Merge "libsnapshot: Use std::unordered_set in DmSnapCowSizeCalculator." am: 6a2963e05a (712be3d6) · Commits · e / os / android_system_core

fs_mgr/libsnapshot/dm_snapshot_internals.h

+13 −46

Original line number	Diff line number	Diff line
		@@ -17,8 +17,9 @@
		#include <android-base/logging.h>
		#include <stdint.h>

		#include <limits>
		#include <optional>
		#include <vector>
		#include <unordered_set>

		namespace android {
		namespace snapshot {
		@@ -37,21 +38,16 @@ class DmSnapCowSizeCalculator {
		return;
		}

		if (modified_chunks_.size() <= chunk_id) {
		if (modified_chunks_.max_size() <= chunk_id) {
		LOG(ERROR) << "Invalid COW size, chunk_id is too large.";
		if (chunk_id > std::numeric_limits<uint32_t>::max()) {
		LOG(ERROR) << "Chunk exceeds maximum size: " << chunk_id;
		valid_ = false;
		return;
		}
		modified_chunks_.resize(chunk_id + 1, false);
		if (modified_chunks_.size() <= chunk_id) {
		LOG(ERROR) << "Invalid COW size, chunk_id is too large.";
		valid_ = false;
		if (modified_chunks_.count(chunk_id) > 0) {
		return;
		}
		}

		modified_chunks_[chunk_id] = true;
		modified_chunks_.emplace(chunk_id);
		}

		std::optional<uint64_t> cow_size_bytes() const {
		@@ -91,23 +87,16 @@ class DmSnapCowSizeCalculator {
		return std::nullopt;
		}

		uint64_t modified_chunks_count = 0;
		uint64_t cow_chunks = 0;

		for (const auto& c : modified_chunks_) {
		if (c) {
		++modified_chunks_count;
		}
		}

		/* disk header + padding = 1 chunk */
		cow_chunks += 1;

		/* snapshot modified chunks */
		cow_chunks += modified_chunks_count;
		cow_chunks += modified_chunks_.size();

		/* snapshot chunks index metadata */
		cow_chunks += 1 + modified_chunks_count / exceptions_per_chunk;
		cow_chunks += 1 + modified_chunks_.size() / exceptions_per_chunk;

		return cow_chunks;
		}
		@@ -150,30 +139,8 @@ class DmSnapCowSizeCalculator {
		/*
		* \|modified_chunks_\| is a container that keeps trace of the modified
		* chunks.
		* Multiple options were considered when choosing the most appropriate data
		* structure for this container. Here follows a summary of why vector<bool>
		* has been chosen, taking as a reference a snapshot partition of 4 GiB and
		* chunk size of 4 KiB.
		* - std::set<uint64_t> is very space-efficient for a small number of
		* operations, but if the whole snapshot is changed, it would need to
		* store
		* 4 GiB / 4 KiB * (64 bit / 8) = 8 MiB
		* just for the data, plus the additional data overhead for the red-black
		* tree used for data sorting (if each rb-tree element stores 3 address
		* and the word-aligne color, the total size grows to 32 MiB).
		* - std::bitset<N> is not a good fit because requires a priori knowledge,
		* at compile time, of the bitset size.
		* - std::vector<bool> is a special case of vector, which uses a data
		* compression that allows reducing the space utilization of each element
		* to 1 bit. In detail, this data structure is composed of a resizable
		* array of words, each of them representing a bitmap. On a 64 bit
		* device, modifying the whole 4 GiB snapshot grows this container up to
		* 4 * GiB / 4 KiB / 64 = 64 KiB
		* that, even if is the same space requirement to change a single byte at
		* the highest address of the snapshot, is a very affordable space
		* requirement.
		*/
		std::vector<bool> modified_chunks_;
		std::unordered_set<uint32_t> modified_chunks_;
		};

		} // namespace snapshot