prioritize sendfile over splice since it results in fewer context switches when sending to pipes

splice returns to userspace when the pipe is full, sendfile
just blocks until it's done, this can achieve much higher throughput

Author: the8472

library/std/src/io/copy.rs

@@ -173,22 +173,6 @@ mod kernel_copy {
                     )
                     .map(|bytes_copied| bytes_copied + bytes_flushed)
                 }
-                (CopyParams(reader_meta, Some(readfd)), CopyParams(writer_meta, Some(writefd)))
-                    if reader_meta.is_fifo() || writer_meta.is_fifo() =>
-                {
-                    // splice
-                    let bytes_flushed = flush()?;
-                    let max_write = reader.min_limit();
-                    let (mut reader, mut writer) =
-                        unsafe { (fd_as_file(readfd), fd_as_file(writefd)) };
-                    crate::sys::fs::sendfile_splice(
-                        crate::sys::fs::SpliceMode::Splice,
-                        &mut reader,
-                        &mut writer,
-                        max_write,
-                    )
-                    .map(|bytes_sent| bytes_sent + bytes_flushed)
-                }
                 (
                     CopyParams(FdMeta::Metadata(reader_meta), Some(readfd)),
                     CopyParams(_, Some(writefd)),
@@ -205,6 +189,22 @@ mod kernel_copy {
                         &mut reader,
                         &mut writer,
                         min(len, max_write),
+                    )
+                        .map(|bytes_sent| bytes_sent + bytes_flushed)
+                }
+                (CopyParams(reader_meta, Some(readfd)), CopyParams(writer_meta, Some(writefd)))
+                    if reader_meta.is_fifo() || writer_meta.is_fifo() =>
+                {
+                    // splice
+                    let bytes_flushed = flush()?;
+                    let max_write = reader.min_limit();
+                    let (mut reader, mut writer) =
+                        unsafe { (fd_as_file(readfd), fd_as_file(writefd)) };
+                    crate::sys::fs::sendfile_splice(
+                        crate::sys::fs::SpliceMode::Splice,
+                        &mut reader,
+                        &mut writer,
+                        max_write,
                     )
                     .map(|bytes_sent| bytes_sent + bytes_flushed)
                 }