Runtime Configuration

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

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

There are two primary decisions to be made configuring APC. First, how much memory is going to be allocated to APC; and second, whether APC will check if a file has been modified on every request. The two ini directives that control these settings are apc.shm_size and apc.stat, respectively. Read the sections on these two directive 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 APC. If GD is enabled in PHP, it will even display some interesting graphs. The first thing to ensure, of course, is that it is actually caching files. If APC 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 APC. Barring that, the apc.filters ought to be used to cache fewer scripts.

When APC is compiled with mmap support (Memory Mapping), it will use only one memory segment, unlike when APC 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.

APC configuration options
Name Default Changeable Changelog
apc.enabled "1" PHP_INI_SYSTEM PHP_INI_SYSTEM in APC 2. PHP_INI_ALL in APC <= 3.0.12.
apc.shm_segments "1" PHP_INI_SYSTEM  
apc.shm_size "32M" PHP_INI_SYSTEM  
apc.optimization "0" PHP_INI_ALL PHP_INI_SYSTEM in APC 2. Removed in APC 3.0.13.
apc.num_files_hint "1000" PHP_INI_SYSTEM  
apc.user_entries_hint "4096" PHP_INI_SYSTEM Available since APC 3.0.0.
apc.ttl "0" PHP_INI_SYSTEM Available since APC 3.0.0.
apc.user_ttl "0" PHP_INI_SYSTEM Available since APC 3.0.0.
apc.gc_ttl "3600" PHP_INI_SYSTEM  
apc.cache_by_default "1" PHP_INI_ALL PHP_INI_SYSTEM in APC <= 3.0.12. Available since APC 3.0.0.
apc.filters NULL PHP_INI_SYSTEM  
apc.mmap_file_mask NULL PHP_INI_SYSTEM  
apc.slam_defense "0" PHP_INI_SYSTEM Available since APC 3.0.0.
apc.file_update_protection "2" PHP_INI_SYSTEM Available since APC 3.0.6.
apc.enable_cli "0" PHP_INI_SYSTEM Available since APC 3.0.7.
apc.max_file_size "1M" PHP_INI_SYSTEM Available since APC 3.0.7.
apc.use_request_time "1" PHP_INI_ALL Available since APC 3.1.3.
apc.stat "1" PHP_INI_SYSTEM Available since APC 3.0.10.
apc.write_lock "1" PHP_INI_SYSTEM Available since APC 3.0.11.
apc.report_autofilter "0" PHP_INI_SYSTEM Available since APC 3.0.11.
apc.include_once_override "0" PHP_INI_SYSTEM Available since APC 3.0.12.
apc.rfc1867 "0" PHP_INI_SYSTEM Available since APC 3.0.13.
apc.rfc1867_prefix "upload_" PHP_INI_SYSTEM  
apc.rfc1867_freq "0" PHP_INI_SYSTEM  
apc.rfc1867_ttl "3600" PHP_INI_SYSTEM Available since APC 3.1.1.
apc.localcache "0" PHP_INI_SYSTEM Available since APC 3.0.14.
apc.localcache.size "512" PHP_INI_SYSTEM Available since APC 3.0.14.
apc.coredump_unmap "0" PHP_INI_SYSTEM Available since APC 3.0.16.
apc.stat_ctime "0" PHP_INI_SYSTEM Available since APC 3.0.13.
apc.preload_path NULL PHP_INI_SYSTEM Available since APC 3.1.1.
apc.file_md5 "0" PHP_INI_SYSTEM Available since APC 3.1.1.
apc.canonicalize "1" PHP_INI_SYSTEM Available since APC 3.1.1.
apc.lazy_functions 0 PHP_INI_SYSTEM Available since APC 3.1.3.
apc.lazy_classes 0 PHP_INI_SYSTEM Available since APC 3.1.3.
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 directives.

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 commented-out).

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 integer

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

apc.optimization integer

The optimization level. Zero disables the optimizer, and higher values use more aggressive optimizations. Expect very modest speed improvements. This is experimental.

apc.num_files_hint integer

A "hint" about the number of distinct source files that will be included or requested on your web server. Set to zero or omit if unsure; this setting is mainly useful for sites that have many thousands of source files.

apc.user_entries_hint integer

Just like apc.num_files_hint, a "hint" about the number of distinct user cache variables to store. Set to zero or omit if not sure.

apc.ttl integer

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.

apc.user_ttl integer

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.

apc.gc_ttl integer

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.cache_by_default boolean

On by default, but can be set to off and used in conjunction with positive apc.filters so that files are only cached if matched by a positive filter.

apc.filters string

A comma-separated list of POSIX extended regular expressions. If any pattern matches the source filename, the file will not be cached. Note that the filename used for matching is the one passed to include/require, not the absolute path. If the first character of the expression is a + then the expression will be additive in the sense that any files matched by the expression will be cached, and if the first character is a - then anything matched will not be cached. The - case is the default, so it can be left off.

