terminal: PageList nodes are pooled individually from the gpa instead of an arena

src/terminal/PageList.zig

@@ -32,14 +32,17 @@ const log = std.log.scoped(.page_list);
 const native_freestanding = builtin.os.tag == .freestanding and
     !builtin.target.cpu.arch.isWasm();
 
-/// The number of PageList.Nodes we preheat the pool with. A node is
-/// a very small struct so we can afford to preheat many, but the exact
-/// number is uncertain. Any number too large is wasting memory, any number
-/// too small will cause the pool to have to allocate more memory later.
-/// This should be set to some reasonable minimum that we expect a terminal
-/// window to scroll into quickly.
+/// The number of pages we preheat the page pool with. For operating systems
+/// that support it, pages are demand-paged (see PagePool) so this only
+/// costs us address space. For other operating systems, we don't preheat.
 const page_preheat = 4;
 
+/// The number of nodes we preheat the node pool with. Unlike pages, nodes
+/// are ordinary heap memory so every idle preheated node costs real
+/// memory. A new PageList needs exactly one node for its first page, so
+/// we preheat that one and let the pool grow on demand.
+const node_preheat = 1;
+
 /// The list of pages in the screen. These are expected to be in order
 /// where the first page is the topmost page (scrollback) and the last is
 /// the bottommost page (the current active page).
@@ -299,7 +302,14 @@ const Node = struct {
 };
 
 /// The memory pool we get page nodes from.
-const NodePool = std.heap.memory_pool.Managed(List.Node);
+///
+/// We don't use a std memory pool here because it is backed by an arena
+/// and we end up paying a lot of wasted memory for the growth factor when
+/// in practice we don't usually use many nodes.
+///
+/// We don't need the "untouched" property of our UntouchedPool but
+/// this gives us a GPA-allocated pool so we reuse it here.
+const NodePool = datastruct.UntouchedPool(List.Node, .of(List.Node));
 
 /// The standard page capacity that we use as a starting point for
 /// all pages. This is chosen as a sane default that fits most terminal
@@ -356,7 +366,7 @@ pub const MemoryPool = struct {
         page_alloc: Allocator,
         preheat: usize,
     ) Allocator.Error!MemoryPool {
-        var node_pool = try NodePool.initCapacity(gen_alloc, preheat);
+        var node_pool = try NodePool.initCapacity(gen_alloc, gen_alloc, node_preheat);
         errdefer node_pool.deinit();
         var page_pool = try PagePool.initCapacity(gen_alloc, page_alloc, preheat);
         errdefer page_pool.deinit();
@@ -899,18 +909,19 @@ fn verifyIntegrity(self: *const PageList) IntegrityError!void {
 
 /// Release every page in the list during a teardown walk (deinit,
 /// reset, or an errdefer unwinding a partially built list): heap-owned
-/// pages go back to the page allocator and pool-owned pages back to the
-/// page pool. The nodes themselves are not touched; they live in the
-/// node pool.
+/// pages go back to the page allocator, pool-owned pages back to the
+/// page pool, and the nodes back to the node pool's free list.
 fn releasePages(pool: *MemoryPool, list: List) void {
     const page_alloc = pool.pages.allocator;
     var it = list.first;
-    while (it) |node| : (it = node.next) {
+    while (it) |node| {
+        it = node.next;
         const page = node.restore(.discard);
         switch (node.owned) {
             .pool => releasePoolPage(pool, page),
             .heap => page_alloc.free(page.memory),
         }
+        pool.nodes.destroy(node);
     }
 }
 
@@ -943,8 +954,7 @@ pub fn deinit(self: *PageList) void {
     // Always deallocate our hashmap.
     self.tracked_pins.deinit(self.pool.alloc);
 
-    // Release every page. We don't need to deallocate the list or
-    // nodes because they all reside in the pool.
+    // Release every page and node back to the pools, then free the pools.
     releasePages(&self.pool, self.pages);
     self.pool.deinit();
 }