OSSP mm - Shared Memory Allocation

OSSP mm 1.3.0 (07-Mar-2003)

 #include "mm.h"

Global Malloc-Replacement API

 int     MM_create(size_t size, const char *file);
 int     MM_permission(mode_t mode, uid_t owner, gid_t group);
 void    MM_destroy(void);
 int     MM_lock(mm_lock_mode mode);
 int     MM_unlock(void);
 void   *MM_malloc(size_t size);
 void   *MM_realloc(void *ptr, size_t size);
 void    MM_free(void *ptr);
 void   *MM_calloc(size_t number, size_t size);
 char   *MM_strdup(const char *str);
 size_t  MM_sizeof(void *ptr);
 size_t  MM_maxsize(void);
 size_t  MM_available(void);
 char   *MM_error(void);

Standard Malloc-Style API

 MM     *mm_create(size_t size, char *file);
 int     mm_permission(MM *mm, mode_t mode, uid_t owner, gid_t group);
 void    mm_destroy(MM *mm);
 int     mm_lock(MM *mm, mm_lock_mode mode);
 int     mm_unlock(MM *mm);
 void   *mm_malloc(MM *mm, size_t size);
 void   *mm_realloc(MM *mm, void *ptr, size_t size);
 void    mm_free(MM *mm, void *ptr);
 void   *mm_calloc(MM *mm, size_t number, size_t size);
 char   *mm_strdup(MM *mm, const char *str);
 size_t  mm_sizeof(void *ptr);
 size_t  mm_maxsize(void);
 size_t  mm_available(MM *mm);
 char   *mm_error(void);
 void    mm_display_info(MM *mm);

Low-level Shared Memory API

 void   *mm_core_create(size_t size, char *file);
 int     mm_core_permission(void *core, mode_t mode, uid_t owner, gid_t group);
 void    mm_core_delete(void *core);
 int     mm_core_lock(void *core, mm_lock_mode mode);
 int     mm_core_unlock(void *core);
 size_t  mm_core_size(void *core);
 size_t  mm_core_maxsegsize(void);
 size_t  mm_core_align2page(size_t size);
 size_t  mm_core_align2click(size_t size);

Internal Library API

 void    mm_lib_error_set(unsigned int, const char *str);
 char   *mm_lib_error_get(void);
 int     mm_lib_version(void);

The OSSP mm library is a 2-layer abstraction library which simplifies the usage of shared memory between forked (and this way strongly related) processes under Unix platforms. On the first (lower) layer it hides all platform dependent implementation details (allocation and locking) when dealing with shared memory segments and on the second (higher) layer it provides a high-level malloc(3)-style API for a convenient and well known way to work with data-structures inside those shared memory segments.

The abbreviation OSSP mm is historically and originally comes from the phrase ``memory mapped'' as used by the POSIX.1 mmap(2) function. Because this facility is internally used by this library on most platforms to establish the shared memory segments.

This library is structured into three main APIs which are internally based on each other:

Global Malloc-Replacement API

This is the most high-level API which directly can be used as replacement API for the POSIX.1 memory allocation API (malloc(2) and friends). This is useful when converting heap based data structures to shared memory based data structures without the need to change the code dramatically. All which is needed is to prefix the POSIX.1 memory allocation functions with `CWMM_', i.e. `CWmalloc' becomes `CWMM_malloc', `CWstrdup' becomes `CWMM_strdup', etc. This API internally uses just a global `CWMM *' pool for calling the corresponding functions (those with prefix `CWmm_') of the Standard Malloc-Style API.

Standard Malloc-Style API

This is the standard high-level memory allocation API. Its interface is similar to the Global Malloc-Replacement API but it uses an explicit `CWMM *' pool to operate on. That is why every function of this API has an argument of type `CWMM *' as its first argument. This API provides a comfortable way to work with small dynamically allocated shared memory chunks inside large statically allocated shared memory segments. It is internally based on the Low-Level Shared Memory API for creating the underlaying shared memory segment.

Low-Level Shared Memory API

This is the basis of the whole OSSP mm library. It provides low-level functions for creating shared memory segments with mutual exclusion (in short mutex) capabilities in a portable way. Internally the shared memory and mutex facility is implemented in various platform-dependent ways. A list of implementation variants follows under the next topic.

Internally the shared memory facility is implemented in various platform-dependent ways. Each way has its own advantages and disadvantages (in addition to the fact that some variants aren't available at all on some platforms). The OSSP mm library's configuration procedure tries hard to make a good decision. The implemented variants are now given for overview and background reasons with their advantages and disadvantages and in an ascending order, i.e. the OSSP mm configuration mechanism chooses the last available one in the list as the preferred variant.

