Single-threaded architecture

Use Rc instead of Arc and replace the concurrent LRU with single-threaded SIEVE. Fixes #23 Fixes #29
2025-12-20 09:54:19 +01:00 · 2024-03-03 11:57:47 +02:00
parent 8f48416de0
commit ed9f3e6d1e
14 changed files with 103 additions and 113 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -289,15 +289,6 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "acbf1af155f9b9ef647e42cdc158db4b64a1b61f743629225fde6f3e0be2a7c7"

-[[package]]
-name = "concurrent_lru"
-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7feb5cb312f774e8a24540e27206db4e890f7d488563671d24a16389cf4c2e4e"
-dependencies = [
- "once_cell",
-]
-
 [[package]]
 name = "cpp_demangle"
 version = "0.4.3"
@@ -850,7 +841,6 @@ version = "0.0.0"
 dependencies = [
 "anyhow",
 "cfg_block",
- "concurrent_lru",
 "criterion",
 "fallible-iterator 0.3.0",
 "io-uring",
@@ -859,6 +849,7 @@ dependencies = [
 "pprof",
 "rstest",
 "rusqlite",
+ "sieve-cache",
 "sqlite3-parser",
 ]

@@ -1431,6 +1422,12 @@ dependencies = [
 "serde",
 ]

+[[package]]
+name = "sieve-cache"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "51bf3a9dccf2c079bf1465d449a485c85b36443caf765f2f127bfec28b180f75"
+
 [[package]]
 name = "siphasher"
 version = "0.3.11"
--- a/bindings/wasm/lib.rs
+++ b/bindings/wasm/lib.rs
@@ -1,5 +1,5 @@
 use anyhow::Result;
-use std::sync::Arc;
+use std::rc::Rc;
 use wasm_bindgen::prelude::*;

 #[wasm_bindgen]
@@ -10,8 +10,8 @@ pub struct Database {
 #[wasm_bindgen]
 impl Database {
    pub fn open(_path: &str) -> Database {
-        let io = Arc::new(IO {});
-        let page_source = limbo_core::PageSource::from_io(Arc::new(PageIO {}));
+        let io = Rc::new(IO {});
+        let page_source = limbo_core::PageSource::from_io(Rc::new(PageIO {}));
        let inner = limbo_core::Database::open(io, page_source).unwrap();
        Database { _inner: inner }
    }
@@ -32,7 +32,7 @@ impl limbo_core::IO for IO {
 pub struct PageIO {}

 impl limbo_core::PageIO for PageIO {
-    fn get(&self, _page_idx: usize, _c: Arc<limbo_core::Completion>) -> Result<()> {
+    fn get(&self, _page_idx: usize, _c: Rc<limbo_core::Completion>) -> Result<()> {
        todo!();
    }
 }
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -2,7 +2,7 @@ use clap::{Parser, ValueEnum};
 use cli_table::{Cell, Table};
 use limbo_core::{Database, RowResult, Value};
 use rustyline::{error::ReadlineError, DefaultEditor};
-use std::{path::PathBuf, sync::Arc};
+use std::{path::PathBuf, rc::Rc};

 #[derive(ValueEnum, Copy, Clone, Debug, PartialEq, Eq)]
 enum OutputMode {
@@ -32,7 +32,7 @@ fn main() -> anyhow::Result<()> {
    env_logger::init();
    let opts = Opts::parse();
    let path = opts.database.to_str().unwrap();
-    let io = Arc::new(limbo_core::PlatformIO::new()?);
+    let io = Rc::new(limbo_core::PlatformIO::new()?);
    let db = Database::open_file(io.clone(), path)?;
    let conn = db.connect();
    if let Some(sql) = opts.sql {
@@ -69,7 +69,7 @@ fn main() -> anyhow::Result<()> {
 }

 fn query(
-    io: Arc<dyn limbo_core::IO>,
+    io: Rc<dyn limbo_core::IO>,
    conn: &limbo_core::Connection,
    sql: &str,
    output_mode: &OutputMode,
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -26,9 +26,9 @@ mimalloc = { version = "*", default-features = false }
 [dependencies]
 anyhow = "1.0.75"
 cfg_block = "0.1.1"
-concurrent_lru = "0.2.0"
 fallible-iterator = "0.3.0"
 log = "0.4.20"
+sieve-cache = "0.1.4"
 sqlite3-parser = "0.11.0"