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.

Deprecated by apc.write_lock.

apc.file_update_protection integer

When a file is modified on a live web server it really ought to be done in an atomic manner. That is, written to a temporary file and renamed (mv) the file into its permanent position when it is ready. Many text editors, cp, tar and other such programs don't do this. This means that there is a chance that a file is accessed (and cached) while it is still being written to. This apc.file_update_protection setting puts a delay on caching brand new files. The default is 2 seconds, which means that if the modification timestamp (mtime) on a file shows that it is less than 2 seconds old when it is accessed, it will not be cached. The unfortunate person who accessed this half-written file will still see weirdness, but at least it won't persist. If all of the webserver's files are atomically updated, via some method like rsync (which updates correctly), this protection can be disabled by setting this directive to 0. If the system is flooded with i/o and some update procedures are taking longer than 2 seconds, this setting should be increased to enable the protection on those slower update operations.

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.max_file_size integer

Prevent files larger than this value from getting cached. Defaults to 1M.

apc.stat integer

Be careful changing this setting. This defaults to on, forcing APC to stat (check) the script on each request to determine if it has been modified. If it has been modified it will recompile and cache the new version. If this setting is off, APC will not check, which usually means that to force APC to recheck files, the web server will have to be restarted or the cache will have to be manually cleared. Note that FastCGI web server configurations may not clear the cache on restart. On a production server where the script files rarely change, a significant performance boost can be achieved by disabled stats.

For included/required files this option applies as well, but note that for relative path includes (any path that doesn't start with / on Unix) APC has to check in order to uniquely identify the file. If you use absolute path includes APC can skip the stat and use that absolute path as the unique identifier for the file.

apc.write_lock boolean

On busy servers, when the web server is first started, or when many files have been modified at the same time, APC may try to compile and cache the same file multiple times. Write_lock guarantees that only one process will attempt to compile and cache an uncached script. The other processes attempting to use the script will run without using the opcode cache, rather than locking and waiting for the cache to prime.

apc.report_autofilter boolean

Logs any scripts that were automatically excluded from being cached due to early/late binding issues.

apc.include_once_override boolean

Optimize include_once() and require_once() calls and avoid the expensive system calls used.

apc.rfc1867 boolean

RFC1867 File Upload Progress hook handler is only available if APC was compiled against PHP 5.2.0 or later. When enabled, any file uploads which includes a field called APC_UPLOAD_PROGRESS before the file field in an upload form will cause APC to automatically create an upload_key user cache entry where key is the value of the APC_UPLOAD_PROGRESS form entry.

Note that the hidden field specified by APC_UPLOAD_PROGRESS must come before the file field, otherwise the upload progress will not work correctly.

Note that the file upload tracking is not threadsafe at this point, so new uploads that happen while a previous one is still going will disable the tracking for the previous.

Example #1 An apc.rfc1867 example


The above example will output something similar to:

    [total] => 1142543
    [current] => 1142543
    [rate] => 1828068.8
    [filename] => test
    [name] => file
    [temp_filename] => /tmp/php8F
    [cancel_upload] => 0
    [done] => 1

apc.rfc1867_prefix string

Key prefix to use for the user cache entry generated by rfc1867 upload progress functionality.

apc.rfc1867_name string

Specify the hidden form entry name that activates APC upload progress and specifies the user cache key suffix.

apc.rfc1867_freq string

The frequency that updates should be made to the user cache entry for upload progress. This can take the form of a percentage of the total file size or a size in bytes optionally suffixed with "k", "m", or "g" for kilobytes, megabytes, or gigabytes respectively (case insensitive). A setting of 0 updates as often as possible, which may cause slower uploads.

apc.rfc1867_ttl bool

TTL for rfc1867 entries.

apc.localcache boolean

This enables a lock-free local process shadow-cache which reduces lock contention when the cache is being written to.

apc.localcache.size integer

The size of the local process shadow-cache, should be set to a sufficiently large value, approximately half of apc.num_files_hint.

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.stat_ctime integer

Verification with ctime will avoid problems caused by programs such as svn or rsync by making sure inodes haven't changed since the last stat. APC will normally only check mtime.

apc.canonicalize bool

If on, then relative paths are canonicalized in no-stat mode.

apc.preload_path string

apc.use_request_time bool

Use the SAPI request start time for TTL.

apc.file_md5 bool

Records a md5 hash of files.

apc.lazy_functions integer

Enables lazy loading for functions.

apc.lazy_classes integer

Enables lazy loading for classes.

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