *pgcookie));

int
DbMpool::trickle(int pct, int *nwrotep);

int
DbMpool::sync(LSN *lsn);

int
DbMpool::stat(DB_MPOOL_STAT **gsp,
.ti +5
DB_MPOOL_FSTAT *(*fsp)[], void *(*db_malloc)(size_t));
\}
.el\{\
import com.sleepycat.db.*;

public DbMpoolStat stat()
.ti +5
throws DbException;

public DbMpoolFStat[] fstat()
.ti +5
throws DbException;

public int trickle(int pct)
.ti +5
throws DbException;

\}
.ft R
.fi
.SH DESCRIPTION
.Gn
.PP
This manual page describes the specific details of the memory pool interface.
.PP
The
.IR DbMpool (3)
.ie '\*[TYPE]'CXX'\{\
and
.IR DbMpoolFile (3)
classes are
\}
.el\{\
class is
\}
the library interface intended to provide general-purpose,
page-oriented buffer management of one or more files.
While designed to work with the other Db classes, this class is
also useful for more general purposes.
The memory pools (DbMpool's) are referred to in this document as
simply ``pools''.
.if '\*[TYPE]'JAVA'\{\
The DbMpool class has a small subset of the corresponding DB/C++
functionality.  This class has been provided to allow certain
administrative actions on underlying Mpool's opened as a consequence
of DbEnv.appinit().  Direct access to other Mpool functionality is
not appropriate for the Java environment.
\}
.if '\*[TYPE]'CXX'\{\
Pools may be shared between processes.
Pools are usually filled by pages from one or more files (DbMpoolFile's).
Pages in the pool are replaced in LRU (least-recently-used) order,
with each new page replacing the page that has been unused the longest.
Pages retrieved from the pool using
.I DbMpoolFile::get
are ``pinned'' in the pool, by default,
until they are returned to the pool's control using the
.I DbMpoolFile::put
method.
.PP
.Co "memory pool" DbMpool
.PP
.Fm
.Sj DB_MPOOL_PRIVATE
Create a private MPOOL that is not shared with any other process (although
it may be shared with other threads).
.Sj DB_NOMMAP
Always copy files in this memory pool into the local cache instead of mapping
them into process memory (see the description of the
.I mp_mmapsize
field of the DbEnv object for further information).
.Ft DbMpool::open DbMpool
.PP
.Mo "memory pool subsystem (other than files created by the \fImemp_fopen\fP method, which are separately specified)"
.PP
The memory pool subsystem is configured
.En "DbMpool::open" "DbMpool::closed"
.TP 5
.Se
.TP 5
size_t mp_mmapsize;
Files that are opened read-only in the pool (and that satisfy a few other
criteria) are, by default,
mapped into the process address space instead of being copied into the local
cache.
This can result in better-than-usual performance,
as available virtual memory is normally much larger than the local cache,
and page faults are faster than page copying on many systems.
However,
in the presence of limited virtual memory it can cause resource starvation,
and in the presence of large databases,
it can result in immense process sizes.
If
.I mp_mmapsize
is non-zero,
it specifies the maximum file size, in bytes,
for a file to be mapped into the process address space.
By default,
it is set to 10Mb.
.TP 5
size_t mp_size;
The suggested size of the pool, in bytes.
This should be the size of the normal working data set of the application,
with some small amount of additional memory for unusual situations.
(Note,
the working set is not the same as the number of simultaneously referenced
pages,
and should be quite a bit larger!)
The default cache size is 128K bytes (16 8K byte pages),
and may not be less than 20K bytes.
.PP
.Rt DbMpool::open
.PP
.Fn DbMpool::close
The
.I DbMpool::close
method closes the pool indicated by the DbMpool object,
as returned by
