Documentation Source Text

<tcl>hd_fragment pagecache {pagecache memory allocator}</tcl>
<h2> Page cache memory</h2>

<p>In most applications, the database page cache subsystem within 
SQLite uses more dynamically allocated memory than all other parts
of SQLite combined.  It is not unusual to see the database page cache
consumes over 10 times more memory than the rest of SQLite combined.</p>

<p>SQLite can be configured to make page cache memory allocations from
a separate and distinct memory pool of fixed-size
slots.  This can have two advantages:</p>

<ul>
<li><p>
................................................................................

<a name="heaplimit"></a>
<h2> Setting memory usage limits</h2>

<p>The [sqlite3_soft_heap_limit64()] interface can be used to set an
upper bound on the total amount of outstanding memory that the
general-purpose memory allocator for SQLite will allow to be outstanding
at one time.  If attempts are made to allocate more memory that specified
by the soft heap limit, then SQLite will first attempt to free cache
memory before continuing with the allocation request.  The soft heap
limit mechanism only works if [memory statistics] are enabled and
it works best
if the SQLite library is compiled with the [SQLITE_ENABLE_MEMORY_MANAGEMENT]
compile-time option.</p>

<p>The soft heap limit is "soft" in this sense:  If SQLite is not able
to free up enough auxiliary memory to stay below the limit, it goes
ahead and allocations the extra memory and exceeds its limit.  This occurs
under the theory that it is better to use additional memory than to fail
outright.</p>

<p>As of SQLite [version 3.6.1] ([dateof:3.6.1]), 
the soft heap limit only applies to the
general-purpose memory allocator.  The soft heap limit does not know
about or interact with the [scratch memory allocator], 

<tcl>hd_fragment pagecache {pagecache memory allocator}</tcl>
<h2> Page cache memory</h2>

<p>In most applications, the database page cache subsystem within 
SQLite uses more dynamically allocated memory than all other parts
of SQLite combined.  It is not unusual to see the database page cache
consume over 10 times more memory than the rest of SQLite combined.</p>

<p>SQLite can be configured to make page cache memory allocations from
a separate and distinct memory pool of fixed-size
slots.  This can have two advantages:</p>

<ul>
<li><p>
................................................................................

<a name="heaplimit"></a>
<h2> Setting memory usage limits</h2>

<p>The [sqlite3_soft_heap_limit64()] interface can be used to set an
upper bound on the total amount of outstanding memory that the
general-purpose memory allocator for SQLite will allow to be outstanding
at one time.  If attempts are made to allocate more memory than specified
by the soft heap limit, then SQLite will first attempt to free cache
memory before continuing with the allocation request.  The soft heap
limit mechanism only works if [memory statistics] are enabled and
it works best
if the SQLite library is compiled with the [SQLITE_ENABLE_MEMORY_MANAGEMENT]
compile-time option.</p>

<p>The soft heap limit is "soft" in this sense:  If SQLite is not able
to free up enough auxiliary memory to stay below the limit, it goes
ahead and allocates the extra memory and exceeds its limit.  This occurs
under the theory that it is better to use additional memory than to fail
outright.</p>

<p>As of SQLite [version 3.6.1] ([dateof:3.6.1]), 
the soft heap limit only applies to the
general-purpose memory allocator.  The soft heap limit does not know
about or interact with the [scratch memory allocator],