 [target.'cfg(not(target_family = "windows"))'.dev-dependencies]
--- a/core/btree.rs
+++ b/core/btree.rs
@@ -5,16 +5,16 @@ use crate::types::OwnedRecord;
 use anyhow::Result;

 use std::cell::{Ref, RefCell};
-use std::sync::Arc;
+use std::rc::Rc;

 pub struct MemPage {
-    parent: Option<Arc<MemPage>>,
+    parent: Option<Rc<MemPage>>,
    page_idx: usize,
    cell_idx: RefCell<usize>,
 }

 impl MemPage {
-    pub fn new(parent: Option<Arc<MemPage>>, page_idx: usize, cell_idx: usize) -> Self {
+    pub fn new(parent: Option<Rc<MemPage>>, page_idx: usize, cell_idx: usize) -> Self {
        Self {
            parent,
            page_idx,
@@ -38,15 +38,15 @@ pub enum CursorResult<T> {
 }

 pub struct Cursor {
-    pager: Arc<Pager>,
+    pager: Rc<Pager>,
    root_page: usize,
-    page: RefCell<Option<Arc<MemPage>>>,
+    page: RefCell<Option<Rc<MemPage>>>,
    rowid: RefCell<Option<u64>>,
    record: RefCell<Option<OwnedRecord>>,
 }

 impl Cursor {
-    pub fn new(pager: Arc<Pager>, root_page: usize) -> Self {
+    pub fn new(pager: Rc<Pager>, root_page: usize) -> Self {
        Self {
            pager,
            root_page,
@@ -62,16 +62,14 @@ impl Cursor {

    pub fn rewind(&mut self) -> Result<CursorResult<()>> {
        let mem_page = MemPage::new(None, self.root_page, 0);
-        self.page.replace(Some(Arc::new(mem_page)));
+        self.page.replace(Some(Rc::new(mem_page)));
        match self.get_next_record()? {
            CursorResult::Ok((rowid, next)) => {
                self.rowid.replace(rowid);
                self.record.replace(next);
                Ok(CursorResult::Ok(()))
            }
-            CursorResult::IO => {
-                Ok(CursorResult::IO)
-            }
+            CursorResult::IO => Ok(CursorResult::IO),
        }
    }

@@ -82,9 +80,7 @@ impl Cursor {
                self.record.replace(next);
                Ok(CursorResult::Ok(()))
            }
-            CursorResult::IO => {
-                Ok(CursorResult::IO)
-            }
+            CursorResult::IO => Ok(CursorResult::IO),
        }
    }

@@ -124,7 +120,7 @@ impl Cursor {
                match page.header.right_most_pointer {
                    Some(right_most_pointer) => {
                        let mem_page = MemPage::new(parent.clone(), right_most_pointer as usize, 0);
-                        self.page.replace(Some(Arc::new(mem_page)));
+                        self.page.replace(Some(Rc::new(mem_page)));
                        continue;
                    }
                    None => match parent {
@@ -147,7 +143,7 @@ impl Cursor {
                    mem_page.advance();
                    let mem_page =
                        MemPage::new(Some(mem_page.clone()), *_left_child_page as usize, 0);
-                    self.page.replace(Some(Arc::new(mem_page)));
+                    self.page.replace(Some(Rc::new(mem_page)));
                    continue;
                }
                BTreeCell::TableLeafCell(TableLeafCell { _rowid, _payload }) => {
--- a/core/io/darwin.rs
+++ b/core/io/darwin.rs
@@ -1,6 +1,6 @@
 use super::{Completion, File, IO};
 use anyhow::{Ok, Result};
-use std::sync::Arc;
+use std::rc::Rc;
 use std::cell::RefCell;
 use std::io::{Read, Seek};
 use log::trace;
@@ -32,7 +32,7 @@ pub struct DarwinFile {
 }

