Using an unrolled linked list to halve memory usage

Author: fulmicoton

src/lib.rs

@@ -1,4 +1,5 @@
 use std::borrow::Cow;
+use std::sync::Arc;
 
 mod block_read_write;
 
@@ -28,6 +29,56 @@ impl<'a> Record<'a> {
     }
 }
 
+#[derive(Clone, Default, Debug, Ord, PartialOrd, Eq, PartialEq)]
+pub struct FileNumber {
+    file_number: Arc<u64>,
+}
+
+impl FileNumber {
+    fn new(file_number: u64) -> Self {
+        FileNumber {
+            file_number: Arc::new(file_number),
+        }
+    }
+
+    /// Returns whether there is no clone of this FileNumber in existance.
+    ///
+    /// /!\ care should be taken to not have some other code store a &FileNumber which could alias
+    /// with self as it might then be sementically incorrect to delete content based only on this
+    /// returning `true`.
+    pub fn can_be_deleted(&self) -> bool {
+        Arc::strong_count(&self.file_number) == 1
+    }
+
+    #[cfg(test)]
+    pub fn unroll(&self, tracker: &crate::rolling::FileTracker) -> Vec<u64> {
+        let mut file = self.clone();
+        let mut file_numbers = Vec::new();
+        loop {
+            file_numbers.push(file.file_number());
+            if let Some(next_file) = tracker.next(&file) {
+                file = next_file;
+            } else {
+                return file_numbers;
+            }
+        }
+    }
+
+    pub fn filename(&self) -> String {
+        format!("wal-{:020}", self.file_number)
+    }
+
+    #[cfg(test)]
+    pub fn file_number(&self) -> u64 {
+        *self.file_number
+    }
+
+    #[cfg(test)]
+    pub fn for_test(file_number: u64) -> Self {
+        FileNumber::new(file_number)
+    }
+}
+
 /// Resources used by mrecordlog
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct ResourceUsage {