Classical mmap(2) on temporary file (MMFILE)

Advantage: maximum portable. Disadvantage: needs a temporary file on the filesystem.

mmap(2) via POSIX.1 shm_open(3) on temporary file (MMPOSX)

Advantage: standardized by POSIX.1 and theoretically portable. Disadvantage: needs a temporary file on the filesystem and is is usually not available on existing Unix platform. Advantage: widely available and mostly portable on SVR4 platforms. Disadvantage: needs the CW/dev/zero device and a mmap(2) which supports memory mapping through this device.

SysV IPC shmget(2) (IPCSHM)

Advantage: does not need a temporary file or external device. Disadvantage: although available on mostly all modern Unix platforms, it has strong restrictions like the maximum size of a single shared memory segment (can be as small as 100KB, but depends on the platform). Advantage: does not need a temporary file or external device. Disadvantage: usually only available on BSD platforms and derivatives.

As for the shared memory facility, internally the locking facility is implemented in various platform-dependent ways. They are again listed in ascending order, i.e. the OSSP mm configuration mechanism chooses the last available one in the list as the preferred variant. The list of implemented variants is:

4.2BSD-style flock(2) on temporary file (FLOCK)

Advantage: exists on a lot of platforms, especially on older Unix derivates. Disadvantage: needs a temporary file on the filesystem and has to re-open file-descriptors to it in each(!) fork(2)'ed child process.

SysV IPC semget(2) (IPCSEM)

Advantage: exists on a lot of platforms and does not need a temporary file. Disadvantage: an unmeant termination of the application leads to a semaphore leak because the facility does not allow a ``remove in advance'' trick (as the IPC shared memory facility does) for safe cleanups.

SVR4-style fcntl(2) on temporary file (FCNTL)

Advantage: exists on a lot of platforms and is also the most powerful variant (although not always the fastest one). Disadvantage: needs a temporary file.

The memory allocation strategy the Standard Malloc-Style API functions use internally is the following:

Allocation

If a chunk of memory has to be allocated, the internal list of free chunks is searched for a minimal-size chunk which is larger or equal than the size of the to be allocated chunk (a best fit strategy). If a chunk is found which matches this best-fit criteria, but is still a lot larger than the requested size, it is split into two chunks: One with exactly the requested size (which is the resulting chunk given back) and one with the remaining size (which is immediately re-inserted into the list of free chunks). If no fitting chunk is found at all in the list of free chunks, a new one is created from the spare area of the shared memory segment until the segment is full (in which case an out of memory error occurs).

Deallocation

If a chunk of memory has to be deallocated, it is inserted in sorted manner into the internal list of free chunks. The insertion operation automatically merges the chunk with a previous and/or a next free chunk if possible, i.e. if the free chunks stay physically seamless (one after another) in memory, to automatically form larger free chunks out of smaller ones. This way the shared memory segment is automatically defragmented when memory is deallocated.

This strategy reduces memory waste and fragmentation caused by small and frequent allocations and deallocations to a minimum.

The internal implementation of the list of free chunks is not specially optimized (for instance by using binary search trees or even splay trees, etc), because it is assumed that the total amount of entries in the list of free chunks is always small (caused both by the fact that shared memory segments are usually a lot smaller than heaps and the fact that we always defragment by merging the free chunks if possible).

In the following, all API functions are described in detail The order directly follows the one in the SYNOPSIS section above.

int MM_create(size_t size, const char *file);

This initializes the global shared memory pool with size and file and has to be called before any fork(2) operations are performed by the application.

int MM_permission(mode_t mode, uid_t owner, gid_t group);

This sets the filesystem mode, owner and group for the global shared memory pool (has effects only if the underlaying shared memory segment implementation is actually based on external auxiliary files). The arguments are directly passed through to chmod(2) and chown(2).

void MM_destroy(void);

This destroys the global shared memory pool and should be called after all child processes were killed.

int MM_lock(mm_lock_mode mode);

This locks the global shared memory pool for the current process in order to perform either shared/read-only (mode is CWMM_LOCK_RD) or exclusive/read-write (mode is CWMM_LOCK_RW) critical operations inside the global shared memory pool.

int MM_unlock(void);

This unlocks the global shared memory pool for the current process after the critical operations were performed inside the global shared memory pool.

void *MM_malloc(size_t size);

Identical to the POSIX.1 malloc(3) function but instead of allocating memory from the heap it allocates it from the global shared memory pool.

void MM_free(void *ptr);

Identical to the POSIX.1 free(3) function but instead of deallocating memory in the heap it deallocates it in the global shared memory pool.

void *MM_realloc(void *ptr, size_t size);

Identical to the POSIX.1 realloc(3) function but instead of reallocating memory in the heap it reallocates it inside the global shared memory pool.

