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Leverage emulated sector size on Windows to decrease usage of the interface bandwidth with SSD #2597

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Mar 11, 2024
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Leverage emulated sector size on Windows to decrease usage of the interface bandwidth with SSD #2597

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69 changes: 49 additions & 20 deletions crates/subspace-farmer/src/single_disk_farm/unbuffered_io_file_windows.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ const MAX_READ_SIZE: usize = 1024 * 1024;
pub struct UnbufferedIoFileWindows {
read_file: File,
write_file: File,
physical_sector_size: usize,
/// Scratch buffer of aligned memory for reads and writes
buffer: Mutex<Vec<[u8; DISK_SECTOR_SIZE]>>,
}
Expand Down Expand Up @@ -66,9 +67,14 @@ impl FileExt for UnbufferedIoFileWindows {
buffer.resize(desired_buffer_size, [0; DISK_SECTOR_SIZE]);
}

// While buffer above is allocated with granularity of `MAX_DISK_SECTOR_SIZE`, reads are
// done with granularity of physical sector size
Comment on lines +70 to +71
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Perhaps the buffer can be just Vec<u8> and allocate according to bytes_to_read + offset_in_buffer, non-blocker.

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Buffer's memory needs to be aligned properly, this is why buffer is allocated to contain arrays of 4096 bytes, forcing memory alignment to 4096 bytes. By extension anything that is aligned to 4096 bytes is also aligned to 512 bytes.

let offset_in_buffer = (offset % self.physical_sector_size as u64) as usize;
self.read_file.read_at(
buffer[..desired_buffer_size].flatten_mut(),
offset / DISK_SECTOR_SIZE as u64 * DISK_SECTOR_SIZE as u64,
&mut buffer.flatten_mut()[..(bytes_to_read + offset_in_buffer)
.div_ceil(self.physical_sector_size)
* self.physical_sector_size],
offset / self.physical_sector_size as u64 * self.physical_sector_size as u64,
)?;

buf.copy_from_slice(&buffer.flatten()[offset_in_buffer..][..bytes_to_read]);
Expand Down Expand Up @@ -103,9 +109,17 @@ impl UnbufferedIoFileWindows {
open_options.advise_unbuffered();
let read_file = open_options.read(true).open(path)?;

// Physical sector size on many SSDs is smaller than 4096 and should improve performance
let physical_sector_size = if read_file.read_at(&mut [0; 512], 512).is_ok() {
512
} else {
DISK_SECTOR_SIZE
};

Ok(Self {
read_file,
write_file,
physical_sector_size,
buffer: Mutex::default(),
})
}
Expand Down Expand Up @@ -134,24 +148,39 @@ mod tests {
thread_rng().fill(data.as_mut_slice());
fs::write(&file_path, &data).unwrap();

let file = UnbufferedIoFileWindows::open(&file_path).unwrap();

let mut buffer = Vec::new();
for (offset, size) in [
(0_usize, 4096_usize),
(0, 4000),
(5, 50),
(5, 4091),
(5, MAX_READ_SIZE * 2),
] {
buffer.resize(size, 0);
file.read_at(buffer.as_mut_slice(), offset as u64)
.unwrap_or_else(|error| panic!("Offset {offset}, size {size}: {error}"));
assert_eq!(
&data[offset..][..size],
buffer.as_slice(),
"Offset {offset}, size {size}"
);
let mut file = UnbufferedIoFileWindows::open(&file_path).unwrap();

for override_physical_sector_size in [None, Some(4096)] {
if let Some(physical_sector_size) = override_physical_sector_size {
file.physical_sector_size = physical_sector_size;
}

let mut buffer = Vec::new();
for (offset, size) in [
(0_usize, 4096_usize),
(0, 4000),
(5, 50),
(5, 4091),
(4091, 5),
(10000, 5),
(5, MAX_READ_SIZE * 2),
] {
buffer.resize(size, 0);
file.read_at(buffer.as_mut_slice(), offset as u64)
.unwrap_or_else(|error| {
panic!(
"Offset {offset}, size {size}, override physical sector size \
{override_physical_sector_size:?}: {error}"
)
});

assert_eq!(
&data[offset..][..size],
buffer.as_slice(),
"Offset {offset}, size {size}, override physical sector size \
{override_physical_sector_size:?}"
);
}
}
}
}
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