try interval-based impl to cross-chec

=> zero batching

https://www.notion.so/neondatabase/benchmarking-notes-143f189e004780c4a630cb5f426e39ba?pvs=4#144f189e00478065a9b3e51726082885

pageserver/src/page_service.rs

@@ -23,7 +23,6 @@ use pq_proto::FeStartupPacket;
 use pq_proto::{BeMessage, FeMessage, RowDescriptor};
 use std::borrow::Cow;
 use std::io;
-use std::pin::Pin;
 use std::str;
 use std::str::FromStr;
 use std::sync::Arc;
@@ -319,7 +318,7 @@ struct PageServerHandler {
     /// See [`PageServerConf::server_side_batch_timeout`]
     server_side_batch_timeout: Option<Duration>,
 
-    server_side_batch_timer: Pin<Box<async_timer::timer::Platform>>,
+    server_side_batch_interval: Option<async_timer::Interval>,
 }
 
 struct Carry {
@@ -589,7 +588,8 @@ impl PageServerHandler {
             timeline_handles: TimelineHandles::new(tenant_manager),
             cancel,
             server_side_batch_timeout,
-            server_side_batch_timer: Box::pin(async_timer::new_timer(Duration::from_secs(999))), // reset each iteration
+            server_side_batch_interval: server_side_batch_timeout
+                .map(|timeout| async_timer::Interval::new(timeout)),
         }
     }
 
@@ -631,50 +631,22 @@ impl PageServerHandler {
     {
         debug_assert_current_span_has_tenant_and_timeline_id_no_shard_id();
 
-        let mut batching_deadline_storage = None; // TODO: can this just be an unsync once_cell?
-
         loop {
             // Create a future that will become ready when we need to stop batching.
             use futures::future::Either;
-            let batching_deadline = match (
-                &*maybe_carry as &Option<Carry>,
-                &mut batching_deadline_storage,
-            ) {
-                (None, None) => Either::Left(futures::future::pending()), // there's no deadline before we have something batched
-                (None, Some(_)) => unreachable!(),
-                (Some(_), Some(fut)) => Either::Right(fut), // below arm already ran
-                (Some(carry), None) => {
-                    match self.server_side_batch_timeout {
-                        None => {
-                            return Ok(BatchOrEof::Batch(smallvec::smallvec![
-                                maybe_carry
-                                    .take()
-                                    .expect("we already checked it's Some")
-                                    .msg
-                            ]))
-                        }
-                        Some(batch_timeout) => {
-                            // Take into consideration the time the carry spent waiting.
-                            let batch_timeout =
-                                batch_timeout.saturating_sub(carry.started_at.elapsed());
-                            if batch_timeout.is_zero() {
-                                // the timer doesn't support restarting with zero duration
-                                return Ok(BatchOrEof::Batch(smallvec::smallvec![
-                                    maybe_carry
-                                        .take()
-                                        .expect("we already checked it's Some")
-                                        .msg
-                                ]));
-                            } else {
-                                self.server_side_batch_timer.restart(batch_timeout);
-                                batching_deadline_storage = Some(&mut self.server_side_batch_timer);
-                                Either::Right(
-                                    batching_deadline_storage.as_mut().expect("we just set it"),
-                                )
-                            }
-                        }
+            let batching_deadline = match &*maybe_carry as &Option<Carry> {
+                None => Either::Left(futures::future::pending()), // there's no deadline before we have something batched
+                Some(carry) => match &mut self.server_side_batch_interval {
+                    None => {
+                        return Ok(BatchOrEof::Batch(smallvec::smallvec![
+                            maybe_carry
+                                .take()
+                                .expect("we already checked it's Some")
+                                .msg
+                        ]))
                     }
-                }
+                    Some(interval) => Either::Right(interval),
+                },
             };
             let msg = tokio::select! {
                 biased;