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Shmop is an easy to use set of functions that allows PHP to read, write, create and delete Unix shared memory segments.
Accessing shared memory objects via the filesystem On Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, normally mounted under /dev/shm. Since kernel 2.6. 19, Linux supports the use of access control lists (ACLs) to control the permissions of objects in the virtual filesystem.
How shared memory is implemented in Linux?
As with all System V IPC objects, access to shared memory areas is controlled via keys and access rights checking. Once the memory is being shared, there are no checks on how the processes are using it. They must rely on other mechanisms, for example System V semaphores, to synchronize access to the memory.
How do I use shared memory between processes?
To use shared memory, we have to perform 2 basic steps:
- Request to the operating system a memory segment that can be shared between processes.
- Associate a part of that memory or the whole memory with the address space of the calling process.
What is SHM cache?
A caching library for storing data in the operating system’s shared memory. Data is persisted across multiple runs of a PHP script. The cache has a FIFO queue which starts removing oldest items once the cache gets full.
Why is shared memory the fastest form of IPC? Once the memory is mapped into the address space of the processes that are sharing the memory region, processes do not execute any system calls into the kernel in passing data between processes, which would otherwise be required.
Why is shared memory faster?
Shared memory is faster because the data is not copied from one address space to another, memory allocation is done only once, andsyncronisation is up to the processes sharing the memory.
How do I write to a shared memory?
Steps : Use ftok to convert a pathname and a project identifier to a System V IPC key. Use shmget which allocates a shared memory segment. Use shmat to attache the shared memory segment identified by shmid to the address space of the calling process.
Shared memory is an efficient means of passing data between programs. Depending on context, programs may run on a single processor or on multiple separate processors. Using memory for communication inside a single program, e.g. among its multiple threads, is also referred to as shared memory.
Disadvantages
- Generally slower to access than non-distributed shared memory.
- Must provide additional protection against simultaneous accesses to shared data.
- May incur a performance penalty.
- Little programmer control over actual messages being generated.
Why do you have to worry about cache coherence if you are using shared memory?
To reduce memory latency, each processor often has its own cache, which contains copies of items from the global memory. Since many caches may have copies of the same item, shared-memory systems maintain cache coherence by regulating how global memory contents are fetched and updated.
What is Memcache PHP?
Memcached is an object caching system. PHP asks Memcached for the page’s data via a Memcached extension. If the data is cached, it is sent back to PHP. If it isn’t cached, Memcached sends the query to the database, returns the data to PHP, and stores it for the next request.