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runtime: update malloc.go documentation
The big documentation comment at the top of malloc.go has gotten woefully out of date. Update it. Change-Id: Ibdb1bdcfdd707a6dc9db79d0633a36a28882301b Reviewed-on: https://go-review.googlesource.com/29731 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com> Reviewed-by: Rick Hudson <rlh@golang.org>
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Memory allocator, based on tcmalloc.
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// Memory allocator.
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//
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// This was originally based on tcmalloc, but has diverged quite a bit.
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// http://goog-perftools.sourceforge.net/doc/tcmalloc.html
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// The main allocator works in runs of pages.
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// Small allocation sizes (up to and including 32 kB) are
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// rounded to one of about 100 size classes, each of which
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// has its own free list of objects of exactly that size.
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// rounded to one of about 70 size classes, each of which
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// has its own free set of objects of exactly that size.
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// Any free page of memory can be split into a set of objects
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// of one size class, which are then managed using free list
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// allocators.
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// of one size class, which are then managed using a free bitmap.
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//
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// The allocator's data structures are:
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//
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// FixAlloc: a free-list allocator for fixed-size objects,
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// fixalloc: a free-list allocator for fixed-size off-heap objects,
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// used to manage storage used by the allocator.
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// MHeap: the malloc heap, managed at page (4096-byte) granularity.
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// MSpan: a run of pages managed by the MHeap.
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// MCentral: a shared free list for a given size class.
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// MCache: a per-thread (in Go, per-P) cache for small objects.
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// MStats: allocation statistics.
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// mheap: the malloc heap, managed at page (8192-byte) granularity.
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// mspan: a run of pages managed by the mheap.
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// mcentral: collects all spans of a given size class.
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// mcache: a per-P cache of mspans with free space.
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// mstats: allocation statistics.
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//
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// Allocating a small object proceeds up a hierarchy of caches:
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//
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// 1. Round the size up to one of the small size classes
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// and look in the corresponding MCache free list.
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// If the list is not empty, allocate an object from it.
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// and look in the corresponding mspan in this P's mcache.
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// Scan the mspan's free bitmap to find a free slot.
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// If there is a free slot, allocate it.
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// This can all be done without acquiring a lock.
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//
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// 2. If the MCache free list is empty, replenish it by
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// taking a bunch of objects from the MCentral free list.
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// Moving a bunch amortizes the cost of acquiring the MCentral lock.
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// 2. If the mspan has no free slots, obtain a new mspan
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// from the mcentral's list of mspans of the required size
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// class that have free space.
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// Obtaining a whole span amortizes the cost of locking
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// the mcentral.
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//
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// 3. If the MCentral free list is empty, replenish it by
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// allocating a run of pages from the MHeap and then
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// chopping that memory into objects of the given size.
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// Allocating many objects amortizes the cost of locking
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// the heap.
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// 3. If the mcentral's mspan list is empty, obtain a run
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// of pages from the mheap to use for the mspan.
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//
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// 4. If the MHeap is empty or has no page runs large enough,
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// 4. If the mheap is empty or has no page runs large enough,
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// allocate a new group of pages (at least 1MB) from the
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// operating system. Allocating a large run of pages
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// operating system. Allocating a large run of pages
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// amortizes the cost of talking to the operating system.
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//
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// Freeing a small object proceeds up the same hierarchy:
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// Sweeping an mspan and freeing objects on it proceeds up a similar
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// hierarchy:
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//
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// 1. Look up the size class for the object and add it to
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// the MCache free list.
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// 1. If the mspan is being swept in response to allocation, it
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// is returned to the mcache to satisfy the allocation.
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//
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// 2. If the MCache free list is too long or the MCache has
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// too much memory, return some to the MCentral free lists.
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// 2. Otherwise, if the mspan still has allocated objects in it,
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// it is placed on the mcentral free list for the mspan's size
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// class.
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//
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// 3. If all the objects in a given span have returned to
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// the MCentral list, return that span to the page heap.
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// 3. Otherwise, if all objects in the mspan are free, the mspan
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// is now "idle", so it is returned to the mheap and no longer
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// has a size class.
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// This may coalesce it with adjacent idle mspans.
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//
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// 4. If the heap has too much memory, return some to the
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// operating system.
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// 4. If an mspan remains idle for long enough, return its pages
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// to the operating system.
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//
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// TODO(rsc): Step 4 is not implemented.
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// Allocating and freeing a large object uses the mheap
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// directly, bypassing the mcache and mcentral.
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//
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// Allocating and freeing a large object uses the page heap
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// directly, bypassing the MCache and MCentral free lists.
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// Free object slots in an mspan are zeroed only if mspan.needzero is
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// false. If needzero is true, objects are zeroed as they are
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// allocated. There are various benefits to delaying zeroing this way:
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//
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// The small objects on the MCache and MCentral free lists
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// may or may not be zeroed. They are zeroed if and only if
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// the second word of the object is zero. A span in the
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// page heap is zeroed unless s->needzero is set. When a span
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// is allocated to break into small objects, it is zeroed if needed
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// and s->needzero is set. There are two main benefits to delaying the
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// zeroing this way:
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// 1. Stack frame allocation can avoid zeroing altogether.
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//
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// 1. stack frames allocated from the small object lists
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// or the page heap can avoid zeroing altogether.
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// 2. the cost of zeroing when reusing a small object is
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// charged to the mutator, not the garbage collector.
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// 2. It exhibits better temporal locality, since the program is
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// probably about to write to the memory.
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//
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// 3. We don't zero pages that never get reused.
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package runtime
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