void *MM_calloc(size_t number, size_t size);

Identical to the POSIX.1 calloc(3) function but instead of allocating and initializing memory from the heap it allocates and initializes it from the global shared memory pool.

char *MM_strdup(const char *str);

Identical to the POSIX.1 strdup(3) function but instead of creating the string copy in the heap it creates it in the global shared memory pool.

size_t MM_sizeof(const void *ptr);

This function returns the size in bytes of the chunk starting at ptr when ptr was previously allocated with MM_malloc(3). The result is undefined if ptr was not previously allocated with MM_malloc(3).

size_t MM_maxsize(void);

This function returns the maximum size which is allowed as the first argument to the MM_create(3) function.

size_t MM_available(void);

Returns the amount in bytes of still available (free) memory in the global shared memory pool.

char *MM_error(void);

Returns the last error message which occurred inside the OSSP mm library.

MM *mm_create(size_t size, const char *file);

This creates a shared memory pool which has space for approximately a total of size bytes with the help of file. Here file is a filesystem path to a file which need not to exist (and perhaps is never created because this depends on the platform and chosen shared memory and mutex implementation). The return value is a pointer to a CWMM structure which should be treated as opaque by the application. It describes the internals of the created shared memory pool. In case of an error CWNULL is returned. A size of 0 means to allocate the maximum allowed size which is platform dependent and is between a few KB and the soft limit of 64MB.

int mm_permission(MM *mm, mode_t mode, uid_t owner, gid_t group);

This sets the filesystem mode, owner and group for the shared memory pool mm (has effects only when the underlaying shared memory segment implementation is actually based on external auxiliary files). The arguments are directly passed through to chmod(2) and chown(2).

void mm_destroy(MM *mm);

This destroys the complete shared memory pool mm and with it all chunks which were allocated in this pool. Additionally any created files on the filesystem corresponding the to shared memory pool are unlinked.

int mm_lock(MM *mm, mm_lock_mode mode);

This locks the shared memory pool mm for the current process in order to perform either shared/read-only (mode is CWMM_LOCK_RD) or exclusive/read-write (mode is CWMM_LOCK_RW) critical operations inside the global shared memory pool.

int mm_unlock(MM *mm);

This unlocks the shared memory pool mm for the current process after critical operations were performed inside the global shared memory pool.

void *mm_malloc(MM *mm, size_t size);

This function allocates size bytes from the shared memory pool mm and returns either a (virtual memory word aligned) pointer to it or CWNULL in case of an error (out of memory). It behaves like the POSIX.1 malloc(3) function but instead of allocating memory from the heap it allocates it from the shared memory segment underlaying mm.

void mm_free(MM *mm, void *ptr);

This deallocates the chunk starting at ptr in the shared memory pool mm. It behaves like the POSIX.1 free(3) function but instead of deallocating memory from the heap it deallocates it from the shared memory segment underlaying mm.

void *mm_realloc(MM *mm, void *ptr, size_t size);

This function reallocates the chunk starting at ptr inside the shared memory pool mm with the new size of size bytes. It behaves like the POSIX.1 realloc(3) function but instead of reallocating memory in the heap it reallocates it in the shared memory segment underlaying mm.

void *mm_calloc(MM *mm, size_t number, size_t size);

This is similar to mm_malloc(3), but additionally clears the chunk. It behaves like the POSIX.1 calloc(3) function. It allocates space for number objects, each size bytes in length from the shared memory pool mm. The result is identical to calling mm_malloc(3) with an argument of ``number * size'', with the exception that the allocated memory is initialized to nul bytes.

char *mm_strdup(MM *mm, const char *str);

This function behaves like the POSIX.1 strdup(3) function. It allocates sufficient memory inside the shared memory pool mm for a copy of the string str, does the copy, and returns a pointer to it. The pointer may subsequently be used as an argument to the function mm_free(3). If insufficient shared memory is available, CWNULL is returned.

size_t mm_sizeof(const void *ptr);

This function returns the size in bytes of the chunk starting at ptr when ptr was previously allocated with mm_malloc(3). The result is undefined when ptr was not previously allocated with mm_malloc(3).

size_t mm_maxsize(void);

This function returns the maximum size which is allowed as the first argument to the mm_create(3) function.

size_t mm_available(MM *mm);

Returns the amount in bytes of still available (free) memory in the shared memory pool mm.

char *mm_error(void);

Returns the last error message which occurred inside the OSSP mm library.

void mm_display_info(MM *mm);

This is debugging function which displays a summary page for the shared memory pool mm describing various internal sizes and counters.

void *mm_core_create(size_t size, const char *file);

