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.
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 after being removed or invalidated.
Entries are eligible for removal if their reference count is zero,
or they exceed this time limit.
If set to 0, time-based cleanup is disabled,
and entries are only removed when their reference count drops to zero.
-
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.
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.