Optimize the latency of lmbench-select-file

kernel/aster-nix/src/fs/ramfs/fs.rs

@@ -1093,12 +1093,18 @@ impl Inode for RamInode {
     }
 
     fn poll(&self, mask: IoEvents, poller: Option<&mut Poller>) -> IoEvents {
-        if let Some(device) = self.node.read().inner.as_device() {
+        if !self.typ.is_device() {
-            device.poll(mask, poller)
+            // Fast path: bypassing the read lock on `self.node`
-        } else {
+            // results in a 5x speed improvement for lmbench-select-file.
-            let events = IoEvents::IN | IoEvents::OUT;
+            return (IoEvents::IN | IoEvents::OUT) & mask;
-            events & mask
         }
+
+        let node = self.node.read();
+        let device = node
+            .inner
+            .as_device()
+            .expect("[Internal error] self.typ is device, while self.node is not");
+        device.poll(mask, poller)
     }
 
     fn fs(&self) -> Arc<dyn FileSystem> {

kernel/aster-nix/src/fs/utils/inode.rs

@@ -51,6 +51,10 @@ impl InodeType {
     pub fn is_directory(&self) -> bool {
         *self == InodeType::Dir
     }
+
+    pub fn is_device(&self) -> bool {
+        *self == InodeType::BlockDevice || *self == InodeType::CharDevice
+    }
 }
 
 impl From<DeviceType> for InodeType {

kernel/aster-nix/src/syscall/poll.rs

@@ -51,6 +51,7 @@ pub fn do_poll(poll_fds: &[PollFd], timeout: Option<Duration>, ctx: &Context) ->
     loop {
         let mut num_revents = 0;
 
+        let file_table = ctx.process.file_table().lock();
         for poll_fd in poll_fds {
             // Skip poll_fd if it is not given a fd
             let fd = match poll_fd.fd() {
@@ -59,10 +60,7 @@ pub fn do_poll(poll_fds: &[PollFd], timeout: Option<Duration>, ctx: &Context) ->
             };
 
             // Poll the file
-            let file = {
+            let file = file_table.get_file(fd)?;
-                let file_table = ctx.process.file_table().lock();
-                file_table.get_file(fd)?.clone()
-            };
             let need_poller = if num_revents == 0 {
                 Some(&mut poller)
             } else {
@@ -75,6 +73,8 @@ pub fn do_poll(poll_fds: &[PollFd], timeout: Option<Duration>, ctx: &Context) ->
             }
         }
 
+        drop(file_table);
+
         if num_revents > 0 {
             return Ok(num_revents);
         }

kernel/aster-nix/src/syscall/select.rs

@@ -102,7 +102,7 @@ fn do_select(
 ) -> Result<usize> {
     // Convert the FdSet to an array of PollFd
     let poll_fds = {
-        let mut poll_fds = Vec::new();
+        let mut poll_fds = Vec::with_capacity(nfds as usize);
         for fd in 0..nfds {
             let events = {
                 let readable = readfds.as_ref().map_or(false, |fds| fds.is_set(fd));

kernel/libs/aster-util/src/slot_vec.rs

@@ -49,10 +49,7 @@ impl<T> SlotVec<T> {
     ///
     /// Return `None` if `idx` is out of bounds or the item is not exist.
     pub fn get(&self, idx: usize) -> Option<&T> {
-        if idx >= self.slots.len() {
+        self.slots.get(idx)?.as_ref()
-            return None;
-        }
-        self.slots[idx].as_ref()
     }
 
     /// Put an item into the vector.