block, virtio-devices: Support request submission in batch

Cache and batch IO requests after parsing all
items in the queue, improving performance—especially
for small block sizes—by reducing per-request overhead.

Introduced two methods in the AsyncIo trait for batch
submission, with implementation in the raw disk backend.
This method should be called during/after parsing all block IO requests
in the available queue. If the batch submission is not enabled, by
default it does the old way of submitting requests.

Signed-off-by: Bo Chen <bchen@crusoe.ai>
Signed-off-by: Muminul Islam <muislam@microsoft.com>

block/src/async_io.rs

@@ -8,7 +8,7 @@ use std::os::fd::{AsRawFd, OwnedFd, RawFd};
 use thiserror::Error;
 use vmm_sys_util::eventfd::EventFd;
 
-use crate::DiskTopology;
+use crate::{BatchRequest, DiskTopology};
 
 #[derive(Error, Debug)]
 pub enum DiskFileError {
@@ -99,4 +99,10 @@ pub trait AsyncIo: Send {
     ) -> AsyncIoResult<()>;
     fn fsync(&mut self, user_data: Option<u64>) -> AsyncIoResult<()>;
     fn next_completed_request(&mut self) -> Option<(u64, i32)>;
+    fn batch_requests_enabled(&self) -> bool {
+        false
+    }
+    fn submit_batch_requests(&mut self, _batch_request: &[BatchRequest]) -> AsyncIoResult<()> {
+        Ok(())
+    }
 }

block/src/lib.rs

@@ -232,9 +232,18 @@ pub struct AlignedOperation {
     layout: Layout,
 }
 
+pub struct BatchRequest {
+    pub offset: libc::off_t,
+    pub iovecs: SmallVec<[libc::iovec; DEFAULT_DESCRIPTOR_VEC_SIZE]>,
+    pub user_data: u64,
+    pub request_type: RequestType,
+}
+
 pub struct ExecuteAsync {
     // `true` if the execution will complete asynchronously
     pub async_complete: bool,
+    // request need to be batched for submission if any
+    pub batch_request: Option<BatchRequest>,
 }
 
 #[derive(Debug)]
@@ -480,6 +489,7 @@ impl Request {
 
         let mut ret = ExecuteAsync {
             async_complete: true,
+            batch_request: None,
         };
         // Queue operations expected to be submitted.
         match request_type {
@@ -490,14 +500,32 @@ impl Request {
                         .bitmap()
                         .mark_dirty(0, *data_len as usize);
                 }
-                disk_image
-                    .read_vectored(offset, &iovecs, user_data)
-                    .map_err(ExecuteError::AsyncRead)?;
+                if disk_image.batch_requests_enabled() {
+                    ret.batch_request = Some(BatchRequest {
+                        offset,
+                        iovecs,
+                        user_data,
+                        request_type,
+                    });
+                } else {
+                    disk_image
+                        .read_vectored(offset, &iovecs, user_data)
+                        .map_err(ExecuteError::AsyncRead)?;
+                }
             }
             RequestType::Out => {
-                disk_image
-                    .write_vectored(offset, &iovecs, user_data)
-                    .map_err(ExecuteError::AsyncWrite)?;
+                if disk_image.batch_requests_enabled() {
+                    ret.batch_request = Some(BatchRequest {
+                        offset,
+                        iovecs,
+                        user_data,
+                        request_type,
+                    });
+                } else {
+                    disk_image
+                        .write_vectored(offset, &iovecs, user_data)
+                        .map_err(ExecuteError::AsyncWrite)?;
+                }
             }
             RequestType::Flush => {
                 disk_image