CascadiaPHP 2024

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 evict entries to free up memory. During eviction, if apc.ttl was specified, APCu will first attempt to remove expired entries, i.e. entries whose TTL has either expired, or entries that have no TTL set and haven't been accessed in the last apc.ttl seconds. If apc.ttl was not set, or removing expired entries did not free up enough space, APCu will clear the entire cache.

The number of evictions should be minimal 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 APCu.

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 recommended 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 INI_SYSTEM  
apc.shm_segments 1 INI_SYSTEM  
apc.shm_size "32M" INI_SYSTEM  
apc.entries_hint 4096 INI_SYSTEM  
apc.ttl 0 INI_SYSTEM  
apc.gc_ttl 3600 INI_SYSTEM  
apc.mmap_file_mask NULL INI_SYSTEM  
apc.slam_defense 0 INI_SYSTEM  
apc.enable_cli 0 INI_SYSTEM  
apc.use_request_time 0 INI_ALL Prior to APCu 5.1.19, the default was 1.
apc.serializer "php" INI_SYSTEM Prior to APCu 5.1.15, the default was "default".
apc.coredump_unmap 0 INI_SYSTEM  
apc.preload_path NULL INI_SYSTEM  
For further details and definitions of the INI_* modes, see the Where a configuration setting may be set.

Here's a short explanation of the configuration directives.

apc.enabled bool

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 commented-out).

apc.shm_segments int

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 FAQ. By default, some systems (including most BSD variants) have very low limits on the size of a shared memory segment.

apc.entries_hint int

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

apc.ttl int

Consider cache entries without an explicit TTL to be expired if they have not been accessed in this many seconds. Effectively, this allows such entries to be removed opportunistically during a cache insert, or prior to a full expunge. Note that because removal is opportunistic, entries can still be readable even if they are older than apc.ttl seconds. This setting has no effect on cache entries that have an explicit TTL specified.

apc.gc_ttl int

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 bool

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. Setting apc.slam_defense to 1 can help prevent multiple processes from caching the same file simultaneously by introducing a probability mechanism. If the same key is attempted to be cached within a short period by different processes, it skips the caching for the current process to mitigate potential cache slams.

apc.enable_cli int

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 serializer.

apc.coredump_unmap bool

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.

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