[pbs-devel] [PATCH v2 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]

`tokio/io-util` is added as dependency in order to use
`tokio::io::BufReader`.

[0]: https://docs.rs/tokio/1.29.1/tokio/io/struct.BufReader.html
[1]: Tested via examples/compare-read.rs on a large (13GB) pxar archive

Before:
> PXAR Read Performance Comparison
> Running in mode: release
>
> First pass:
> With aio::Decoder:   Ok(()) (elapsed: 20.532270177s)
> With sync::Decoder:  Ok(()) (elapsed: 3.498566141s)
> With aio::Accessor:  Ok(()) (elapsed: 3.978160609s)
> With sync::Accessor: Ok(()) (elapsed: 3.885640895s)
>
> Second pass:
> With aio::Decoder:   Ok(()) (elapsed: 18.648986266s)
> With sync::Decoder:  Ok(()) (elapsed: 3.617167922s)
> With aio::Accessor:  Ok(()) (elapsed: 4.083678211s)
> With sync::Accessor: Ok(()) (elapsed: 4.103763507s)

After:
> PXAR Read Performance Comparison
> Running in mode: release
>
> First pass:
> With aio::Decoder:   Ok(()) (elapsed: 9.546522171s)
> With sync::Decoder:  Ok(()) (elapsed: 3.535062119s)
> With aio::Accessor:  Ok(()) (elapsed: 3.926439101s)
> With sync::Accessor: Ok(()) (elapsed: 3.905232916s)
>
> Second pass:
> With aio::Decoder:   Ok(()) (elapsed: 10.633561678s)
> With sync::Decoder:  Ok(()) (elapsed: 3.528989778s)
> With aio::Accessor:  Ok(()) (elapsed: 3.831093917s)
> With sync::Accessor: Ok(()) (elapsed: 3.848684845s)

Signed-off-by: Max Carrara <m.carrara at proxmox.com>
---
 Changes v1 --> v2:
  * Include addition of `tokio/io-util` as dependency
  * Use new examples/compare-read.rs instead of old custom tool to
    measure performance impact
  * Use default buffer size (8K) instead of 16K
    (I wasn't able to reproduce the performance gains, so ...)

 Cargo.toml         |  2 +-
 src/decoder/aio.rs | 10 +++++++---
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 8669e30..08c0973 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -63,7 +63,7 @@ libc = "0.2"

 [features]
 default = [ "tokio-io" ]
-tokio-io = [ "tokio" ]
+tokio-io = [ "tokio", "tokio/io-util" ]
 tokio-fs = [ "tokio-io", "tokio/fs" ]

 full = [ "tokio-fs"]
diff --git a/src/decoder/aio.rs b/src/decoder/aio.rs
index 200dd3d..7cb9c12 100644
--- a/src/decoder/aio.rs
+++ b/src/decoder/aio.rs
@@ -79,14 +79,18 @@ 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 }
+            Self {
+                inner: tokio::io::BufReader::new(inner),
+            }
         }
     }

--
2.39.2