Project #1 - Buffer Pool

Do not post your project on a public Github repository.

Overview

During the semester, you will build a disk-oriented storage manager for the BusTub DBMS. In such a storage manager, the primary storage location of the database is on disk.

The first programming project is to implement a buffer pool in your storage manager. The buffer pool is responsible for moving physical pages back and forth from main memory to disk. It allows a DBMS to support databases that are larger than the amount of memory available to the system. The buffer pool's operations are transparent to other parts in the system. For example, the system asks the buffer pool for a page using its unique identifier (page_id_t) and it does not know whether that page is already in memory or whether the system has to retrieve it from disk.

Your implementation will need to be thread-safe. Multiple threads will concurrently access the internal data structures and must make sure that their critical sections are protected with latches (these are called "locks" in operating systems).

You must implement the following storage manager components:

This is a single-person project that must be completed individually (i.e. no groups).

  • Release Date: Jan 30, 2023
  • Due Date: Feb 19, 2023 @ 11:59pm

Remember to pull latest code from the bustub repository.

Project Specification

For each of the following components, we provide stub classes that contain the API that you must implement. You should not modify the signatures for the pre-defined functions in these classes. If you modify the signatures, our grading test code will not work and you will get no credit for the project.

If a class already contains data members, you should not remove them. For example, the BufferPoolManager contains the DiskManager and LRUKReplacer objects. These are required to implement functionality needed by the rest of the system. You may add data members and helper functions to these classes to correctly implement the required functionality.

You may use any built-in C++17 containers in your project unless specified otherwise. It is up to you to decide which ones you want to use. Be warned that these containers are not thread-safe; will need to use latches to protect access to them. You may not use additional third-party libraries (e.g. boost).

Task #1 - LRU-K Replacement Policy

This component is responsible for tracking page usage in the buffer pool. You will implement a new class called LRUKReplacer in src/include/buffer/lru_k_replacer.h and its corresponding implementation file in src/buffer/lru_k_replacer.cpp. Note that LRUKReplacer is a stand-alone class and is not related to any of the other Replacer classes. You are expected to implement only the LRU-K replacement policy. You don't have to implement LRU or a clock replacement policy, even if there is a corresponding file for it.

The LRU-K algorithm evicts a frame whose backward k-distance is maximum of all frames in the replacer. Backward k-distance is computed as the difference in time between current timestamp and the timestamp of kth previous access. A frame with fewer than k historical accesses is given +inf as its backward k-distance. When multiple frames have +inf backward k-distance, the replacer evicts the frame with the earliest overall timestamp (i.e., the frame whose least-recent recorded access is the overall least recent access, overall, out of all frames).

The maximum size for the LRUKReplacer is the same as the size of the buffer pool since it contains placeholders for all of the frames in the BufferPoolManager. However, at any given moment, not all the frames in the replacer are considered to be evictable. The size of LRUKReplacer is represented by the number of evictable frames. The LRUKReplacer is initialized to have no frames in it. Then, only when a frame is marked as evictable, replacer's size will increase.

You will need to implement the LRU-K policy discussed in this course. You will need to implement the following methods as defined in the header file (src/include/buffer/lru_k_replacer.h) and in the source file (src/buffer/lru_k_replacer.cpp):

  • Evict(frame_id_t* frame_id) : Evict the frame with largest backward k-distance compared to all other evictable frames being tracked by the Replacer. Store the frame id in the output parameter and return True. If there are no evictable frames return False.
  • RecordAccess(frame_id_t frame_id) : Record that given frame id is accessed at current timestamp. This method should be called after a page is pinned in the BufferPoolManager.
  • Remove(frame_id_t frame_id) : Clear all access history associated with a frame. This method should be called only when a page is deleted in the BufferPoolManager.
  • SetEvictable(frame_id_t frame_id, bool set_evictable) : This method controls whether a frame is evictable or not. It also controls LRUKReplacer's size. You'll know when to call this function when you implement the BufferPoolManager. To be specific, when pin count of a page reaches 0, its corresponding frame is marked evictable and replacer's size is incremented.
  • Size() : This method returns the number of evictable frames that are currently in the LRUKReplacer.

The implementation details are up to you. You are allowed to use built-in STL containers. You may assume that you will not run out of memory, but you must make sure that your implementation is thread-safe.

Task #2 - Buffer Pool Manager

Next, implement the buffer pool manager (BufferPoolManager). The BufferPoolManager is responsible for fetching database pages from the DiskManager and storing them in memory. The BufferPoolManager can also write dirty pages out to disk when it is either explicitly instructed to do so or when it needs to evict a page to make space for a new page.

To make sure that your implementation works correctly with the rest of the system, we will provide you with some functions already filled in. You will also not need to implement the code that actually reads and writes data to disk (this is called the DiskManager in our implementation). We will provide that functionality.

