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. */
12 #include <boost/utility.hpp>
13 #include <boost/unordered_map.hpp>
14 #include <boost/unordered_set.hpp>
21 #ifndef MC_PAGE_STORE_H
22 #define MC_PAGE_STORE_H
24 struct s_mc_pages_store;
28 /** @brief Storage for snapshot memory pages
30 * The first (lower) layer of the per-page snapshot mechanism is a page
31 * store: it's responsibility is to store immutable shareable
32 * reference-counted memory pages independently of the snapshoting
33 * logic. Snapshot management and representation, soft-dirty tracking is
34 * handled to an higher layer. READMORE
38 * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
39 * region holding the memory pages (the address of the first page).
41 * We want to keep this memory region aligned on the memory pages (so
42 * that we might be able to create non-linear memory mappings on those
43 * pages in the future) and be able to expand it without coyping the
44 * data (there will be a lot of pages here): we will be able to
45 * efficiently expand the memory mapping using `mremap()`, moving it
46 * to another virtual address if necessary.
48 * Because we will move this memory mapping on the virtual address
49 * space, only the index of the page will be stored in the snapshots
50 * and the page will always be looked up by going through `memory`:
52 * void* page = (char*) page_store->memory + page_index << pagebits;
54 * * The number of pages mapped in virtual memory (`capacity_`). Once all
55 * those pages are used, we need to expand the page store with
58 * * A reference count for each memory page `page_counts_`. Each time a
59 * snapshot references a page, the counter is incremented. If a
60 * snapshot is freed, the reference count is decremented. When the
61 * reference count, of a page reaches 0 it is added to a list of available
62 * pages (`free_pages_`).
64 * * A list of free pages `free_pages_` which can be reused. This avoids having
65 * to scan the reference count list to find a free page.
67 * * When we are expanding the memory map we do not want to add thousand of page
68 * to the `free_pages_` list and remove them just afterwards. The `top_index_`
69 * field is an index after which all pages are free and are not in the `free_pages_`
72 * * When we are adding a page, we need to check if a page with the same
73 * content is already in the page store in order to reuse it. For this
74 * reason, we maintain an index (`hash_index_`) mapping the hash of a
75 * page to the list of page indices with this hash.
76 * We use a fast (non cryptographic) hash so there may be conflicts:
77 * we must be able to store multiple indices for the same hash.
80 struct s_mc_pages_store {
82 typedef uint64_t hash_type;
83 typedef boost ::unordered_set<size_t> page_set_type;
84 typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
89 * mc_page_store_get_page expects that this is the first field.
92 /** Number of available pages in virtual memory */
94 /** Top of the used pages (index of the next available page) */
96 /** Page reference count */
97 std::vector<uint64_t> page_counts_;
98 /** Index of available pages before the top */
99 std::vector<size_t> free_pages_;
100 /** Index from page hash to page index */
101 pages_map_type hash_index_;
104 void resize(size_t size);
106 void remove_page(size_t pageno);
108 public: // Constructors
109 explicit s_mc_pages_store(size_t size);
114 /** @brief Decrement the reference count for a given page
116 * Decrement the reference count of this page. Used when a snapshot is
119 * If the reference count reaches zero, the page is recycled:
120 * it is added to the `free_pages_` list and removed from the `hash_index_`.
123 void unref_page(size_t pageno);
125 /** @brief Increment the refcount for a given page
127 * This method used to increase a reference count of a page if we know
128 * that the content of a page is the same as a page already in the page
131 * This will be the case if a page if soft clean: we know that is has not
132 * changed since the previous cnapshot/restoration and we can avoid
133 * hashing the page, comparing byte-per-byte to candidates.
135 void ref_page(size_t pageno);
137 /** @brief Store a page in the page store */
138 size_t store_page(void* page);
140 /** @brief Get a page from its page number
142 * @param Number of the memory page in the store
143 * @return Start of the page
145 const void* get_page(size_t pageno) const;
147 public: // Debug/test methods
149 /** @brief Get the number of references for a page */
150 size_t get_ref(size_t pageno);
152 /** @brief Get the number of used pages */
155 /** @brief Get the capacity of the page store
157 * The capacity is expanded by a system call (mremap).
163 inline __attribute__((always_inline))
164 void s_mc_pages_store::unref_page(size_t pageno) {
165 if ((--this->page_counts_[pageno]) == 0) {
166 this->remove_page(pageno);
170 inline __attribute__((always_inline))
171 void s_mc_pages_store::ref_page(size_t pageno) {
172 ++this->page_counts_[pageno];
175 inline __attribute__((always_inline))
176 const void* s_mc_pages_store::get_page(size_t pageno) const {
177 return mc_page_from_number(this->memory_, pageno);
180 inline __attribute__((always_inline))
181 size_t s_mc_pages_store::get_ref(size_t pageno) {
182 return this->page_counts_[pageno];
185 inline __attribute__((always_inline))
186 size_t s_mc_pages_store::size() {
187 return this->top_index_ - this->free_pages_.size();
190 inline __attribute__((always_inline))
191 size_t s_mc_pages_store::capacity() {
192 return this->capacity_;
199 typedef struct s_mc_pages_store s_mc_pages_store_t, * mc_pages_store_t;
200 mc_pages_store_t mc_pages_store_new();
204 static inline __attribute__((always_inline))
205 const void* mc_page_store_get_page(mc_pages_store_t page_store, size_t pageno)
207 // This is page_store->memory_:
208 void* memory = *(void**)page_store;
209 return mc_page_from_number(memory, pageno);