+++ /dev/null
-/* Copyright (c) 2015-2020. The SimGrid Team. All rights reserved. */
-
-/* This program is free software; you can redistribute it and/or modify it
- * under the terms of the license (GNU LGPL) which comes with this package. */
-
-#ifndef SIMGRID_MC_PAGESTORE_HPP
-#define SIMGRID_MC_PAGESTORE_HPP
-
-#include "src/mc/mc_forward.hpp"
-#include "src/mc/mc_mmu.hpp"
-
-#include <unordered_map>
-#include <unordered_set>
-#include <vector>
-
-#ifndef XBT_ALWAYS_INLINE
-#define XBT_ALWAYS_INLINE inline __attribute__((always_inline))
-#endif
-
-namespace simgrid {
-namespace mc {
-
-/** @brief Storage for snapshot memory pages
- *
- * The first (lower) layer of the per-page snapshot mechanism is a page store:
- * its responsibility is to store immutable sharable reference-counted memory
- * pages independently of the snapshotting logic. Snapshot management and
- * representation is handled to an higher layer. READMORE
- *
- * Data structure:
- *
- * * A pointer (`memory_`) to a (currently anonymous) `mmap()`ed memory
- * region holding the memory pages (the address of the first page).
- *
- * We want to keep this memory region aligned on the memory pages (so
- * that we might be able to create non-linear memory mappings on those
- * pages in the future) and be able to expand it without copying the
- * data (there will be a lot of pages here): we will be able to
- * efficiently expand the memory mapping using `mremap()`, moving it
- * to another virtual address if necessary.
- *
- * Because we will move this memory mapping on the virtual address
- * space, only the index of the page will be stored in the snapshots
- * and the page will always be looked up by going through `memory`:
- *
- * void* page = (char*) page_store->memory + page_index << pagebits;
- *
- * * The number of pages mapped in virtual memory (`capacity_`). Once all
- * those pages are used, we need to expand the page store with
- * `mremap()`.
- *
- * * A reference count for each memory page `page_counts_`. Each time a
- * snapshot references a page, the counter is incremented. If a
- * snapshot is freed, the reference count is decremented. When the
- * reference count, of a page reaches 0 it is added to a list of available
- * pages (`free_pages_`).
- *
- * * A list of free pages `free_pages_` which can be reused. This avoids having
- * to scan the reference count list to find a free page.
- *
- * * When we are expanding the memory map we do not want to add thousand of page
- * to the `free_pages_` list and remove them just afterwards. The `top_index_`
- * field is an index after which all pages are free and are not in the `free_pages_`
- * list.
- *
- * * When we are adding a page, we need to check if a page with the same
- * content is already in the page store in order to reuse it. For this
- * reason, we maintain an index (`hash_index_`) mapping the hash of a
- * page to the list of page indices with this hash.
- * We use a fast (non cryptographic) hash so there may be conflicts:
- * we must be able to store multiple indices for the same hash.
- *
- */
-class PageStore {
-public: // Types
- typedef std::uint64_t hash_type;
-
-private:
- // Types
- // We are using a cheap hash to index a page.
- // We should expect collision and we need to associate multiple page indices
- // to the same hash.
- typedef std::unordered_set<std::size_t> page_set_type;
- typedef std::unordered_map<hash_type, page_set_type> pages_map_type;
-
- // Fields:
- /** First page */
- void* memory_;
- /** Number of available pages in virtual memory */
- std::size_t capacity_;
- /** Top of the used pages (index of the next available page) */
- std::size_t top_index_;
- /** Page reference count */
- std::vector<std::uint64_t> page_counts_;
- /** Index of available pages before the top */
- std::vector<std::size_t> free_pages_;
- /** Index from page hash to page index */
- pages_map_type hash_index_;
-
- // Methods
- void resize(std::size_t size);
- std::size_t alloc_page();
- void remove_page(std::size_t pageno);
-
-public:
- // Constructors
- PageStore(PageStore const&) = delete;
- PageStore& operator=(PageStore const&) = delete;
- explicit PageStore(std::size_t size);
- ~PageStore();
-
- // Methods
-
- /** @brief Decrement the reference count for a given page
- *
- * Decrement the reference count of this page. Used when a snapshot is destroyed.
- *
- * If the reference count reaches zero, the page is recycled:
- * it is added to the `free_pages_` list and removed from the `hash_index_`.
- *
- * */
- void unref_page(std::size_t pageno);
-
- /** @brief Increment the refcount for a given page
- *
- * This method used to increase a reference count of a page if we know
- * that the content of a page is the same as a page already in the page
- * store.
- *
- * This will be the case if a page if soft clean: we know that is has not
- * changed since the previous snapshot/restoration and we can avoid
- * hashing the page, comparing byte-per-byte to candidates.
- * */
- void ref_page(size_t pageno);
-
- /** @brief Store a page in the page store */
- std::size_t store_page(const void* page);
-
- /** @brief Get a page from its page number
- *
- * @param pageno Number of the memory page in the store
- * @return Start of the page
- */
- void* get_page(std::size_t pageno) const;
-
- // Debug/test methods
-
- /** @brief Get the number of references for a page */
- std::size_t get_ref(std::size_t pageno);
-
- /** @brief Get the number of used pages */
- std::size_t size();
-
- /** @brief Get the capacity of the page store
- *
- * The capacity is expanded by a system call (mremap).
- * */
- std::size_t capacity();
-};
-
-XBT_ALWAYS_INLINE void PageStore::unref_page(std::size_t pageno)
-{
- if ((--this->page_counts_[pageno]) == 0)
- this->remove_page(pageno);
-}
-
-XBT_ALWAYS_INLINE void PageStore::ref_page(size_t pageno)
-{
- ++this->page_counts_[pageno];
-}
-
-XBT_ALWAYS_INLINE void* PageStore::get_page(std::size_t pageno) const
-{
- return (void*)simgrid::mc::mmu::join(pageno, (std::uintptr_t)this->memory_);
-}
-
-XBT_ALWAYS_INLINE std::size_t PageStore::get_ref(std::size_t pageno)
-{
- return this->page_counts_[pageno];
-}
-
-XBT_ALWAYS_INLINE std::size_t PageStore::size()
-{
- return this->top_index_ - this->free_pages_.size();
-}
-
-XBT_ALWAYS_INLINE std::size_t PageStore::capacity()
-{
- return this->capacity_;
-}
-
-} // namespace mc
-} // namespace simgrid
-
-#endif