 impl File for DarwinFile {
-    fn pread(&self, pos: usize, c: Arc<Completion>) -> Result<()> {
+    fn pread(&self, pos: usize, c: Rc<Completion>) -> Result<()> {
        let mut file = self.file.borrow_mut();
        file.seek(std::io::SeekFrom::Start(pos as u64))?;
        {
--- a/core/io/linux.rs
+++ b/core/io/linux.rs
@@ -3,7 +3,6 @@ use anyhow::Result;
 use std::cell::RefCell;
 use std::os::unix::io::AsRawFd;
 use std::rc::Rc;
-use std::sync::Arc;
 use log::trace;

 pub struct LinuxIO {
@@ -36,7 +35,7 @@ impl IO for LinuxIO {
        loop {
            match ring.completion().next() {
                Some(cqe) => {
-                    let c = unsafe { Arc::from_raw(cqe.user_data() as *const Completion) };
+                    let c = unsafe { Rc::from_raw(cqe.user_data() as *const Completion) };
                    c.complete();
                }
                None => break,
@@ -52,14 +51,14 @@ pub struct LinuxFile {
 }

 impl File for LinuxFile {
-    fn pread(&self, pos: usize, c: Arc<Completion>) -> Result<()> {
+    fn pread(&self, pos: usize, c: Rc<Completion>) -> Result<()> {
        trace!("pread(pos = {}, length = {})", pos, c.buf().len());
        let fd = io_uring::types::Fd(self.file.as_raw_fd());
        let read_e = {
            let mut buf = c.buf_mut();
            let len = buf.len();
            let buf = buf.as_mut_ptr();
-            let ptr = Arc::into_raw(c.clone());
+            let ptr = Rc::into_raw(c.clone());
            io_uring::opcode::Read::new(fd, buf, len as u32 )
                .offset(pos as u64)
                .build()
--- a/core/io/mod.rs
+++ b/core/io/mod.rs
@@ -4,11 +4,11 @@ use std::{
    cell::{Ref, RefCell, RefMut},
    mem::ManuallyDrop,
    pin::Pin,
-    sync::Arc,
+    rc::Rc,
 };

 pub trait File {
-    fn pread(&self, pos: usize, c: Arc<Completion>) -> Result<()>;
+    fn pread(&self, pos: usize, c: Rc<Completion>) -> Result<()>;
 }

 pub trait IO {
@@ -46,7 +46,7 @@ impl Completion {

 pub type BufferData = Pin<Vec<u8>>;

-pub type BufferDropFn = Arc<dyn Fn(BufferData)>;
+pub type BufferDropFn = Rc<dyn Fn(BufferData)>;

 pub struct Buffer {
    data: ManuallyDrop<BufferData>,
--- a/core/io/windows.rs
+++ b/core/io/windows.rs
@@ -1,6 +1,6 @@
 use super::{Completion, File, IO};
 use anyhow::{Ok, Result};
-use std::sync::Arc;
+use std::rc::Rc;
 use std::cell::RefCell;
 use std::io::{Read, Seek};
 use log::trace;
@@ -32,7 +32,7 @@ pub struct WindowsFile {
 }

 impl File for WindowsFile {
-    fn pread(&self, pos: usize, c: Arc<Completion>) -> Result<()> {
+    fn pread(&self, pos: usize, c: Rc<Completion>) -> Result<()> {
        let mut file = self.file.borrow_mut();
        file.seek(std::io::SeekFrom::Start(pos as u64))?;
        {
--- a/core/lib.rs
+++ b/core/lib.rs
@@ -18,7 +18,7 @@ use fallible_iterator::FallibleIterator;
 use pager::Pager;
 use schema::Schema;
 use sqlite3_parser::{ast::Cmd, lexer::sql::Parser};
-use std::sync::Arc;
+use std::rc::Rc;

 #[cfg(feature = "fs")]
 pub use io::PlatformIO;
@@ -27,23 +27,23 @@ pub use storage::{PageIO, PageSource};
 pub use types::Value;

 pub struct Database {
-    pager: Arc<Pager>,
-    schema: Arc<Schema>,
+    pager: Rc<Pager>,
+    schema: Rc<Schema>,
 }

 impl Database {
    #[cfg(feature = "fs")]
-    pub fn open_file(io: Arc<dyn crate::io::IO>, path: &str) -> Result<Database> {
+    pub fn open_file(io: Rc<dyn crate::io::IO>, path: &str) -> Result<Database> {
        let file = io.open_file(path)?;
        let storage = storage::PageSource::from_file(file);
        Self::open(io, storage)
    }