.IR DbMpool::open .
This method does not imply a call to
.IR DbMpoolFile::sync ,
but does imply a call to
.I DbMpoolFile::close
for any remaining open DbMpoolFile objects returned to this process by
calls to
.IR DbMpoolFile::open .
.PP
.Cc DbMpool
.PP
When multiple threads are using the DbMpool handle concurrently,
only a single thread may call the
.I DbMpool::close
method.
.PP
.Rt DbMpool::close
.PP
.Un "memory pool" DbMpool
.PP
.Fn DbMpool::db_register
The
.I DbMpool::db_register
method registers page-in and page-out functions for files of type
.I ftype
in the specified pool.
.PP
If the
.I pgin
function is non-NULL,
it is called each time a page is read into the memory pool from a file
of type
.IR ftype ,
or a page is created for a file of type
.I ftype
(see the DB_MPOOL_CREATE flag for the
.I DbMpoolFile::get
method).
If the
.I pgout
function is non-NULL,
it is called each time a page is written to a file of type
.IR ftype .
.PP
Both the
.I pgin
and
.I pgout
functions are called with the page number,
a pointer to the page being read or written,
and any argument
.I pgcookie
that was specified to the
.I DbMpoolFile::open
method when the file was opened.
The
.I pgin
and
.I pgout
functions should return 0 on success,
and an applicable non-zero
.I errno
value on failure,
in which case the
.I DbMpool
method calling it will also fail,
returning that
.I errno
value.
.PP
The purpose of the
.I DbMpool::db_register
method is to support processing when pages are entered into,
or flushed from,
the pool.
A file type must be specified to make it possible for unrelated
threads or processes,
that are sharing a pool,
to evict each other's pages from the pool.
Applications should call
.IR DbMpool::db_register ,
during initialization,
for each type of file requiring input or output processing that will be
sharing the underlying pool.
(No registry is necessary for the standard access method types,
btree, hash and recno, as
.IR Db::open (3)
registers them separately.)
.PP
If a thread or process does not call
.I DbMpool::db_register
for a file type,
it is impossible for it to evict pages for any file requiring input or
output processing from the pool.
For this reason,
.I DbMpool::db_register
should always be called by each application sharing a pool for each type of
file included in the pool,
regardless of whether or not the application itself uses files of that type.
.PP
There are no standard values for
.IR ftype ,
.IR pgin ,
.I pgout
and
.IR pgcookie ,
except that the
.I ftype
value for a file must be a non-zero positive number,
as negative numbers are reserved for internal use by the DB library.
For this reason,
applications sharing a pool must coordinate their values amongst themselves.
.PP
.Rt DbMpool::db_register
\}
.PP
.Fn DbMpool::trickle
The
.I DbMpool::trickle
method ensures that at least
.I pct
percent of the pages in the shared memory pool are clean by writing dirty
pages to their backing files.
.ie '\*[TYPE]'CXX'\{\
If the
.I nwrotep
argument is non-NULL,
the number of pages that were written to reach the correct percentage is
returned in the memory location it references.
\}
.el\{\
The number of pages that were written to reach the correct percentage is
returned.
\}
.PP
The purpose of the
.I DbMpool::trickle
method is to enable a memory pool manager to ensure that a page is
always available for reading in new information without having to wait
for a write.
.PP
.Rt DbMpool::trickle
.if '\*[TYPE]'CXX'\{\
.PP
.Fn DbMpool::sync
The
.I DbMpool::sync
method ensures that all the modified pages in the pool with log sequence
numbers (LSNs) less than the
.I lsn
