memory management

April 28, 2012 — Leave a comment

the performance of most applications depends on the efficiency that memory is managed by the operating system. because memory is not only used by applications but also by the kernel itself for storing its data and structures some portions of the memory are reserved by the kernel. the rest of the memory is called the dynamic memory.

memory gets addressed by the kernel in pages which are typically 4kb in size. you can check the pagesize in linux with the following command:

getconf PAGESIZE
4096

to track the status of all the pages there is a so called page descriptor. as pages may be used by several processes, the kernel itself or not used at all this descriptor contains the information the kernel needs when dealing with all the pages. one of the fields of the pages descriptor is called “lru” which stands for? same as in the oracle database: least recently used. the lru contains pointers to the least recently used double linked list of pages. remember: the same concepts, over and over again.

another fields describes the status of a page, which, for example, can be:

  • locked
  • dirty
  • active

note that oracle uses the word “dirty” in exactly the same manner: the page has been modified ( in oracle syntax it is a buffer )

when there is a request for allocating a page two things can happen:

  1. there is enough free space and the request is successful immediately
  2. before the allocation request may succeed some cleanup work must be done ( which usually blocks the request until finished ). you can compare this to the “buffer wait” events in oracle

because some critical requests can not be blocked there are exceptions to the second case ( for example when handling interrupts ). in these cases the request will be atomic and fail if no free pages are available. to minimize the chance for failing requests some pages are reserved by the kernel for these atomic requests. the amount of pages which will be reserved is calculated at system initialization and can be changed later by modifying a file in the proc filesystem:

cat /proc/sys/vm/min_free_kbytes
67584

oracle uses a similar concept for the shared_pool_reserved_size. this parameter defines some portion of the shared_pool to be reserved for large contiguous memory allocations. the goal is the same as with the linux kernel: try to block others as less as possible and make sure the allocation request succeeds.

as time goes by and requests for memory allocations come and go, memory will be allocated and released. this leads to a common issue with memory management: fragmentation. frequent allocations and releases may lead to situations that although there is enough free memory for a request the request will fail. this is because the remaining free memory is scattered through already allocated pages ( internal fragmentation ) or there is no free contiguous free memory that can satisfy the request ( external fragmentation ). and that is whats happening when you face the “ORA-04031: unable to allocate x bytes of shared memory” in the alertlog.

memory fragmentation

while internal fragmentation is waste of memory external fragmentation may lead to failing allocation requests. to avoid external fragmentation as much as possible the linux kernel groups the free pages into lists of 1,2,4,8,16,32,64,128,256,512 and 1024 contiguous chunks. if, for example, a request for 128 of contiguous memory page frames arrives the kernel will first check the 128 list for a free block. if a free block exists the memory gets allocated. if no free block exists in that list the next bigger list ( the 256 ) will be check for free blocks. if a free block exists there the kernel allocates 128 from the 256 page frames and inserts the remaining 128 page frames to the 128 list. this, if no free block is found in the next bigger list, will continue until the last group is reached ( the 1024 list ) and if this list is empty an error will be signaled.
the other way around the kernel tries to merge free blocks into bigger blocks when memory is released ( if the blocks have the same size and are located next to each other ).

there is much more to say about memory management ( e.g. slabs ), but this will be a topic for another post ….

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