-    pub fn open(io: Arc<dyn crate::io::IO>, page_source: PageSource) -> Result<Database> {
+    pub fn open(io: Rc<dyn crate::io::IO>, page_source: PageSource) -> Result<Database> {
        let db_header = Pager::begin_open(&page_source)?;
        io.run_once()?;
-        let pager = Arc::new(Pager::finish_open(db_header, page_source)?);
-        let bootstrap_schema = Arc::new(Schema::new());
+        let pager = Rc::new(Pager::finish_open(db_header, page_source)?);
+        let bootstrap_schema = Rc::new(Schema::new());
        let conn = Connection {
            pager: pager.clone(),
            schema: bootstrap_schema.clone(),
@@ -74,7 +74,7 @@ impl Database {
                }
            }
        }
-        let schema = Arc::new(schema);
+        let schema = Rc::new(schema);
        Ok(Database { pager, schema })
    }

@@ -87,8 +87,8 @@ impl Database {
 }

 pub struct Connection {
-    pager: Arc<Pager>,
-    schema: Arc<Schema>,
+    pager: Rc<Pager>,
+    schema: Rc<Schema>,
 }

 impl Connection {
@@ -99,7 +99,7 @@ impl Connection {
        if let Some(cmd) = cmd {
            match cmd {
                Cmd::Stmt(stmt) => {
-                    let program = Arc::new(translate::translate(&self.schema, stmt)?);
+                    let program = Rc::new(translate::translate(&self.schema, stmt)?);
                    Ok(Statement::new(program, self.pager.clone()))
                }
                Cmd::Explain(_stmt) => todo!(),
@@ -117,7 +117,7 @@ impl Connection {
        if let Some(cmd) = cmd {
            match cmd {
                Cmd::Stmt(stmt) => {
-                    let program = Arc::new(translate::translate(&self.schema, stmt)?);
+                    let program = Rc::new(translate::translate(&self.schema, stmt)?);
                    let stmt = Statement::new(program, self.pager.clone());
                    Ok(Some(Rows { stmt }))
                }
@@ -156,13 +156,13 @@ impl Connection {
 }

 pub struct Statement {
-    program: Arc<vdbe::Program>,
+    program: Rc<vdbe::Program>,
    state: vdbe::ProgramState,
-    pager: Arc<Pager>,
+    pager: Rc<Pager>,
 }

 impl Statement {
-    pub fn new(program: Arc<vdbe::Program>, pager: Arc<Pager>) -> Self {
+    pub fn new(program: Rc<vdbe::Program>, pager: Rc<Pager>) -> Self {
        let state = vdbe::ProgramState::new(program.max_registers);
        Self {
            program,
@@ -174,15 +174,9 @@ impl Statement {
    pub fn step(&mut self) -> Result<RowResult<'_>> {
        let result = self.program.step(&mut self.state, self.pager.clone())?;
        match result {
-            vdbe::StepResult::Row(row) => {
-                Ok(RowResult::Row(Row { values: row.values }))
-            }
-            vdbe::StepResult::IO => {
-                Ok(RowResult::IO)
-            }
-            vdbe::StepResult::Done => {
-                Ok(RowResult::Done)
-            }
+            vdbe::StepResult::Row(row) => Ok(RowResult::Row(Row { values: row.values })),
+            vdbe::StepResult::IO => Ok(RowResult::IO),
+            vdbe::StepResult::Done => Ok(RowResult::Done),
        }
    }

--- a/core/pager.rs
+++ b/core/pager.rs
@@ -2,14 +2,12 @@ use crate::buffer_pool::BufferPool;
 use crate::sqlite3_ondisk::BTreePage;
 use crate::sqlite3_ondisk::{self, DatabaseHeader};
 use crate::PageSource;
-use concurrent_lru::unsharded::LruCache;
 use log::trace;
+use sieve_cache::SieveCache;
 use std::cell::RefCell;
+use std::rc::Rc;
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::RwLock;
-use std::sync::{
-    atomic::{AtomicUsize, Ordering},
-    Arc,
-};