argument are written to disk.
.PP
.Rc DbMpool::sync
and DB_INCOMPLETE if there were pages which need to be written but which
.I DbMpool::sync
was unable to write immediately.
In addition,
if
.I DbMpool::sync
returns success,
the value of
.I lsn
will be overwritten with the largest LSN from any page which was written by
.I DbMpool::sync
to satisfy this request.
.PP
The purpose of the
.I DbMpool::sync
method is to enable a transaction manager to ensure,
as part of a checkpoint,
that all pages modified by a certain time have been written to disk.
Pages in the pool which cannot be written back to disk immediately (e.g.,
are currently pinned) are written to disk as soon as it is possible to do
so.
The expected behavior of the transaction manager is to call the
.I DbMpool::sync
method and then,
if the return indicates that some pages could not be written immediately,
to wait briefly and retry again with the same LSN until the
.I DbMpool::sync
method returns that all pages have been written.
.PP
To support the
.I DbMpool::sync
functionality,
it is necessary that the pool methods know the location of the LSN on
the page for each file type.
This location should be specified when the file is opened using the
.I DbMpoolFile::open
method.
(Note, it is not required that the LSN be aligned on the page in any way.)
\}
.PP
.Fn DbMpool::stat
.ie '\*[TYPE]'CXX'\{
The
.I DbMpool::stat
method creates statistical structures and copies
pointers to them into
user-specified memory locations.
\}
.el\{\
.Fn DbMpool::fstat
The
.I DbMpool::stat
and
.I DbMpool::fstat
methods create statistical structures and return
to the caller.
\}
The statistics include the number of files participating in the pool,
the active pages in the pool,
and information as to how effective the cache has been.
.PP
.Ma "Statistical structures"
.PP
.ie '\*[TYPE]'CXX'\{\
If
.I gsp
is non-NULL, the global statistics for the memory pool
object
are copied into the memory location it references.
The global statistics are stored in a structure of type
DB_MPOOL_STAT (typedef'd in <db_cxx.h>).
.PP
The following DB_MPOOL_STAT fields will be filled in:
.TP 5
u_int32_t st_refcnt;
The number of references to the region.
.Nt
u_int32_t st_regsize;
The size of the region.
.Nt
size_t st_cachesize;
\}
.el\{\
The
.I stat
method creates a DbMpoolStat object containing global
statistics.  The fields can be accessed via methods:
.TP 5
long get_st_cachesize();
\}
Cache size in bytes.
.Nt
.Sf st_cache_hit
Requested pages found in the cache.
.Nt
.Sf st_cache_miss
Requested pages not found in the cache.
.Nt
.Sf st_map
Requested pages mapped into the process' address space (there is no
available information as to whether or not this request caused disk I/O,
although examining the application page fault rate may be helpful).
.Nt
.Sf st_page_create
Pages created in the cache.
.Nt
.Sf st_page_in
Pages read into the cache.
.Nt
.Sf st_page_out
Pages written from the cache to the backing file.
.Nt
.Sf st_ro_evict
Clean pages forced from the cache.
.Nt
.Sf st_rw_evict
Dirty pages forced from the cache.
.Nt
.Sf st_hash_buckets
Number of hash buckets in buffer hash table.
.Nt
.Sf st_hash_searches
Total number of buffer hash table lookups.
.Nt
.Sf st_hash_longest
The longest chain ever encountered in buffer hash table lookups.
.Nt
.Sf st_hash_examined
Total number of hash elements traversed during hash table lookups.
.Nt
.Sf st_page_clean
Clean pages currently in the cache.
.Nt
.Sf st_page_dirty
Dirty pages currently in the cache.
.Nt
.Sf st_page_trickle
Dirty pages written using the
.I memp_trickle
interface.
.PP
.ie '\*[TYPE]'CXX'\{\
If
.I fsp
is non-NULL,
