• Installing/Configuring
  • Runtime Configuration

  • Runtime Configuration
  • Runtime Configuration

    Runtime Configuration

    The behaviour of these functions is affected by
    settings in php.ini.

    Although the default APCu settings are fine for
    many installations, serious users should consider tuning the
    following parameters.

    There is one decision to be made configuring APCu.
    How much memory is going to be allocated to APCu. The ini directive
    that controls this is apc.shm_size Read the sections on
    this carefully below.

    Once the server is running, the apc.php
    script that is bundled with the extension should be copied
    somewhere into the docroot and viewed with a browser as it provides
    a detailed analysis of the internal workings of APCu. If GD is
    enabled in PHP, it will even display some interesting graphs. If
    APCu is working, the Cache full count number (on the left)
    will display the number of times the cache has reached maximum
    capacity and has had to forcefully clean any entries that haven’t
    been accessed in the last apc.ttl seconds. This number is
    minimized in a well-configured cache. If the cache is constantly
    being filled, and thusly forcefully freed, the resulting churning
    will have disparaging effects on script performance. The easiest
    way to minimize this number is to allocate more memory for

    When APCu is compiled with mmap support (Memory
    Mapping), it will use only one memory segment, unlike when APCu is
    built with SHM (SysV Shared Memory) support that uses multiple
    memory segments. MMAP does not have a maximum limit like SHM does
    in /proc/sys/kernel/shmmax. In general MMAP support is
    recommeded because it will reclaim the memory faster when the
    webserver is restarted and all in all reduces memory allocation
    impact at startup.

    APCu configuration options
    Name Default Changeable Changelog
    apc.enabled “1” PHP_INI_SYSTEM  
    apc.shm_segments “1” PHP_INI_SYSTEM  
    apc.shm_size “32M” PHP_INI_SYSTEM  
    apc.entries_hint “4096” PHP_INI_SYSTEM  
    apc.ttl “0” PHP_INI_SYSTEM  
    apc.gc_ttl “3600” PHP_INI_SYSTEM  
    apc.mmap_file_mask NULL PHP_INI_SYSTEM  
    apc.slam_defense “1” PHP_INI_SYSTEM  
    apc.enable_cli “0” PHP_INI_SYSTEM  
    apc.use_request_time “1” PHP_INI_ALL  
    apc.serializer “default” PHP_INI_SYSTEM  
    apc.coredump_unmap “0” PHP_INI_SYSTEM  
    apc.preload_path NULL PHP_INI_SYSTEM  

    For further details and definitions of the PHP_INI_* modes, see the
    Where a
    configuration setting may be set

    Here’s a short explanation of the configuration

    apc.enabled boolean

    apc.enabled can be set to 0 to disable
    APC. This is primarily useful when APC is statically compiled into
    PHP, since there is no other way to disable it (when compiled as a
    DSO, the extension line in php.ini can just be

    apc.shm_segments integer

    The number of shared memory segments to allocate
    for the compiler cache. If APC is running out of shared memory but
    apc.shm_size is set as high as the system allows, raising
    this value might prevent APC from exhausting its memory.

    apc.shm_size string

    The size of each shared memory segment given by a
    shorthand notation as described in this
    . By default, some systems (including most BSD variants)
    have very low limits on the size of a shared memory segment.

    apc.entries_hint integer

    A “hint” about the number of distinct variables
    that might be stored. Set to zero or omit if not sure.


    The number of seconds a cache entry is allowed to
    idle in a slot in case this cache entry slot is needed by another
    entry. Leaving this at zero means that APC’s cache could
    potentially fill up with stale entries while newer entries won’t be
    cached. In the event of a cache running out of available memory,
    the cache will be completely expunged if ttl is equal to 0.
    Otherwise, if the ttl is greater than 0, APC will attempt to remove
    expired entries.


    The number of seconds that a cache entry may remain
    on the garbage-collection list. This value provides a fail-safe in
    the event that a server process dies while executing a cached
    source file; if that source file is modified, the memory allocated
    for the old version will not be reclaimed until this TTL reached.
    Set to zero to disable this feature.

    apc.mmap_file_mask string

    If compiled with MMAP support by using
    –enable-mmap this is the mktemp-style file_mask to pass
    to the mmap module for determining whether your mmap’ed memory
    region is going to be file-backed or shared memory backed. For
    straight file-backed mmap, set it to something like
    /tmp/apc.XXXXXX (exactly 6 Xs). To use
    POSIX-style shm_open/mmap put a .shm somewhere in your
    mask. e.g. /apc.shm.XXXXXX You can also set it to
    /dev/zero to use your kernel’s /dev/zero
    interface to anonymous mmap’ed memory. Leaving it undefined will
    force an anonymous mmap.

    apc.slam_defense integer

    On very busy servers whenever you start the server
    or modify files you can create a race of many processes all trying
    to cache the same file at the same time. This option sets the
    percentage of processes that will skip trying to cache an uncached
    file. Or think of it as the probability of a single process to skip
    caching. For example, setting apc.slam_defense to
    75 would mean that there is a 75% chance that the process
    will not cache an uncached file. So, the higher the setting the
    greater the defense against cache slams. Setting this to 0
    disables this feature.

    apc.enable_cli integer

    Mostly for testing and debugging. Setting this
    enables APC for the CLI version of PHP. Under normal circumstances,
    it is not ideal to create, populate and destroy the APC cache on
    every CLI request, but for various test scenarios it is useful to
    be able to enable APC for the CLI version of PHP easily.

    apc.serializer string

    Used to configure APC to use a third party

    apc.coredump_unmap boolean

    Enables APC handling of signals, such as SIGSEGV,
    that write core files when signaled. When these signals are
    received, APC will attempt to unmap the shared memory segment in
    order to exclude it from the core file. This setting may improve
    system stability when fatal signals are received and a large APC
    shared memory segment is configured.


    This feature is potentially dangerous. Unmapping
    the shared memory segment in a fatal signal handler may cause
    undefined behaviour if a fatal error occurs.


    Although some kernels may provide a facility to
    ignore various types of shared memory when generating a core dump
    file, these implementations may also ignore important shared memory
    segments such as the Apache scoreboard.

    apc.preload_path string

    Optionally, set a path to the directory that APC
    will load cache data at startup.

    apc.use_request_time bool

    Use the SAPI request
    start time for TTL.