 pub struct Page {
    flags: AtomicUsize,
@@ -70,23 +68,23 @@ impl Page {

 pub struct Pager {
    page_source: PageSource,
-    page_cache: LruCache<usize, Arc<Page>>,
-    buffer_pool: Arc<BufferPool>,
+    page_cache: RefCell<SieveCache<usize, Rc<Page>>>,
+    buffer_pool: Rc<BufferPool>,
 }

 impl Pager {
-    pub fn begin_open(page_source: &PageSource) -> anyhow::Result<Arc<RefCell<DatabaseHeader>>> {
+    pub fn begin_open(page_source: &PageSource) -> anyhow::Result<Rc<RefCell<DatabaseHeader>>> {
        sqlite3_ondisk::begin_read_database_header(page_source)
    }

    pub fn finish_open(
-        db_header: Arc<RefCell<DatabaseHeader>>,
+        db_header: Rc<RefCell<DatabaseHeader>>,
        page_source: PageSource,
    ) -> anyhow::Result<Self> {
        let db_header = db_header.borrow();
        let page_size = db_header.page_size as usize;
-        let buffer_pool = Arc::new(BufferPool::new(page_size));
-        let page_cache = LruCache::new(10);
+        let buffer_pool = Rc::new(BufferPool::new(page_size));
+        let page_cache = RefCell::new(SieveCache::new(10).unwrap());
        Ok(Self {
            page_source,
            buffer_pool,
@@ -94,20 +92,28 @@ impl Pager {
        })
    }

-    pub fn read_page(&self, page_idx: usize) -> anyhow::Result<Arc<Page>> {
+    pub fn read_page(&self, page_idx: usize) -> anyhow::Result<Rc<Page>> {
        trace!("read_page(page_idx = {})", page_idx);
+        let mut page_cache = self.page_cache.borrow_mut();
+        if let Some(page) = page_cache.get(&page_idx) {
+            return Ok(page.clone());
+        }
+        let page = Rc::new(Page::new());
+        page.set_locked();
+        sqlite3_ondisk::begin_read_btree_page(
+            &self.page_source,
+            self.buffer_pool.clone(),
+            page.clone(),
+            page_idx,
+        )
+        .unwrap();
+        page_cache.insert(page_idx, page.clone());
+        Ok(page)
+        /*
        let handle = self.page_cache.get_or_try_init(page_idx, 1, |_idx| {
-            let page = Arc::new(Page::new());
-            page.set_locked();
-            sqlite3_ondisk::begin_read_btree_page(
-                &self.page_source,
-                self.buffer_pool.clone(),
-                page.clone(),
-                page_idx,
-            )
-            .unwrap();
-            Ok::<Arc<Page>, anyhow::Error>(page)
+            Ok::<Rc<Page>, anyhow::Error>(page)
        })?;
        Ok(handle.value().clone())
+        */
    }
 }
--- a/core/sqlite3_ondisk.rs
+++ b/core/sqlite3_ondisk.rs
@@ -31,7 +31,7 @@ use crate::PageSource;
 use anyhow::{anyhow, Result};
 use log::trace;
 use std::cell::RefCell;
-use std::sync::Arc;
+use std::rc::Rc;

 /// The size of the database header in bytes.
 pub const DATABASE_HEADER_SIZE: usize = 100;
@@ -63,23 +63,21 @@ pub struct DatabaseHeader {
    version_number: u32,
 }

-pub fn begin_read_database_header(
-    page_source: &PageSource,
-) -> Result<Arc<RefCell<DatabaseHeader>>> {
-    let drop_fn = Arc::new(|_buf| {});
+pub fn begin_read_database_header(page_source: &PageSource) -> Result<Rc<RefCell<DatabaseHeader>>> {
+    let drop_fn = Rc::new(|_buf| {});
    let buf = Buffer::allocate(512, drop_fn);
-    let result = Arc::new(RefCell::new(DatabaseHeader::default()));
+    let result = Rc::new(RefCell::new(DatabaseHeader::default()));
    let header = result.clone();
    let complete = Box::new(move |buf: &Buffer| {
        let header = header.clone();
        finish_read_database_header(buf, header).unwrap();
    });
-    let c = Arc::new(Completion::new(buf, complete));
+    let c = Rc::new(Completion::new(buf, complete));
    page_source.get(1, c.clone())?;
    Ok(result)
 }

