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
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
recommeded because it will reclaim the memory faster when the
webserver is restarted and all in all reduces memory allocation
impact at startup.

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 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 integer

A “hint” about the number of distinct variables
that might be stored. 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.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.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
serializer.

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.

Warning

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

Note:

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.