These kalloc.n zones provide memory quickly by making it an off-the-shelf commodity. There are also a few, such as kalloc.48 and kalloc.80, which are additional to that power-of-two rule. These exist – in 64-bit macOS at least – in sizes which are powers of 2, such as kalloc.16 (the smallest), kalloc.256, up to kalloc.16384 (largest). When kalloc() is used, that memory will be allocated from one of the kalloc.n zones. Once the Mach zones have been set up, the kernel and its extensions may use calls such as zalloc() and kalloc() which provide fast access to memory. mcache and others, which support BSD’s Mcaches.maps and other map zones, used by kernel VM mapping.kalloc.16 to kalloc.16384, which are kernel allocations made by kalloc() calls.ipc.ports and others, which support InterProcess Communication.APFS_BTREE_NODES and others, which are used by the APFS file system. HFS.node and others, which are used for the HFS+ file system.buf.512 to buf.16384, which are buffers for the VFS file system.Which lists the zone name, the size of the memory blocks it contains, its current total size and the maximum, the current number of elements and the maximum, and more.Īmong the more important and prominent Mach zones are: Each has a name, and can be inspected using the command Typically, in Catalina there will be more than 300 Mach zones once the system is up and running. macOS provides a special mechanism for doing this: Mach zones, which are similar to what are known as memory caches in Linux, or Pools in Windows.ĭuring kernel boot, many standard Mach zones are created and set up ready for the kernel and its extensions to use, and others can be created later when required. Doing so requires that they can be allocated and deallocated memory very quickly. The kernel and its more than 300 extensions are kept extremely busy handling all the essential tasks that user processes expect. When a user process wants some memory, it asks to be allocated some by the kernel using a call such as malloc() when a kernel extension wants some memory allocated to it, it uses a different call such as zalloc() or kalloc(), which isn’t available to user processes, so that the memory is allocated from that reserved for kernel mode processes. This is reflected in one of the central tasks performed by the kernel, managing memory. MacOS consists of two major divisions: the kernel and its extensions which run in kernel mode, and user processes which are run in user mode even if they’re running as root user. In this article, I try to explain how the kernel manages memory for itself and its extensions in Mach zones, and how a memory leak within those can cause a panic. The bug in macOS Catalina 10.15.6 which can result in kernel panics isn’t simple to grok.
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