All in-memory pages in the system are represented by Page objects. The BufferPoolManager does not need to understand the contents of these pages. But it is important for you as the system developer to understand that Page objects are just containers for memory in the buffer pool and thus are not specific to a unique page. That is, each Page object contains a block of memory that the DiskManager will use as a location to copy the contents of a physical page that it reads from disk. The BufferPoolManager will reuse the same Page object to store data as it moves back and forth to disk. This means that the same Page object may contain a different physical page throughout the life of the system. The Page object's identifer (page_id) keeps track of what physical page it contains; if a Page object does not contain a physical page, then its page_id must be set to INVALID_PAGE_ID.

Each Page object also maintains a counter for the number of threads that have "pinned" that page. Your BufferPoolManager is not allowed to free a Page that is pinned. Each Page object also keeps track of whether it is dirty or not. It is your job to record whether a page was modified before it is unpinned. Your BufferPoolManager must write the contents of a dirty Page back to disk before that object can be reused.

Your BufferPoolManager implementation will use the LRUKReplacer class that you created in the previous steps of this assignment. The LRUKReplacer will keep track of when Page objects are accessed so that it can decide which one to evict when it must free a frame to make room for copying a new physical page from disk. When mapping page_id to frame_id in the BufferPoolManager, again be warned that STL containers are not thread-safe.

You will need to implement the following functions defined in the header file (src/include/buffer/buffer_pool_manager.h) and in the source file (src/buffer/buffer_pool_manager.cpp):

  • FetchPage(page_id_t page_id)
  • UnpinPage(page_id_t page_id, bool is_dirty)
  • FlushPage(page_id_t page_id)
  • NewPage(page_id_t* page_id)
  • DeletePage(page_id_t page_id)
  • FlushAllPages()

For FetchPage, you should return nullptr if no page is available in the free list and all other pages are currently pinned. FlushPage should flush a page regardless of its pin status.

For UnpinPage, the is_dirty parameter keeps track of whether a page was modified while it was pinned.

The AllocatePage private method provides the BufferPoolManager a unique new page id when you want to create a new page in NewPage(). On the other hand, the DeallocatePage() method is a no-op that imitates freeing a page on the disk and you should call this in your DeletePage() implementation.

Disk Manager

The Disk Manager class (src/include/storage/disk/disk_manager.h) reads and writes the page data from and to the disk. Your buffer pool manager will use DiskManager::ReadPage() and DiskManager::WritePage() whenever it needs to fetch a page to the buffer pool or flush a page to the disk.

Task #3 - Read/Write Page Guards

In the Buffer Pool Manager, FetchPage and NewPage functions return pointers to pages that are already pinned. The pinning mechanism ensures that the pages are not evicted until there are no more reads and writes on the page. To indicate that the page is no longer needed in memory, the programmer has to manually call UnpinPage.

On the other hand, if the programmer forgets to call UnpinPage, the page will never be evicted out of the buffer pool. As the buffer pool operates with an effectively smaller number of frames, there will be more swapping of pages in and out of the disk. Not only the performance takes a hit, the bug is also difficult to be detected.

You will implement BasicPageGuard which store the pointers to BufferPoolManager and Page objects. A page guard ensures that UnpinPage is called on the corresponding Page object as soon as it goes out of scope. Note that it should still expose a method for a programmer to manually unpin the page.

As BasicPageGuard hides the underlying Page pointer, it can also provide read-only/write data APIs that provide compile-time checks to ensure that the is_dirty flag is set correctly for each use case.

In the future projects, multiple threads will be reading and writing from the same pages, thus reader-writer latches are required to ensure the correctness of the data. Note that in the Page class, there are relevant latching methods for this purpose. Similar to unpinning of a page, a programmer can forget to unlatch a page after use. To mitigate the problem, you will implement ReadPageGuard and WritePageGuard which automatically unlatch the pages as soon as they go out of scope.

You will need to implement the following functions for all BasicPageGuard, ReadPageGuard and WritePageGuard.

  • PageGuard(PageGuard &&that) : Move constructor.
  • operator=(PageGuard &&that) : Move operator.
  • Drop() : Unpin and/or unlatch.
  • ~PageGuard() : Destructor.

With the new page guards, implement the following wrappers in BufferPoolManager.

  • FetchPageBasic(page_id_t page_id)
  • FetchPageRead(page_id_t page_id)
  • FetchPageWrite(page_id_t page_id)
  • NewPageGuarded(page_id_t *page_id)

Please refer to the header files (lru_k_replacer.h, buffer_pool_manager.h, page_guard.h) for more detailed specs and documentations.

Leaderboard Task (Optional)

For this project's leaderboard challenge, we are doing a benchmark on your buffer pool manager with a special storage backend.

Optimizing for the leaderboard is optional (i.e., you can get a perfect score in the project after finishing all previous tasks). However, your solution must finish the leaderboard test with a correct result and without deadlock and segfault.

The leaderboard test is compiled with the release profile:

mkdir cmake-build-relwithdebinfo
cd cmake-build-relwithdebinfo
cmake .. -DCMAKE_BUILD_TYPE=RelWithDebInfo
make -j`nproc` bpm-bench
./bin/bustub-bpm-bench --duration 5000 --latency 1

We strongly recommend you to checkpoint your code before optimizing for leaderboard tests, so that if these optimizations cause problems in future projects, you can always revert them.

