Memory Allocation Mechanisms in AIX

Memory allocation mechanisms in AIXSkill Level: Intermediate Uma Chandolu ([email protected]) Senior Staff Software Engineer IBM Gargi Srinivas ([email protected]) System Software Engineer IBM 21 Sep 2010 Memory management is one of the most important responsibilities of an operating system. It allocates portions of memory to programs at their request and frees it for reuse when no longer needed. This article describes different allocation algorithms which are available in AIX® for memory management and their features. Introduction An allocation policy refers to the set of data structures and algorithms used to represent and manage the heap in terms of implementing the memory management functions of allocating, freeing and reallocating memory dynamically. AIX malloc subsystem supports the following allocation policies: • Default (Yorktown) allocation policy • Watson allocation policy • Malloc 3.1 allocation policy Overview of default allocation policy The default allocation policy in AIX is also called Yorktown policy. It maintains the Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 1 of 12 Figure 1. • Since it is maintained as a tree with varying sized nodes. It is the most widely used and most stable of all allocation policies. The nodes are arranged left-to-right by address (increasing address to the right) and top-to-bottom by length (such that no child is larger than its parent). Cartesian binary search tree for Yorktown policy When to use Yorktown • Yorktown is enabled by default on AIX. type the following command: export MALLOCTYPE=Yorktown It can be verified for enablement in two ways: 1. Using DBX sub command malloc Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 2 of 12 .developerWorks® ibm. and therefore is suitable for almost all cases.com/developerWorks free space in heap as nodes in a Cartesian binary search tree format. there are no limitations on the allocation block sizes. To enable Yorktown. The required block size is then returned back to the caller. 2. alignment). Overview of Watson allocation policy The Watson allocation policy maintains free space in the heap as nodes in two separate "red-black trees": 1.ibm. Yorktown free algorithm Freeing of memory involves inserting the freed node back to the free tree.com/developerWorks developerWorks® 2. the node from the free tree with the lowest address (which is greater than or equal to the requested block size) is removed. and the new block is returned to the caller. A new block of the requested size is then allocated. because a lot of free nodes. The data is moved from the original block to the new block. the block is split and the extra block is returned to the free tree with the required size block being returned to the caller. Sorted by address Sorted by size Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 3 of 12 . it is joined to that node. where: • n equals number of bytes requested by the user • prefix_size equals metadata information First. This follows the standard root insertion algorithm. The old block is freed and is no longer valid. the original block is returned to the free tree so that any possible joining can occur with the adjoining nodes. Running command echo $MALLOCTYPE Yorktown allocation algorithm Total memory allocated = roundup (n + prefix_size. it is broken into two pieces. If this is larger than what is wanted. would be scattered in the tree and will not be available for contiguous allocation. even though they are adjacent with respect to addresses. and if it adjoins any other node in terms of the address. The extra broken block is returned back to the tree and is inserted in the appropriate place in the free tree. Yorktown reallocation algorithm If the size of the reallocated block turns out larger than the original block. This joining of adjacent nodes prevents fragmentation to some extent. Each node in the tree is searched in comparison to the node being freed. If the size of the reallocated block is smaller than the original block. Red-black tree used in Watson allocation policy based on "Sorted by Size" Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 4 of 12 . Figure 3. Figure 2.developerWorks® ibm.com/developerWorks Both trees have to be maintained properly for consistency and correctness. Red-black tree used in Watson allocation policy based on "Sorted by Address" Every node in the address tree will have lesser address nodes in its left subtree and higher address nodes in its right subtree. then the process heap is expanded using the sbrk() system call. a block of the expanded size is added to the size and address trees. and allocation continues as before. it provides more efficient tree operations like insertion and searching as compared to other policies. the block is removed from both the size and the address tree and then returned to the caller. If a block of sufficient size is not found in the free tree. Watson free algorithm Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 5 of 12 . To enable it. When to use Watson Since Watson maintains its space in a red-black tree. If the block found in the size tree is exactly the required size.ibm. type the following command: export MALLOCTYPE=Watson Watson allocation algorithm Similar to the Yorktown policy.com/developerWorks developerWorks® Every node in the size tree will have lesser size nodes in its left subtree and greater size nodes in the right subtree. Watson searches the size tree and finds the smallest possible block suitable for the request. The tree is traversed to check if the node adjoins any other node in the tree with respect to the address. If the size of the reallocated block is smaller than the original block. The size of the block is calculated using the following formula (i identifies the bucket number): size = 2 ^ (i + 4) Figure 4. the data is moved from the original block to the new block. the heap memory is maintained as a hash bucket where each bucket points to a linked list having nodes or blocks of only a certain specific size.developerWorks® ibm. the block is split and the remainder is returned to the free tree after sending the required amount to the caller. Heap memory as a hash bucket in malloc 3. Adjoining of nodes in this manner prevents fragmentation of memory in heap. Overview of malloc 3. the freed node is joined with the node in the tree.1. the original block is returned to the free tree so that any possible coalescence can occur with the adjoining nodes. If so.1 allocation policy In malloc 3.1 Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 6 of 12 . and the new block is returned to the caller. Watson reallocation algorithm If the size of the reallocated block would be larger than the original block.com/developerWorks The node is returned to the address tree first at the root. A new block of the requested size is then allocated. then bucket = (log(needed)/log(2) rounded down to the nearest integer) .ibm. use the following command: export MALLOCTYPE=3.1 for 32 bit programs export MALLOCTYPE=3.3 Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 7 of 12 .1 allocation algorithm A block is allocated