1 /* Copyright (c) 2015. 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. */
12 #include <boost/array.hpp>
13 #include <boost/utility.hpp>
14 #include <boost/unordered_map.hpp>
15 #include <boost/unordered_set.hpp>
21 #include "mc_forward.h"
23 #ifndef MC_PAGE_STORE_H
24 #define MC_PAGE_STORE_H
29 /** @brief Storage for snapshot memory pages
31 * The first (lower) layer of the per-page snapshot mechanism is a page
32 * store: it's responsibility is to store immutable shareable
33 * reference-counted memory pages independently of the snapshoting
34 * logic. Snapshot management and representation, soft-dirty tracking is
35 * handled to an higher layer. READMORE
39 * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
40 * region holding the memory pages (the address of the first page).
42 * We want to keep this memory region aligned on the memory pages (so
43 * that we might be able to create non-linear memory mappings on those
44 * pages in the future) and be able to expand it without coyping the
45 * data (there will be a lot of pages here): we will be able to
46 * efficiently expand the memory mapping using `mremap()`, moving it
47 * to another virtual address if necessary.
49 * Because we will move this memory mapping on the virtual address
50 * space, only the index of the page will be stored in the snapshots
51 * and the page will always be looked up by going through `memory`:
53 * void* page = (char*) page_store->memory + page_index << pagebits;
55 * * The number of pages mapped in virtual memory (`capacity_`). Once all
56 * those pages are used, we need to expand the page store with
59 * * A reference count for each memory page `page_counts_`. Each time a
60 * snapshot references a page, the counter is incremented. If a
61 * snapshot is freed, the reference count is decremented. When the
62 * reference count, of a page reaches 0 it is added to a list of available
63 * pages (`free_pages_`).
65 * * A list of free pages `free_pages_` which can be reused. This avoids having
66 * to scan the reference count list to find a free page.
68 * * When we are expanding the memory map we do not want to add thousand of page
69 * to the `free_pages_` list and remove them just afterwards. The `top_index_`
70 * field is an index after which all pages are free and are not in the `free_pages_`
73 * * When we are adding a page, we need to check if a page with the same
74 * content is already in the page store in order to reuse it. For this
75 * reason, we maintain an index (`hash_index_`) mapping the hash of a
76 * page to the list of page indices with this hash.
77 * We use a fast (non cryptographic) hash so there may be conflicts:
78 * we must be able to store multiple indices for the same hash.
83 #ifdef MC_PAGE_STORE_MD4
84 typedef boost::array<uint64_t,2> hash_type;
86 typedef uint64_t hash_type;
89 #ifdef MC_PAGE_STORE_MD4
90 // We are using a secure hash to identify a page.
91 // We assume there will not be any collision: we need to map a hash
92 // to a single page index.
93 typedef boost::unordered_map<hash_type, size_t> pages_map_type;
95 // We are using a cheap hash to index a page.
96 // We should expect collision and we need to associate multiple page indices
98 typedef boost::unordered_set<size_t> page_set_type;
99 typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
105 * mc_page_store_get_page expects that this is the first field.
108 /** Number of available pages in virtual memory */
110 /** Top of the used pages (index of the next available page) */
112 /** Page reference count */
113 std::vector<uint64_t> page_counts_;
114 /** Index of available pages before the top */
115 std::vector<size_t> free_pages_;
116 /** Index from page hash to page index */
117 pages_map_type hash_index_;
120 void resize(size_t size);
122 void remove_page(size_t pageno);
124 public: // Constructors
125 PageStore(PageStore const&) = delete;
126 PageStore& operator=(PageStore const&) = delete;
127 explicit PageStore(size_t size);
132 /** @brief Decrement the reference count for a given page
134 * Decrement the reference count of this page. Used when a snapshot is
137 * If the reference count reaches zero, the page is recycled:
138 * it is added to the `free_pages_` list and removed from the `hash_index_`.
141 void unref_page(size_t pageno);
143 /** @brief Increment the refcount for a given page
145 * This method used to increase a reference count of a page if we know
146 * that the content of a page is the same as a page already in the page
149 * This will be the case if a page if soft clean: we know that is has not
150 * changed since the previous cnapshot/restoration and we can avoid
151 * hashing the page, comparing byte-per-byte to candidates.
153 void ref_page(size_t pageno);
155 /** @brief Store a page in the page store */
156 size_t store_page(void* page);
158 /** @brief Get a page from its page number
160 * @param Number of the memory page in the store
161 * @return Start of the page
163 const void* get_page(size_t pageno) const;
165 public: // Debug/test methods
167 /** @brief Get the number of references for a page */
168 size_t get_ref(size_t pageno);
170 /** @brief Get the number of used pages */
173 /** @brief Get the capacity of the page store
175 * The capacity is expanded by a system call (mremap).
181 inline __attribute__((always_inline))
182 void PageStore::unref_page(size_t pageno) {
183 if ((--this->page_counts_[pageno]) == 0) {
184 this->remove_page(pageno);
188 inline __attribute__((always_inline))
189 void PageStore::ref_page(size_t pageno) {
190 ++this->page_counts_[pageno];
193 inline __attribute__((always_inline))
194 const void* PageStore::get_page(size_t pageno) const {
195 return mc_page_from_number(this->memory_, pageno);
198 inline __attribute__((always_inline))
199 size_t PageStore::get_ref(size_t pageno) {
200 return this->page_counts_[pageno];
203 inline __attribute__((always_inline))
204 size_t PageStore::size() {
205 return this->top_index_ - this->free_pages_.size();
208 inline __attribute__((always_inline))
209 size_t PageStore::capacity() {
210 return this->capacity_;