1 /* Copyright (c) 2014. The SimGrid Team.
2 * All rights reserved. */
4 /* This program is free software; you can redistribute it and/or modify it
5 * under the terms of the license (GNU LGPL) which comes with this package. */
11 #include <boost/utility.hpp>
12 #include <boost/unordered_map.hpp>
13 #include <boost/unordered_set.hpp>
17 #include "mc_private.h"
20 #ifndef MC_PAGE_SNAPSHOT_H
21 #define MC_PAGE_SNAPSHOT_H
23 /** @brief Storage for snapshot memory pages
25 * The first (lower) layer of the per-page snapshot mechanism is a page
26 * store: it's responsibility is to store immutable shareable
27 * reference-counted memory pages independently of the snapshoting
28 * logic. Snapshot management and representation, soft-dirty tracking is
29 * handled to an higher layer. READMORE
33 * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
34 * region holding the memory pages (the address of the first page).
36 * We want to keep this memory region aligned on the memory pages (so
37 * that we might be able to create non-linear memory mappings on those
38 * pages in the future) and be able to expand it without coyping the
39 * data (there will be a lot of pages here): we will be able to
40 * efficiently expand the memory mapping using `mremap()`, moving it
41 * to another virtual address if necessary.
43 * Because we will move this memory mapping on the virtual address
44 * space, only the index of the page will be stored in the snapshots
45 * and the page will always be looked up by going through `memory`:
47 * void* page = (char*) page_store->memory + page_index << pagebits;
49 * * The number of pages mapped in virtual memory (`capacity_`). Once all
50 * those pages are used, we need to expand the page store with
53 * * A reference count for each memory page `page_counts_`. Each time a
54 * snapshot references a page, the counter is incremented. If a
55 * snapshot is freed, the reference count is decremented. When the
56 * reference count, of a page reaches 0 it is added to a list of available
57 * pages (`free_pages_`).
59 * * A list of free pages `free_pages_` which can be reused. This avoids having
60 * to scan the reference count list to find a free page.
62 * * When we are expanding the memory map we do not want to add thousand of page
63 * to the `free_pages_` list and remove them just afterwards. The `top_index_`
64 * field is an index after which all pages are free and are not in the `free_pages_`
67 * * When we are adding a page, we need to check if a page with the same
68 * content is already in the page store in order to reuse it. For this
69 * reason, we maintain an index (`hash_index_`) mapping the hash of a
70 * page to the list of page indices with this hash.
71 * We use a fast (non cryptographic) hash so there may be conflicts:
72 * we must be able to store multiple indices for the same hash.
75 struct s_mc_pages_store {
77 typedef uint64_t hash_type;
78 typedef boost ::unordered_set<size_t> page_set_type;
79 typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
84 /** Number of available pages in virtual memory */
86 /** Top of the used pages (index of the next available page) */
88 /** Page reference count */
89 std::vector<uint64_t> page_counts_;
90 /** Index of available pages before the top */
91 std::vector<size_t> free_pages_;
92 /** Index from page hash to page index */
93 pages_map_type hash_index_;
96 void resize(size_t size);
98 void remove_page(size_t pageno);
100 public: // Constructors
101 explicit s_mc_pages_store(size_t size);
106 /** @brief Decrement the reference count for a given page
108 * Decrement the reference count of this page. Used when a snapshot is
111 * If the reference count reaches zero, the page is recycled:
112 * it is added to the `free_pages_` list and removed from the `hash_index_`.
115 void unref_page(size_t pageno) {
116 if ((--this->page_counts_[pageno]) == 0) {
117 this->remove_page(pageno);
121 /** @brief Increment the refcount for a given page
123 * This method used to increase a reference count of a page if we know
124 * that the content of a page is the same as a page already in the page
127 * This will be the case if a page if soft clean: we know that is has not
128 * changed since the previous cnapshot/restoration and we can avoid
129 * hashing the page, comparing byte-per-byte to candidates.
131 void ref_page(size_t pageno) {
132 ++this->page_counts_[pageno];
135 /** @brief Store a page in the page store */
136 size_t store_page(void* page);
138 /** @brief Get a page from its page number
140 * @param Number of the memory page in the store
141 * @return Start of the page
143 const void* get_page(size_t pageno) const {
144 return mc_page_from_number(this->memory_, pageno);
147 public: // Debug/test methods
149 /** @brief Get the number of references for a page */
150 size_t get_ref(size_t pageno) {
151 return this->page_counts_[pageno];
154 /** @brief Get the number of used pages */
156 return this->top_index_ - this->free_pages_.size();
159 /** @brief Get the capacity of the page store
161 * The capacity is expanded by a system call (mremap).
164 return this->capacity_;