This creates a shared memory area which is at least size bytes in size with the help of file. The value size has to be greater than 0 and less or equal the value returned by mm_core_maxsegsize(3). Here file is a filesystem path to a file which need not to exist (and perhaps is never created because this depends on the platform and chosen shared memory and mutex implementation). The return value is either a (virtual memory word aligned) pointer to the shared memory segment or CWNULL in case of an error. The application is guaranteed to be able to access the shared memory segment from byte 0 to byte size-1 starting at the returned address.

int mm_core_permission(void *core, mode_t mode, uid_t owner, gid_t group);

This sets the filesystem mode, owner and group for the shared memory segment code (has effects only when the underlaying shared memory segment implementation is actually based on external auxiliary files). The arguments are directly passed through to chmod(2) and chown(2).

void mm_core_delete(void *core);

This deletes a shared memory segment core (as previously returned by a mm_core_create(3) call). After this operation, accessing the segment starting at core is no longer allowed and will usually lead to a segmentation fault.

int mm_core_lock(const void *core, mm_lock_mode mode);

This function acquires an advisory lock for the current process on the shared memory segment core for either shared/read-only (mode is CWMM_LOCK_RD) or exclusive/read-write (mode is CWMM_LOCK_RW) critical operations between fork(2)'ed child processes.

int mm_core_unlock(const void *core);

This function releases a previously acquired advisory lock for the current process on the shared memory segment core.

size_t mm_core_size(const void *core);

This returns the size in bytes of core. This size is exactly the size which was used for creating the shared memory area via mm_core_create(3). The function is provided just for convenience reasons to not require the application to remember the memory size behind core itself.

size_t mm_core_maxsegsize(void);

This returns the number of bytes of a maximum-size shared memory segment which is allowed to allocate via the MM library. It is between a few KB and the soft limit of 64MB.

size_t mm_core_align2page(size_t size);

This is just a utility function which can be used to align the number size to the next virtual memory page boundary used by the underlaying platform. The memory page boundary under Unix platforms is usually somewhere between 2048 and 16384 bytes. You do not have to align the size arguments of other OSSP mm library functions yourself, because this is already done internally. This function is exported by the OSSP mm library just for convenience reasons in case an application wants to perform similar calculations for other purposes.

size_t mm_core_align2word(size_t size);

This is another utility function which can be used to align the number size to the next virtual memory word boundary used by the underlaying platform. The memory word boundary under Unix platforms is usually somewhere between 4 and 16 bytes. You do not have to align the size arguments of other OSSP mm library functions yourself, because this is already done internally. This function is exported by the OSSP mm library just for convenience reasons in case an application wants to perform simular calculations for other purposes.

void mm_lib_error_set(unsigned int, const char *str);

This is a function which is used internally by the various MM function to set an error string. It's usually not called directly from applications.

char *mm_lib_error_get(void);

This is a function which is used internally by MM_error(3) and mm_error(3) functions to get the current error string. It is usually not called directly from applications.

int mm_lib_version(void);

This function returns a hex-value ``0xVRRTLL'' which describes the current OSSP mm library version. V is the version, RR the revisions, LL the level and T the type of the level (alphalevel=0, betalevel=1, patchlevel=2, etc). For instance OSSP mm version 1.0.4 is encoded as 0x100204. The reason for this unusual mapping is that this way the version number is steadily increasing.

The maximum size of a continuous shared memory segment one can allocate depends on the underlaying platform. This cannot be changed, of course. But currently the high-level malloc(3)-style API just uses a single shared memory segment as the underlaying data structure for an CWMM object which means that the maximum amount of memory an CWMM object represents also depends on the platform.

This could be changed in later versions by allowing at least the high-level malloc(3)-style API to internally use multiple shared memory segments to form the CWMM object. This way CWMM objects could have arbitrary sizes, although the maximum size of an allocatable continous chunk still is bounded by the maximum size of a shared memory segment.

mm-config(1).

malloc(3), calloc(3), realloc(3), strdup(3), free(3), mmap(2), shmget(2), shmctl(2), flock(2), fcntl(2), semget(2), semctl(2), semop(2).

http://www.ossp.org/pkg/lib/mm/

This library was originally written in January 1999 by Ralf S. Engelschall <rse@engelschall.com> for use in the Extended API (EAPI) of the Apache HTTP server project (see http://www.apache.org/), which was originally invented for mod_ssl (see http://www.modssl.org/).

Its base idea (a malloc-style API for handling shared memory) was originally derived from the non-publically available mm_malloc library written in October 1997 by Charles Randall <crandall@matchlogic.com> for MatchLogic, Inc.

In 2000 this library joined the OSSP project where all other software development projects of Ralf S. Engelschall are located.

 Ralf S. Engelschall
 rse@engelschall.com
 www.engelschall.com