What is slab in Linux kernel?
What Is a Slab Page? Slab allocation is a form of memory management, within the Linux kernel, used with the intention of making memory allocation of objects efficient. This type of memory management reduces fragmentation caused by allocations and deallocations.
What is SLUB in kernel?
SLUB (the unqueued slab allocator) is a memory management mechanism intended for the efficient memory allocation of kernel objects which displays the desirable property of eliminating fragmentation caused by allocations and deallocations.
What is slab and SLUB?
The SLOB (simple list of blocks) allocator is one of three available memory allocators in the Linux kernel. The other two are SLAB (slab allocator) and SLUB. The SLOB allocator is designed to require little memory for the implementation and housekeeping, for use in small systems such as embedded systems.
What is Slub_debug?
SLUB can enable debugging only for selected slabs in order to avoid an impact on overall system performance which may make a bug more difficult to find. In order to switch debugging on one can add an option “slub_debug” to the kernel command line. That will enable full debugging for all slabs.
What is a slab memory?
A slab is a set of one or more contiguous pages of memory set aside by the slab allocator for an individual cache. This memory is further divided into equal segments the size of the object type that the cache is managing.
What is a slab in OS?
Slabs. A slab is the amount by which a cache can grow or shrink. It represents one memory allocation to the cache from the machine, and whose size is customarily a multiple of the page size.
What are the differences between slab allocator and slub allocator?
Slub is simpler than Slab. SLOB (Simple List Of Blocks) is a memory allocator optimized for embedded systems with very little memory—on the order of megabytes. It applies a very simple first-fit algorithm on a list of blocks, not unlike the old K&R-style heap allocator.
How does the slub allocator work?
The SLOB Allocator It is primarily used in small embedded systems where memory is expensive, and SLOB on a whole, uses very little memory in its implementation. It works by using a first-fit type of algorithm. This is where it places the object in the first possible place in memory which it will fit.
What is kernel memory in OS?
Kernel memory is the memory used by the Windows kernel. It includes memory used by core components of Windows along with any device drivers. Typically, the number will be very small, in the hundreds of megabytes.
What is kernel memory allocation?
The kernel should allocate memory dynamically to other kernel subsystems and to user processes. The KMA (kernel memory allocation) API usually consists of two functions: void* malloc(size_t nbytes); void free(void* ptr); There are various issues associated with allocation of memory.
What is slab allocation in OS?
What is Fibonacci buddy system?
A Fibonacci buddy system is a system in which blocks are divided into sizes which are Fibonacci numbers. It satisfies the following relation: Zi = Z(i-1)+Z(i-2) The original procedure for the Fibonacci buddy system was either limited to a small, fixed number of block sizes or a time-consuming computation.
What is Linux Kmalloc?
kmalloc is the normal method of allocating memory for objects smaller than page size in the kernel. The flags argument may be one of: GFP_USER – Allocate memory on behalf of user. May sleep.
Is kernel a RAM?
The kernel memory in the task manager is a part of the total memory available in a computer that is blocked off for the operating system’s processes. The total memory consists of the RAM (random-access memory) and the virtual memory.
What is true for Slab in terms of allocating kernel memory?
A second strategy for allocating kernel memory is known as slab allocation. It eliminates fragmentation caused by allocations and deallocations. This method is used to retain allocated memory that contains a data object of a certain type for reuse upon subsequent allocations of objects of the same type.
What is kernel data structure?
The kernel stores and organizes a lot of information. So it has data about which processes are running in the system, their memory requirements, files in use etc. To handle all this, three important structures are used. These are process table, file table and v node/ i node information.
Why buddy algorithm is fast?
The buddy memory allocation system is implemented with the use of a binary tree to represent used or unused split memory blocks. The buddy system is very fast to allocate or deallocate memory. In buddy systems, the cost to allocate and free a block of memory is low compared to that of best-fit or first-fit algorithms.
What is slab allocator OS?
The slab allocator has three principle aims: The allocation of small blocks of memory to help eliminate internal fragmentation that would be otherwise caused by the buddy system; The caching of commonly used objects so that the system does not waste time allocating, initialising and destroying objects.
What is Kmalloc and Vmalloc?
The kmalloc() function guarantees that the pages are physically contiguous (and virtually contiguous). The vmalloc() function works in a similar fashion to kmalloc() , except it allocates memory that is only virtually contiguous and not necessarily physically contiguous.
What is Copy_to_user and Copy_from_user?
copy_to_user. Copies a block of data from the kernel to user space. copy_from_user. Copies a block of data from user space to the kernel.
Where kernel is stored?
All of kernel memory and user process memory is stored in physical memory in the computer (or perhaps on disk if data has been swapped from memory). The key to answering the rest of your questions is to understand the difference between physical memory and virtual memory.
What is slabs in memory?
A slab is the amount by which a cache can grow or shrink. It represents one memory allocation to the cache from the machine, and whose size is customarily a multiple of the page size.
How does slab allocation work?
The slab allocation algorithm uses caches to store kernel objects. When a cache is created a number of objects which are initially marked as free are allocated to the cache. The number of objects in the cache depends on size of the associated slab.
How to determine the slab and cache an object belongs to?
Today, the slab and cache an object belongs to is determined by the struct pageand kmem_bufctl_tis simply an integer array of object indices. The number of elements in the array is the same as the number of objects on the slab.
What is the Linux slab allocator?
It is a slab allocator which is very similar in many respects to the general kernel allocator used in Solaris [MM01]. Linux’s implementation is heavily based on the first slab allocator paper by Bonwick [Bon94] with many improvements that bear a close resemblance to those described in his later paper [BA01].
How to calculate the number of objects in a slab?
The calculation is quite basic and takes the following steps Initialise wastageto be the total size of the slab i.e. PAGE_SIZEgfp_order; Subtract the amount of space required to store the slab descriptor; Count up the number of objects that may be stored.
When a slab is created what happens to its initialised state?
When a slab is created, all the objects in it are put in an initialised state. If a constructor is available, it is called for each object and it is expected that objects are left in an initialised state upon free.