[pbs-devel] [PATCH pxar 2/2] decoder: aio: improve performance of async file reads

[Max Carrara]

In order to bring `aio::Decoder` on par with its `sync` counterpart
as well as `sync::Accessor` and `aio::Accessor`, its input is now
buffered.

As the `tokio` docs mention themselves [0], it can be really
inefficient to directly work with an (unbuffered) `AsyncRead`
instance.

The other aforementioned types already buffer their reads in one way
or another, so wrapping the input reader in `tokio::io::BufReader`
results in a substantial performance gain. [1]

[0]: https://docs.rs/tokio/1.29.1/tokio/io/struct.BufReader.html
[1]: Tested via a custom CLI utility that opens and traverses a
     large (13GB) pxar archive with each decoder and accessor

Before:
> First pass
> With aio::Decoder:   Ok(()) (elapsed: 25.827150007s)
> With sync::Decoder:  Ok(()) (elapsed: 3.577611655s)
> With aio::Accessor:  Ok(()) (elapsed: 3.962754675s)
> With sync::Accessor: Ok(()) (elapsed: 3.961245996s)
>
> Second pass
> With aio::Decoder:   Ok(()) (elapsed: 21.045064325s)
> With sync::Decoder:  Ok(()) (elapsed: 3.644003471s)
> With aio::Accessor:  Ok(()) (elapsed: 4.054085818s)
> With sync::Accessor: Ok(()) (elapsed: 4.036097687s)

After:
> First pass:
> With aio::Decoder:   Ok(()) (elapsed: 7.07321221s)
> With sync::Decoder:  Ok(()) (elapsed: 3.431787191s)
> With aio::Accessor:  Ok(()) (elapsed: 3.930457465s)
> With sync::Accessor: Ok(()) (elapsed: 4.007415416s)
>
> Second pass:
> With aio::Decoder:   Ok(()) (elapsed: 6.826005792s)
> With sync::Decoder:  Ok(()) (elapsed: 3.437391887s)
> With aio::Accessor:  Ok(()) (elapsed: 3.833275725s)
> With sync::Accessor: Ok(()) (elapsed: 3.909827322s)

Signed-off-by: Max Carrara <m.carrara at proxmox.com>
---
 src/decoder/aio.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/decoder/aio.rs b/src/decoder/aio.rs
index 200dd3d..174551b 100644
--- a/src/decoder/aio.rs
+++ b/src/decoder/aio.rs
@@ -79,14 +79,20 @@ mod tok {
     use std::pin::Pin;
     use std::task::{Context, Poll};

-    /// Read adapter for `futures::io::AsyncRead`
+    use tokio::io::AsyncRead;
+
+    /// Read adapter for `tokio::io::AsyncRead`
     pub struct TokioReader<T> {
-        inner: T,
+        inner: tokio::io::BufReader<T>,
     }

     impl<T: tokio::io::AsyncRead> TokioReader<T> {
         pub fn new(inner: T) -> Self {
-            Self { inner }
+            // buffer size "sweet spot" - larger sizes don't seem to provide any benefit
+            const BUF_SIZE: usize = 1024 * 16;
+            Self {
+                inner: tokio::io::BufReader::with_capacity(BUF_SIZE, inner),
+            }
         }
     }

--
2.39.2