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>
19 #include "mc_private.h"
22 #ifndef MC_PAGE_SNAPSHOT_H
23 #define MC_PAGE_SNAPSHOT_H
25 struct s_mc_pages_store;
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.
81 struct s_mc_pages_store {
83 typedef uint64_t hash_type;
84 typedef boost ::unordered_set<size_t> page_set_type;
85 typedef boost::unordered_map<hash_type, page_set_type> pages_map_type;
90 * mc_page_store_get_page expects that this is the first field.
93 /** Number of available pages in virtual memory */
95 /** Top of the used pages (index of the next available page) */
97 /** Page reference count */
98 std::vector<uint64_t> page_counts_;
99 /** Index of available pages before the top */
100 std::vector<size_t> free_pages_;
101 /** Index from page hash to page index */
102 pages_map_type hash_index_;
105 void resize(size_t size);
107 void remove_page(size_t pageno);
109 public: // Constructors
110 explicit s_mc_pages_store(size_t size);
115 /** @brief Decrement the reference count for a given page
117 * Decrement the reference count of this page. Used when a snapshot is
120 * If the reference count reaches zero, the page is recycled:
121 * it is added to the `free_pages_` list and removed from the `hash_index_`.
124 void unref_page(size_t pageno);
126 /** @brief Increment the refcount for a given page
128 * This method used to increase a reference count of a page if we know
129 * that the content of a page is the same as a page already in the page
132 * This will be the case if a page if soft clean: we know that is has not
133 * changed since the previous cnapshot/restoration and we can avoid
134 * hashing the page, comparing byte-per-byte to candidates.
136 void ref_page(size_t pageno);
138 /** @brief Store a page in the page store */
139 size_t store_page(void* page);
141 /** @brief Get a page from its page number
143 * @param Number of the memory page in the store
144 * @return Start of the page
146 const void* get_page(size_t pageno) const;
148 public: // Debug/test methods
150 /** @brief Get the number of references for a page */
151 size_t get_ref(size_t pageno);
153 /** @brief Get the number of used pages */
156 /** @brief Get the capacity of the page store
158 * The capacity is expanded by a system call (mremap).
164 inline __attribute__((always_inline))
165 void s_mc_pages_store::unref_page(size_t pageno) {
166 if ((--this->page_counts_[pageno]) == 0) {
167 this->remove_page(pageno);
171 inline __attribute__((always_inline))
172 void s_mc_pages_store::ref_page(size_t pageno) {
173 ++this->page_counts_[pageno];
176 inline __attribute__((always_inline))
177 const void* s_mc_pages_store::get_page(size_t pageno) const {
178 return mc_page_from_number(this->memory_, pageno);
181 inline __attribute__((always_inline))
182 size_t s_mc_pages_store::get_ref(size_t pageno) {
183 return this->page_counts_[pageno];
186 inline __attribute__((always_inline))
187 size_t s_mc_pages_store::size() {
188 return this->top_index_ - this->free_pages_.size();
191 inline __attribute__((always_inline))
192 size_t s_mc_pages_store::capacity() {
193 return this->capacity_;
200 static inline __attribute__((always_inline))
201 const void* mc_page_store_get_page(mc_pages_store_t page_store, size_t pageno)
203 // This is page_store->memory_:
204 void* memory = *(void**)page_store;
205 return mc_page_from_number(memory, pageno);