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/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
26 struct s_mc_pages_store;
30 /** @brief Storage for snapshot memory pages
32 * The first (lower) layer of the per-page snapshot mechanism is a page
33 * store: it's responsibility is to store immutable shareable
34 * reference-counted memory pages independently of the snapshoting
35 * logic. Snapshot management and representation, soft-dirty tracking is
36 * handled to an higher layer. READMORE
40 * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
41 * region holding the memory pages (the address of the first page).
43 * We want to keep this memory region aligned on the memory pages (so
44 * that we might be able to create non-linear memory mappings on those
45 * pages in the future) and be able to expand it without coyping the
46 * data (there will be a lot of pages here): we will be able to
47 * efficiently expand the memory mapping using `mremap()`, moving it
48 * to another virtual address if necessary.
50 * Because we will move this memory mapping on the virtual address
51 * space, only the index of the page will be stored in the snapshots
52 * and the page will always be looked up by going through `memory`:
54 * void* page = (char*) page_store->memory + page_index << pagebits;
56 * * The number of pages mapped in virtual memory (`capacity_`). Once all
57 * those pages are used, we need to expand the page store with
60 * * A reference count for each memory page `page_counts_`. Each time a
61 * snapshot references a page, the counter is incremented. If a
62 * snapshot is freed, the reference count is decremented. When the
63 * reference count, of a page reaches 0 it is added to a list of available
64 * pages (`free_pages_`).
66 * * A list of free pages `free_pages_` which can be reused. This avoids having
67 * to scan the reference count list to find a free page.
69 * * When we are expanding the memory map we do not want to add thousand of page
70 * to the `free_pages_` list and remove them just afterwards. The `top_index_`
71 * field is an index after which all pages are free and are not in the `free_pages_`
74 * * When we are adding a page, we need to check if a page with the same
75 * content is already in the page store in order to reuse it. For this
76 * reason, we maintain an index (`hash_index_`) mapping the hash of a
77 * page to the list of page indices with this hash.
78 * We use a fast (non cryptographic) hash so there may be conflicts:
79 * we must be able to store multiple indices for the same hash.
82 struct s_mc_pages_store {
84 #ifdef MC_PAGE_STORE_MD4
85 typedef boost::array<uint64_t,2> hash_type;
87 typedef uint64_t hash_type;
90 #ifdef MC_PAGE_STORE_MD4
91 // We are using a secure hash to identify a page.
92 // We assume there will not be any collision: we need to map a hash
93 // to a single page index.
94 typedef boost::unordered_map<hash_type, size_t> pages_map_type;
96 // We are using a cheap hash to index a page.
97 // We should expect collision and we need to associate multiple page indices
99 typedef boost::unordered_set<size_t> page_set_type;
100 typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
106 * mc_page_store_get_page expects that this is the first field.
109 /** Number of available pages in virtual memory */
111 /** Top of the used pages (index of the next available page) */
113 /** Page reference count */
114 std::vector<uint64_t> page_counts_;
115 /** Index of available pages before the top */
116 std::vector<size_t> free_pages_;
117 /** Index from page hash to page index */
118 pages_map_type hash_index_;
121 void resize(size_t size);
123 void remove_page(size_t pageno);
125 public: // Constructors
126 explicit s_mc_pages_store(size_t size);
131 /** @brief Decrement the reference count for a given page
133 * Decrement the reference count of this page. Used when a snapshot is
136 * If the reference count reaches zero, the page is recycled:
137 * it is added to the `free_pages_` list and removed from the `hash_index_`.
140 void unref_page(size_t pageno);
142 /** @brief Increment the refcount for a given page
144 * This method used to increase a reference count of a page if we know
145 * that the content of a page is the same as a page already in the page
148 * This will be the case if a page if soft clean: we know that is has not
149 * changed since the previous cnapshot/restoration and we can avoid
150 * hashing the page, comparing byte-per-byte to candidates.
152 void ref_page(size_t pageno);
154 /** @brief Store a page in the page store */
155 size_t store_page(void* page);
157 /** @brief Get a page from its page number
159 * @param Number of the memory page in the store
160 * @return Start of the page
162 const void* get_page(size_t pageno) const;
164 public: // Debug/test methods
166 /** @brief Get the number of references for a page */
167 size_t get_ref(size_t pageno);
169 /** @brief Get the number of used pages */
172 /** @brief Get the capacity of the page store
174 * The capacity is expanded by a system call (mremap).
180 inline __attribute__((always_inline))
181 void s_mc_pages_store::unref_page(size_t pageno) {
182 if ((--this->page_counts_[pageno]) == 0) {
183 this->remove_page(pageno);
187 inline __attribute__((always_inline))
188 void s_mc_pages_store::ref_page(size_t pageno) {
189 ++this->page_counts_[pageno];
192 inline __attribute__((always_inline))
193 const void* s_mc_pages_store::get_page(size_t pageno) const {
194 return mc_page_from_number(this->memory_, pageno);
197 inline __attribute__((always_inline))
198 size_t s_mc_pages_store::get_ref(size_t pageno) {
199 return this->page_counts_[pageno];
202 inline __attribute__((always_inline))
203 size_t s_mc_pages_store::size() {
204 return this->top_index_ - this->free_pages_.size();
207 inline __attribute__((always_inline))
208 size_t s_mc_pages_store::capacity() {
209 return this->capacity_;
216 mc_pages_store_t mc_pages_store_new(void);
217 void mc_pages_store_delete(mc_pages_store_t store);
221 static inline __attribute__((always_inline))
222 const void* mc_page_store_get_page(mc_pages_store_t page_store, size_t pageno)
224 // This is page_store->memory_:
225 void* memory = *(void**)page_store;
226 return mc_page_from_number(memory, pageno);