src/mem/arena.rs

@@ -0,0 +1,203 @@
+use std::time::{Instant, Duration};
+
+#[cfg(not(test))]
+pub const PAGE_SIZE: usize = 1 << 20;
+
+#[cfg(test)]
+pub const PAGE_SIZE: usize = 7;
+
+
+// TODO make it an array once we get a way to allocate array on the heap.
+pub type Page = Box<[u8]>;
+
+#[derive(Clone, Copy, Eq, PartialEq, Debug)]
+pub struct PageId(usize);
+
+/// An arena of fixed sized pages.
+#[derive(Default)]
+pub struct Arena {
+    /// We use an array to store the list of pages.
+    /// It can be seen as an efficient map from page id to pages.
+    ///
+    /// This map's len can-only grows. Its size is therefore the maximum number of pages
+    /// that was ever allocated. One page being 1MB long, this is not a problem.
+    ///
+    /// If a page is not allocated, the corresponding entry is `None`.
+    pages: Vec<Option<Page>>,
+    /// `free_slots` slots keeps track of the pages that are not allocated.
+    free_slots: Vec<PageId>,
+    /// `free_page_ids` keeps track of the allocated pages that are
+    /// available.
+    free_page_ids: Vec<PageId>,
+    /// Arena stats used to track how many pages should be freed.
+    stats: ArenaStats,
+}
+
+// The idea here is that we keep track of the maximum number of pages used through time.
+// To identify if it is worth deallocating pages, we look at the maximum number of pages
+// in use in the last few minutes minutes.
+//
+// We then allow ourselves to free memory down to this value.
+// Tracking exactly the maximum number of pages in use in the last 5 minutes is unnecessarily
+// complicated.
+//
+// For instance, we could run an extra task or thread.
+//
+// Instead, we just run a routine whenever someone interacts with the GC.
+// This routine only checks time 1 out of 256 calls.
+//
+// Pitfall: If pages are requests way less often than 256 times per minutes,
+// this arena may take way too much time to release its memory.
+struct ArenaStats {
+    max_num_used_pages_former: usize,
+    max_num_used_pages_current: usize,
+    call_counter: u8,
+    next_window_start: Instant,
+}
+
+const WINDOW: Duration = Duration::from_secs(60);
+
+impl Default for ArenaStats {
+    fn default() -> ArenaStats {
+        ArenaStats {
+            // We arbitrarily initialize num used pages former to 100.
+            max_num_used_pages_former: 100,
+            max_num_used_pages_current: 0,
+            call_counter: 0u8,
+            next_window_start: Instant::now(),
+        }
+    }
+}
+
+impl ArenaStats {
+    /// This method happens when we are changing time window.
+    fn roll(&mut self, now: Instant) {
+        self.max_num_used_pages_former = self.max_num_used_pages_current;
+        self.max_num_used_pages_current = 0;
+        self.next_window_start = now + WINDOW;
+    }
+
+    pub fn record_num_used_page(&mut self, num_used_pages: usize) -> usize {
+        // The only function of the call counter is to avoid calling `Instant::now()`
+        // at every single call.
+        self.call_counter = self.call_counter.wrapping_add(1);
+        if self.call_counter == 0u8 {
+            let now = Instant::now();
+            if now > self.next_window_start {
+                self.roll(now);
+            }
+        }
+        self.max_num_used_pages_current = self.max_num_used_pages_current.max(num_used_pages);
+        self.target_num_pages()
+    }
+
+    // This method returns a target number of pages.
+    //
+    // If we currently have a number of allocated pages higher than this, we need to free
+    // pages until we reach this number.
+    fn target_num_pages(&self) -> usize {
+        let max_over_both_windows = self.max_num_used_pages_former.max(self.max_num_used_pages_current);
+        (max_over_both_windows + 10).max(max_over_both_windows * 105 / 100)
+    }
+}
+
+
+impl Arena {
+    /// Returns an allocated page id.
+    pub fn get_page_id(&mut self) -> PageId {
+        if let Some(page_id) = self.free_page_ids.pop() {
+            assert!(self.pages[page_id.0].is_some());
+            return page_id;
+        }
+        let page: Page = vec![0u8; PAGE_SIZE].into_boxed_slice();
+        if let Some(free_slot) = self.free_slots.pop() {
+            let slot = &mut self.pages[free_slot.0];
+            assert!(slot.is_none());
+            *slot = Some(page);
+            return free_slot;
+        } else {
+            let new_page_id = self.pages.len();
+            self.pages.push(Some(page));
+            PageId(new_page_id)
+        }
+    }
+
+    #[inline]
+    pub fn page(&self, page_id: PageId) -> &[u8] {
+        self.pages[page_id.0].as_ref().unwrap()
+    }
+
+    #[inline]
+    pub fn page_mut(&mut self, page_id: PageId) -> &mut [u8] {
+        self.pages[page_id.0].as_mut().unwrap()
+    }
+
+    pub fn release_page(&mut self, page_id: PageId) {
+        self.free_page_ids.push(page_id);
+        assert!(self.pages[page_id.0].is_some());
+        self.gc();
+    }
+
+    /// `gc` releases memory by deallocating ALL of the free pages.
+    pub fn gc(&mut self) {
+        let num_used_pages = self.num_used_pages();
+        let target_num_pages = self.stats.record_num_used_page(num_used_pages);
+        assert!(target_num_pages >= num_used_pages);
+        let num_pages_to_free = self.num_allocated_pages() - target_num_pages;
+        assert!(num_pages_to_free >= self.free_page_ids.len());
+        for _ in 0..num_pages_to_free {
+            let page_id = self.free_page_ids.pop().unwrap();
+            self.pages[page_id.0] = None;
+            self.free_slots.push(page_id);
+        }
+    }
+
+    /// Returns the number of pages that are allocated
+    /// (regardless of whether they are in use or not).
+    pub fn num_allocated_pages(&self) -> usize {
+        self.pages.len() - self.free_slots.len()
+    }
+
+    /// Returns the number of pages that are allocated AND actually used.
+    pub fn num_used_pages(&self) -> usize {
+        self.pages.len() - self.free_slots.len() - self.free_page_ids.len()
+    }
+
+    pub fn capacity(&self) -> usize {
+        self.num_allocated_pages() * PAGE_SIZE
+    }
+
+    pub fn unused_capacity(&self) -> usize {
+        self.free_page_ids.len() * PAGE_SIZE
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_arena_simple() {
+        let mut arena = Arena::default();
+        assert_eq!(arena.capacity(), 0);
+        assert_eq!(arena.get_page_id(), PageId(0));
+        assert_eq!(arena.get_page_id(), PageId(1));
+        arena.release_page(PageId(0));
+        assert_eq!(arena.get_page_id(), PageId(0));
+    }
+
+    #[test]
+    fn test_arena_gc() {
+        let mut arena = Arena::default();
+        assert_eq!(arena.capacity(), 0);
+        assert_eq!(arena.get_page_id(), PageId(0));
+        assert_eq!(arena.get_page_id(), PageId(1));
+        arena.release_page(PageId(1));
+        assert_eq!(arena.num_allocated_pages(), 2);
+        arena.gc();
+        assert_eq!(arena.num_allocated_pages(), 1);
+        assert_eq!(arena.get_page_id(), PageId(1));
+        assert_eq!(arena.num_allocated_pages(), 2);
+    }
+}

src/mem/mod.rs

@@ -1,8 +1,14 @@
 mod queue;
 mod queues;
+mod arena;
+mod rolling_buffer;
 
+
+use self::arena::{Arena, PAGE_SIZE};
+use self::rolling_buffer::RollingBuffer;
 pub(crate) use self::queue::MemQueue;
 pub(crate) use self::queues::MemQueues;
 
+
 #[cfg(test)]
 mod tests;