from the free pool by first converting the requested bytes to an index in the bucket array.com/developerWorks developerWorks® To enable malloc. then bucket = 0 If needed >16.1_64BIT for 64 bit programs Malloc 3. using the following equation: needed = requested + 8 If needed <= 16. the block is freed.1 Free algorithm When a block of memory is returned to the free pool. and it is not recommended for use in most cases. This policy rounds off the size of the allocation request to the next highest available block. memory is allocated using the sbrk() subroutine to add blocks to the list. let's assume that there is a program which incorrectly overwrites the number of bytes it has been allocated by malloc. It may function correctly when using the malloc 3. If the needed size happens to be greater than the existing block. The same program can and should fail when it runs with different allocation policy. Therefore. In such cases. Malloc 3.developerWorks® ibm. If the list in the bucket is null.1 may have allocated more memory than it requested. the new block size can fall into the same bucket as that of the original block.1 allocation policy. we can be allocated more memory than we want. Because of the wide difference among the sizes handled by each bucket. The block to be freed is then added to the head of the free list for that bucket.1 reallocation algorithm During reallocation. the requested size is compared against the existing size of the allocated block. Additional malloc options Some additional malloc options are: • Malloc multiheap Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 8 of 12 . However.1 may perform better than the other allocation algorithms by reducing the overhead of data movement during a reallocation. 3. in essence. If the free list is not empty. the block at the head of the list is returned to the caller. a new block is allocated from a new bucket.1 policy because. Malloc 3. the same block is returned. the bucket index is calculated just like it is done with allocation. 3. Sometimes application programs may depend unknowingly on the side effects of the malloc 3.com/developerWorks The size of each block in the linked list pointed by the bucket is block size = 2 ^ (bucket + 4). in some cases. For example. because it have been initially allocated more memory than it actually requested. Malloc 3.1 limitations The malloc allocation policy is less efficient than the default. and the data is moved from the old block to the new block. The next block on the list then becomes the new head. com/developerWorks developerWorks® • Pool allocation • Malloc buckets Malloc multiheap By default. Pool allocation creates Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 9 of 12 . • Multiheap option is not supported for malloc 3. this option can be useful. • Since the considersize option requires some extra processing. unnecessary enabling of malloc multiheap can cause severe fragmentation. In such cases. use the following: MALLOCOPTIONS=[multiheap:n] | [considersize] The options are as described: • multiheap:n The maximum number of heaps available to malloc multiheap is 32. In multithreaded enviornments multiple threads access the same heap. malloc multiheap selects the next available heap.1 and user defined allocation policies. Malloc multiheap limitations • Since a single heap is actually divided into many heaps (up to 32). the other threads will have to wait for their turn to access the heap. because when one thread is being serviced and has taken the heap lock. • considersize By default. In this scenario using a single heap is not very efficient. If the considersize option is specified. Pool allocation When memory size is less than or equal to 512 bytes. You can specify "n" number of heaps between 1 to 32 for malloc to use.ibm. malloc multiheap will use a different algorithm and select the heap which has enough free space to handle the request. because the available memory is fragmented and is not available contiguously. the malloc subsystem treats the entire process heap as a single entity. using this option may slow down the process to some extent. It can also result in a situation where the system will seem to have run out of memory. To enable the multiheap option. AIX malloc provides a more efficient frontend to the actual backend functions of malloc. Pool allocation does not work effectively if one thread of the application allocates memory and other thread frees the memory. Conclusion This article covers the essence of the various AIX memory allocation schemes and their salient features. use the following: MALLOCOPTIONS=pool<:max_size> This basically creates a fixed number of pools with increasing size. malloc buckets is more commonly used.developerWorks® ibm.number_of_buckets:n Only allocation requests up to 512 bytes can be serviced by the bucket allocator. To enable malloc buckets. Having an overall idea about the various allocation schemes can help you decide on the allocation scheme that best suits your application at hand in terms of execution efficiency and performance. It can be used only along with Yorktown malloc policy. and any greater requests are automatically forwarded to the Yorktown malloc. Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 10 of 12 . At present. There can be 128 pools for a 32-bit application and 64 pools for a 64-bit application. To enable pool allocation. Malloc buckets Malloc buckets is an extension to the Yorktown allocation policy.com/developerWorks for each thread its own malloc pool which avoids lock contention with other threads. the maximum pool size per process for this allocator is limited to 512MB. use the following: MALLOCOPTIONS=buckets. When an application requires a large number of small allocation requests. Connect with other developerWorks users while exploring the developer-driven blogs. • Get involved in the My developerWorks community.com/developerWorks developerWorks® Resources Learn • The AIX InfoCenter provides more information on system memory allocation using malloc subsystem. • Stay current with developerWorks technical events and webcasts focused on a variety of IBM products and IT industry topics. and wikis. • Watch developerWorks on-demand demos ranging from product installation and setup demos for beginners. to advanced functionality for experienced developers. try a product online. forums. schemas. 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Chandolu works as a Development Support Specialist on AIX.ibm. He has experience interfacing with customers and handling customer-critical situations.ibm. He has five years of extensive hands-on experience in AIX environments and demonstrated expertise in AIX system administration and other subsystems. Gargi Srinivas Gargi Srinivas works as an AIX L3 developer for IBM. You can reach her at gargi.com Memory allocation mechanisms in AIX © Copyright IBM Corporation 2010 Trademarks Page 12 of 12 .developerWorks® ibm.com.com/developerWorks About the authors Uma Chandolu Uma M. She has three years of experience on AIX user space components. He has been recognized as an IBM developerWorks contributing author.srinivas@in.