-fn finish_read_database_header(buf: &Buffer, header: Arc<RefCell<DatabaseHeader>>) -> Result<()> {
+fn finish_read_database_header(buf: &Buffer, header: Rc<RefCell<DatabaseHeader>>) -> Result<()> {
    let buf = buf.as_slice();
    let mut header = header.borrow_mut();
    header.magic.copy_from_slice(&buf[0..16]);
@@ -149,13 +147,13 @@ pub struct BTreePage {

 pub fn begin_read_btree_page(
    page_source: &PageSource,
-    buffer_pool: Arc<BufferPool>,
-    page: Arc<Page>,
+    buffer_pool: Rc<BufferPool>,
+    page: Rc<Page>,
    page_idx: usize,
 ) -> Result<()> {
    trace!("begin_read_btree_page(page_idx = {})", page_idx);
    let buf = buffer_pool.get();
-    let drop_fn = Arc::new(move |buf| {
+    let drop_fn = Rc::new(move |buf| {
        let buffer_pool = buffer_pool.clone();
        buffer_pool.put(buf);
    });
@@ -166,12 +164,12 @@ pub fn begin_read_btree_page(
            page.set_error();
        }
    });
-    let c = Arc::new(Completion::new(buf, complete));
+    let c = Rc::new(Completion::new(buf, complete));
    page_source.get(page_idx, c.clone())?;
    Ok(())
 }

-fn finish_read_btree_page(page_idx: usize, buf: &Buffer, page: Arc<Page>) -> Result<()> {
+fn finish_read_btree_page(page_idx: usize, buf: &Buffer, page: Rc<Page>) -> Result<()> {
    trace!("finish_read_btree_page(page_idx = {})", page_idx);
    let mut pos = if page_idx == 1 {
        DATABASE_HEADER_SIZE
--- a/core/storage.rs
+++ b/core/storage.rs
@@ -2,31 +2,31 @@ use crate::io::Completion;
 #[cfg(feature = "fs")]
 use crate::io::File;
 use anyhow::Result;
-use std::sync::Arc;
+use std::rc::Rc;

 pub struct PageSource {
-    io: Arc<dyn PageIO>,
+    io: Rc<dyn PageIO>,
 }

 impl PageSource {
-    pub fn from_io(io: Arc<dyn PageIO>) -> Self {
+    pub fn from_io(io: Rc<dyn PageIO>) -> Self {
        Self { io }
    }

    #[cfg(feature = "fs")]
    pub fn from_file(file: Box<dyn File>) -> Self {
        Self {
-            io: Arc::new(FileStorage::new(file)),
+            io: Rc::new(FileStorage::new(file)),
        }
    }

-    pub fn get(&self, page_idx: usize, c: Arc<Completion>) -> Result<()> {
+    pub fn get(&self, page_idx: usize, c: Rc<Completion>) -> Result<()> {
        self.io.get(page_idx, c)
    }
 }

 pub trait PageIO {
-    fn get(&self, page_idx: usize, c: Arc<Completion>) -> Result<()>;
+    fn get(&self, page_idx: usize, c: Rc<Completion>) -> Result<()>;
 }

 #[cfg(feature = "fs")]
@@ -36,7 +36,7 @@ struct FileStorage {

 #[cfg(feature = "fs")]
 impl PageIO for FileStorage {
-    fn get(&self, page_idx: usize, c: Arc<Completion>) -> Result<()> {
+    fn get(&self, page_idx: usize, c: Rc<Completion>) -> Result<()> {
        let page_size = c.buf().len();
        assert!(page_idx > 0);
        assert!(page_size >= 512);
--- a/core/vdbe.rs
+++ b/core/vdbe.rs
@@ -5,7 +5,7 @@ use crate::types::{OwnedValue, Record};
 use anyhow::Result;
 use std::cell::RefCell;
 use std::collections::BTreeMap;
-use std::sync::Arc;
+use std::rc::Rc;

 pub type BranchOffset = usize;

@@ -195,7 +195,7 @@ impl Program {
    pub fn step<'a>(
        &self,
        state: &'a mut ProgramState,
-        pager: Arc<Pager>,
+        pager: Rc<Pager>,
    ) -> Result<StepResult<'a>> {
        loop {
            let insn = &self.insns[state.pc];