In the leaderboard test, we will have multiple threads accessing the pages on the disk. There are two types of threads running in the benchmark:

  1. Scan threads. Each scan thread will update all pages on the disk sequentially. There will be 8 scan threads.
  2. Get threads. Each get thread will randomly select a page for access using the zipfian distribution. There will be 8 get threads.

We will run the benchmark twice, each time for 30 seconds. For the first time, it will run directly on the in-memory storage backend. For the second time, we will add a 1-millisecond latency to each of the read / write operation. The final score is computed as a weighted QPS of scan and get operations, with and without latency respectively:

scan_qps_0ms / 10000 + get_qps_0ms / 10000 + scan_qps_1ms + get_qps_1ms * 10

Recommended Optimizations

  1. Better replacer algorithm. Given that get workload is skewed (i.e., some pages are more frequently accessed than others), you can design your LRU-k replacer to take page access type into consideration, so as to reduce page miss.
  2. Parallel I/O operations. Instead of holding a global lock when accessing the disk manager, you can issue multiple requests to the disk manager at the same time. This optimization will be very useful in modern storage devices, where concurrent access to the disk can make better use of the disk bandwidth.

Instructions

See the Project #0 instructions on how to create your private repository and setup your development environment.

Testing

You can test the individual components of this assigment using our testing framework. We use GTest for unit test cases. There are three separate files that contain tests for each component:

  • LRUKReplacer: test/buffer/lru_k_replacer_test.cpp
  • BufferPoolManager: test/buffer/buffer_pool_manager_test.cpp
  • PageGuard: test/storage/page_guard_test.cpp

You can compile and run each test individually from the command-line:

$ make lru_k_replacer_test -j$(nproc)
$ ./test/lru_k_replacer_test

You can also run make check-tests to run ALL of the test cases. Note that some tests are disabled as you have not implemented future projects. You can disable tests in GTest by adding a DISABLED_ prefix to the test name.

Important: These tests are only a subset of the all the tests that we will use to evaluate and grade your project. You should write additional test cases on your own to check the complete functionality of your implementation.

Formatting

Your code must follow the Google C++ Style Guide. We use Clang to automatically check the quality of your source code. Your project grade will be zero if your submission fails any of these checks.

Execute the following commands to check your syntax. The format target will automatically correct your code. The check-lint and check-clang-tidy-p1 targets will print errors and instruct you how to fix it to conform to our style guide.

$ make format
$ make check-lint
$ make check-clang-tidy-p1

Memory Leaks

For this project, we use LLVM Address Sanitizer (ASAN) and Leak Sanitizer (LSAN) to check for memory errors. To enable ASAN and LSAN, configure CMake in debug mode and run tests as you normally would. If there is memory error, you will see a memory error report. Note that macOS only supports address sanitizer without leak sanitizer.

In some cases, address sanitizer might affect the usability of the debugger. In this case, you might need to disable all sanitizers by configuring the CMake project with:

$ cmake -DCMAKE_BUILD_TYPE=Debug -DBUSTUB_SANITIZER= ..

Development Hints

You can use BUSTUB_ASSERT for assertions in debug mode. Note that the statements within BUSTUB_ASSERT will NOT be executed in release mode. If you have something to assert in all cases, use BUSTUB_ENSURE instead.

Post all of your questions about this project on Piazza. Do not email the TAs directly with questions.

We encourage you to use a graphical debugger to debug your project if you are having problems.

If you are having compilation problems, running make clean does not completely reset the compilation process. You will need to delete your build directory and run cmake .. again before you rerun make.

Grading Rubric

Each project submission will be graded based on the following criteria:

  1. Does the submission successfully execute all of the test cases and produce the correct answer?
  2. Does the submission execute without any memory leaks?
  3. Does the submission follow the code formatting and style policies?

Note that we will use additional test cases to grade your submission that are more complex than the sample test cases that we provide you.

Late Policy

See the late policy in the syllabus.

Submission

After completing the assignment, you can submit your implementation to Gradescope:

Running make submit-p1 in your build/ directory will generate a zip archive called project1-submission.zip under your project root directory that you can submit to Gradescope.

You can submit your answers as many times as you like and get immediate feedback.

Notes on Gradescope and Autograder

  1. If you are timing out on Gradescope, it's likely because you have a deadlock in your code or your code is too slow and does not run in 60 seconds. If your code is too slow it may be because your LRUKReplacer is not efficient enough.
  2. The autograder will not work if you are printing too many logs in your submissions.
  3. If the autograder did not work properly, make sure that your formatting commands work and that you are submitting the right files.
  4. The leaderboard benchmark score will be calculated by stress testing your buffer_pool_manager implementation.

CMU students should use the Gradescope course code announced on Piazza.

Collaboration Policy

  • Every student has to work individually on this assignment.
  • Students are allowed to discuss high-level details about the project with others.
  • Students are not allowed to copy the contents of a white-board after a group meeting with other students.
  • Students are not allowed to copy the solutions from another colleague.

WARNING: All of the code for this project must be your own. You may not copy source code from other students or other sources that you find on the web. Plagiarism will not be tolerated. See CMU's Policy on Academic Integrity for additional information.