a pointer to a NULL-terminated variable length array of statistics for
individual files
in the memory pool
is copied into the memory location it references.
If no individual files currently exist in the memory pool,
.I fsp
will be set to NULL.
.PP
The per-file statistics are stored in structures of type
DB_MPOOL_FSTAT (typedef'd in <db_cxx.h>).
The following DB_MPOOL_FSTAT fields will be filled in for each file in the
pool, i.e., each element of the array:
.TP 5
char *file_name;
The name of the file.
.Nt
size_t st_pagesize;
\}
.el\{\
The
.I fstat
method creates an array of DbMpoolFStat objects containing
statistics for individual files in the pool.  Each DbMpoolFStat
object contains statistics for an individual DbMpoolFile, and
the statistics can be accessed via methods:
.TP 5
String get_file_name();
The name of the file.
.Nt
long get_st_pagesize();
\}
Page size in bytes.
.Nt
.Sf st_cache_hit
Requested pages found in the cache.
.Nt
.Sf st_cache_miss
Requested pages not found in the cache.
.Nt
.Sf st_map
Requested pages mapped into the process' address space.
.Nt
.Sf st_page_create
Pages created in the cache.
.Nt
.Sf st_page_in
Pages read into the cache.
.Nt
.Sf st_page_out
Pages written from the cache to the backing file.
.PP
.Rt DbMpool::stat
.SH "ENVIRONMENT VARIABLES"
The following environment variables affect the behavior of
.IR DbMpoolFile :
.TP 5
.Eh DbMpool::open
.TP 5
.Ev "memory pool" DbMpool
.SH ERRORS
.Ek
.if '\*[TYPE]'CXX'\{\
.PP
.Ee DbMpool::open
.na
.Nh
DBmemp->pgin(3), 
DBmemp->pgout(3), 
DbLog::compare(3), 
DbLog::flush(3), 
DbMpool::close(3), 
DbMpool::unlink(3), 
close(2), 
db_version(3), 
fcntl(2), 
fflush(3), 
fsync(2), 
lseek(2), 
malloc(3), 
memcmp(3), 
memcpy(3), 
memset(3), 
mmap(2), 
munmap(2), 
open(2), 
sigfillset(3), 
sigprocmask(2), 
stat(2), 
strcpy(3), 
strdup(3), 
strerror(3), 
strlen(3), 
time(3), 
unlink(2), 
and
write(2). 
.Hy
.ad
.PP
.Ec DbMpool::open
.TP 5
.Em
.TP 5
.Ei
.sp
.Et
.sp
A NULL pathname was specified without the DB_MPOOL_PRIVATE flag.
.sp
The specified cache size was impossibly small.
.PP
.Ee DbMpool::close
.na
.Nh
DbMpoolFile::close(3), 
close(2), 
fcntl(2), 
fflush(3), 
munmap(2), 
and
strerror(3). 
.Hy
.ad
.PP
.Ee DbMpool::unlink
.na
.Nh
close(2), 
fcntl(2), 
fflush(3), 
malloc(3), 
memcpy(3), 
memset(3), 
mmap(2), 
munmap(2), 
open(2), 
sigfillset(3), 
sigprocmask(2), 
stat(2), 
strcpy(3), 
strdup(3), 
strerror(3), 
strlen(3), 
and
unlink(2). 
.Hy
.ad
.PP
.Ec DbMpool::unlink
.TP 5
.Eb
.PP
.Ee DbMpool::db_register
.na
.Nh
fcntl(2), 
and
malloc(3). 
.Hy
.ad
\}
.PP
.Ee DbMpool::trickle
.na
.Nh
DBmemp->pgin(3), 
DBmemp->pgout(3), 
DbLog::compare(3), 
DbLog::flush(3), 
close(2), 
fcntl(2), 
fflush(3), 
fsync(2), 
lseek(2), 
malloc(3), 
memcmp(3), 
memcpy(3), 
memset(3), 
mmap(2), 
open(2), 
sigfillset(3), 
sigprocmask(2), 
stat(2), 
strcpy(3), 
strdup(3), 
strerror(3), 
strlen(3), 
time(3), 
unlink(2), 
and
write(2). 
.Hy
.ad
.PP
.Ec DbMpool::trickle
.TP 5
.Ei
.if '\*[TYPE]'CXX'\{\
.PP
.Ee DbMpool::sync
.na
.Nh
DBmemp->pgin(3), 
DBmemp->pgout(3), 
DbLog::compare(3), 
DbLog::flush(3), 
close(2), 
fcntl(2), 
fflush(3), 
fsync(2), 
lseek(2), 
malloc(3), 
memcmp(3), 
memcpy(3), 
memset(3), 
mmap(2), 
open(2), 
qsort(3), 
realloc(3), 
sigfillset(3), 
sigprocmask(2), 
stat(2), 
strcpy(3), 
strdup(3), 
strerror(3), 
strlen(3), 
time(3), 
unlink(2), 
and
write(2). 
.Hy
.ad
.PP
.Ec DbMpool::sync
.TP 5
.Ei
.sp
The
.I DbMpool::sync
method was called without logging having been initialized in the environment.
\}
.PP
.Ee DbMpool::stat
.na
.Nh
fcntl(2), 
malloc(3), 
memcpy(3), 
and
strlen(3). 
.Hy
.ad
.SH "SEE ALSO"
.Sa