X-Git-Url: http://info.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/blobdiff_plain/9ae3c710a6a8ac04570f77d88eb8e0c7a13f101c..4b0aa1525597fd0800c4d00c6df3f3c49d53ee2c:/src/mc/sosp/PageStore.hpp diff --git a/src/mc/sosp/PageStore.hpp b/src/mc/sosp/PageStore.hpp new file mode 100644 index 0000000000..02e9e07a3d --- /dev/null +++ b/src/mc/sosp/PageStore.hpp @@ -0,0 +1,197 @@ +/* Copyright (c) 2015-2018. 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 +#include + +#include +#include + +#include "src/mc/mc_forward.hpp" +#include "src/mc/mc_mmu.hpp" + +#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 coyping 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 page_set_type; + typedef std::unordered_map 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 page_counts_; + /** Index of available pages before the top */ + std::vector 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 cnapshot/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(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 + */ + const 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 const 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