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OSv malloc - English version

Introduction to OSv malloc.

It's writtein in English.

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OSv malloc - English version

  1. 1. OSv malloc(English ver) Satoru Takeuchi <satoru.takeuchi@gmail.com>
  2. 2. Introduction ● This article is for OSv Advent Calendar [1] day 21 ● Prerequisite ○ The basic knowledge about OS/OSv [2] ○ The knowledge about C++ 11 specification ○ The basic knowledge about algorithm and data structure ○ It”s better If you’re familiar with Linux kernel source ● Source tree: The latest master branch [3] at Dec 21, 2014 ○ HEAD commit: f7d3ddd648b38789daa8287626a66863a780f139 ○ For simplicity、omit debug feature, trace feature and exclusive control mechanism ● Pathnames in this document are the relative path from the top of OSv source tree
  3. 3. Summary ● Defined in core/mempool.c: Less than 2K lines ○ The implementation is very simple ○ A comment says ”Our malloc() is very coarse” :-) ● Specialized to small object (I guess it’s for JVM) ● One malloc() code is used for both kernel and user apps: It’s OSv way! ○ It’s like calling kmalloc() directly from user application in linux kernel ● Allocating size is aligned to 2^n >=8 (n is a positive integer) ○ e.g) malloc(4) returns a 8byte memory object
  4. 4. The structure of memory management subsystem ● Use the different mechanisms depend on claiming size ○ <= 1KiB: mempool ○ > 1KiB: page allocator user or kernel mempool (It ‘s like sl[auo]b allocator in Linux kernel) memory management subsystem page allocator (It’s like buddy allocator in Linux kernel) malloc()/free()
  5. 5. Simplified call trace malloc(size) - > std_malloc(size, align) # Useb by other memory allocation APIs(*1) Align size to 2^n >= 8 byte (n is a positive integer) if (size <= 1KiB && after SMP initialization(always true for applications)) malloc_pools[lg(n)].alloc() # Allocate from mempool else if (1KiB < size <= 4KiB) memory::alloc_page() # Allocate one page from page allocator else malloc_large() # Allocate pages from page allocator # Use the most complex logic among # the three ways here (Omit to explain here) *1) If you explicitly set alignment, call trace becomes more complex. For more information, please refer to the implementation of std_malloc()
  6. 6. Allocating an object <= 1KiB ● Use mempool as I described before ○ Managed by class malloc_pool (inherit from class pool) ● Definition: malloc_pool malloc_pools[] ○ One pool for each object size ■ # of this array is lg(page size)+1. In x86_64 (its page size is 4KiB), it’s 12+1=13 ■ Each pool corresponds to allocation size, 1,2,4,8,...,4KiB,8KiB ● I wonder why last entry corresponds to 8KiB exists... ○ malloc() from applications never use mempool[11,…,(page_size+1)]
  7. 7. mempool ● slab allocator[4] in OSv. Used for small object <= page size ○ Managed by class pool # I consider it’s name is too abstract ● Handle memory objects from 8byte to 1page(4KiB) ○ [8,1KiB) => Multiple objects in one page ○ (1KiB,4KiB] => one object in one page ● Have per-CPU cache for improving scalability in MP system ○ Because of the locality of reference, improving the probability of allocating object which callee CPU recently used ○ Exclusive control is not necessary on memory allocation by mempool ● For more information, please refer to the comment beginning from ” Memory allocation strategy” and source code
  8. 8. page allocator ● Used for page size or more object ○ Managed by “class page_range_allocator” ● Have two level caches, named L1 and L2 ○ L1: per-CPU cache ○ L2: global cache ● Global page allocator: The bottom layer under these two caches kernel subsystem (including mempool) L1 cache (per-CPU cache) L2 cache (Shared among all CPUs) global page allocator page allocator
  9. 9. page allocator: L1 cache ● per-CPU cache ● Managed by “struct l1” ○ Cache up to “l1::max(=512)” pages ● Definition: “l1 percpu_l1[<# of CPUs>]” ● UI: “*_local” don’t synchronous fill/refill pages ○ Page allocation: l1::alloc_page{,_local} ○ Page free: l1::free_page{,_local} ● Interface to L2 cache ○ Async: Use per-CPU thread, “page_pool_l1_<cpu>” ■ # of pages < l1::max*¼ => Fill some pages ■ # of pages > l1::max*¾ => Refill some pages ○ Sync: If # of pages becomes 0 or l1::max, fill/refill some pages
  10. 10. page allocator: L2 cache ● Cache shared among all CPUs ● Managed by class l2 ○ Cache up to “l2::max” pages ● UI (used by mempool): “try_*” don’t synchronously fill/refill pages ○ Multiple page allocation: l2::{try_,}alloc_page_batch ○ Multiple page free: l2::{try_,}free_page_batch ● Interface to global page allocator ○ Async: Use per-CPU thread, “page_pool_l2” ■ # of pages < l2::max*¼ => Fill some pages ■ # of pages > l2::max*¾ => Refill some pages ○ Sync: If # of pages becomes 0 or l2::max, fill/refill some pages
  11. 11. page allocator: global page allocator ● The deepest component in the OSv’s memory management subsystem ● Manage whole free pages in whole system ● Omit to explain it here due to lack of my extra time… ;-(
  12. 12. References 1. OSv Advent Calendar2014 http://qiita.com/advent-calendar/2014/osv 2. OSvのご紹介 in OSC2014 Tokyo/Fall, Takuya ASADA, Cloudius Systems http://www.slideshare.net/syuu1228/osv-in-osc2014-tokyofall 3. The source tree of OSv https://github.com/cloudius-systems/osv 4. slab allocation at Wikipedia http://en.wikipedia.org/wiki/Slab_allocation 5. mallocの旅(Glibc編), こさき@ぬまづ http://www.slideshare.net/kosaki55tea/glibc-malloc
  13. 13. Extra: My impression after reading OSv’s code ● First I tried to just the code of malloc(). However, finally, I read most of memory management code. ○ I forgot that kernel and user apps run on the same memory space ● The source is simple and easy to read (than the giant linux kernel code) ○ There seems to be plenty of room to improve performance ○ Nice code to learn OS ● There is malloc(size, align) which is similar to posix_memalign() ○ Only kernel can use it because it’s not exported to user applications ○ It can be accomplished by C++’s overload feature. Viva C++! ■ When I encountered these two functions, I couldn’t understand why it works since I forgot the overload feature at that time ● My last experience in C++ is 10 years ago (C++89 era)...

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