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Use tokio locks in VirtualFile and turn with_file into macro (#5247)

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## Problem

For #4743, we want to convert everything up to the actual I/O operations
of `VirtualFile` to `async fn`.

## Summary of changes

This PR is the last change in a series of changes to `VirtualFile`:
#5189, #5190, #5195, #5203, and #5224.

It does the last preparations before the I/O operations are actually
made async. We are doing the following things:

* First, we change the locks for the file descriptor cache to tokio's
locks that support Send. This is important when one wants to hold locks
across await points (which we want to do), otherwise the Future won't be
Send. Also, one shouldn't generally block in async code as executors
don't like that.
* Due to the lock change, we now take an approach for the `VirtualFile`
destructors similar to the one proposed by #5122 for the page cache, to
use `try_write`. Similarly to the situation in the linked PR, one can
make an argument that if we are in the destructor and the slot has not
been reused yet, we are the only user accessing the slot due to owning
the lock mutably. It is still possible that we are not obtaining the
lock, but the only cause for that is the clock algorithm touching the
slot, which should be quite an unlikely occurence. For the instance of
`try_write` failing, we spawn an async task to destroy the lock. As just
argued however, most of the time the code path where we spawn the task
should not be visited.
* Lastly, we split `with_file` into a macro part, and a function part
that contains most of the logic. The function part returns a lock
object, that the macro uses. The macro exists to perform the operation
in a more compact fashion, saving code from putting the lock into a
variable and then doing the operation while measuring the time to run
it. We take the locks approach because Rust has no support for async
closures. One can make normal closures return a future, but that
approach gets into lifetime issues the moment you want to pass data to
these closures via parameters that has a lifetime (captures work). For
details, see
[this](https://smallcultfollowing.com/babysteps/blog/2023/03/29/thoughts-on-async-closures/)
and
[this](https://users.rust-lang.org/t/function-that-takes-an-async-closure/61663)
link. In #5224, we ran into a similar problem with the `test_files`
function, and we ended up passing the path and the `OpenOptions`
by-value instead of by-ref, at the expense of a few extra copies. This
can be done as the data is cheaply copyable, and we are in test code.
But here, we are not, and while `File::try_clone` exists, it [issues
system calls
internally](1e746d7741/library/std/src/os/fd/owned.rs (L94-L111)).
Also, it would allocate an entirely new file descriptor, something that
the fd cache was built to prevent.
* We change the `STORAGE_IO_TIME` metrics to support async.

Part of #4743.
This commit is contained in:
Arpad Müller
2023-09-11 17:35:05 +02:00
committed by GitHub
parent c0ed362790
commit 76cc87398c
1 changed files with 94 additions and 56 deletions
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150
pageserver/src/virtual_file.rs
View File

@@ -18,7 +18,8 @@ use std::io::{Error, ErrorKind, Seek, SeekFrom};
use std::os::unix::fs::FileExt;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::{RwLock, RwLockWriteGuard};
use tokio::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
use tokio::time::Instant;
///
/// A virtual file descriptor. You can use this just like std::fs::File, but internally
@@ -110,7 +111,7 @@ impl OpenFiles {
///
/// On return, we hold a lock on the slot, and its 'tag' has been updated
/// recently_used has been set. It's all ready for reuse.
fn find_victim_slot(&self) -> (SlotHandle, RwLockWriteGuard<SlotInner>) {
async fn find_victim_slot(&self) -> (SlotHandle, RwLockWriteGuard<SlotInner>) {
//
// Run the clock algorithm to find a slot to replace.
//
@@ -142,7 +143,7 @@ impl OpenFiles {
}
retries += 1;
} else {
slot_guard = slot.inner.write().unwrap();
slot_guard = slot.inner.write().await;
index = next;
break;
}
@@ -208,6 +209,19 @@ impl CrashsafeOverwriteError {
}
}
macro_rules! with_file {
($this:expr, $op:expr, | $ident:ident | $($body:tt)*) => {{
let $ident = $this.lock_file().await?;
let instant = Instant::now();
let result = $($body)*;
let elapsed = instant.elapsed().as_secs_f64();
STORAGE_IO_TIME_METRIC
.get($op)
.observe(elapsed);
result
}};
}
impl VirtualFile {
/// Open a file in read-only mode. Like File::open.
pub async fn open(path: &Path) -> Result<VirtualFile, std::io::Error> {
@@ -244,7 +258,7 @@ impl VirtualFile {
tenant_id = "*".to_string();
timeline_id = "*".to_string();
}
let (handle, mut slot_guard) = get_open_files().find_victim_slot();
let (handle, mut slot_guard) = get_open_files().find_victim_slot().await;
let file = STORAGE_IO_TIME_METRIC
.get(StorageIoOperation::Open)
@@ -331,22 +345,24 @@ impl VirtualFile {
/// Call File::sync_all() on the underlying File.
pub async fn sync_all(&self) -> Result<(), Error> {
self.with_file(StorageIoOperation::Fsync, |file| file.sync_all())
.await?
with_file!(self, StorageIoOperation::Fsync, |file| file
.as_ref()
.sync_all())
}
pub async fn metadata(&self) -> Result<fs::Metadata, Error> {
self.with_file(StorageIoOperation::Metadata, |file| file.metadata())
.await?
with_file!(self, StorageIoOperation::Metadata, |file| file
.as_ref()
.metadata())
}
/// Helper function that looks up the underlying File for this VirtualFile,
/// opening it and evicting some other File if necessary. It calls 'func'
/// with the physical File.
async fn with_file<F, R>(&self, op: StorageIoOperation, mut func: F) -> Result<R, Error>
where
F: FnMut(&File) -> R,
{
/// Helper function internal to `VirtualFile` that looks up the underlying File,
/// opens it and evicts some other File if necessary. The passed parameter is
/// assumed to be a function available for the physical `File`.
///
/// We are doing it via a macro as Rust doesn't support async closures that
/// take on parameters with lifetimes.
async fn lock_file(&self) -> Result<FileGuard<'_>, Error> {
let open_files = get_open_files();
let mut handle_guard = {
@@ -356,27 +372,23 @@ impl VirtualFile {
// We only need to hold the handle lock while we read the current handle. If
// another thread closes the file and recycles the slot for a different file,
// we will notice that the handle we read is no longer valid and retry.
let mut handle = *self.handle.read().unwrap();
let mut handle = *self.handle.read().await;
loop {
// Check if the slot contains our File
{
let slot = &open_files.slots[handle.index];
let slot_guard = slot.inner.read().unwrap();
if slot_guard.tag == handle.tag {
if let Some(file) = &slot_guard.file {
// Found a cached file descriptor.
slot.recently_used.store(true, Ordering::Relaxed);
return Ok(STORAGE_IO_TIME_METRIC
.get(op)
.observe_closure_duration(|| func(file)));
}
let slot_guard = slot.inner.read().await;
if slot_guard.tag == handle.tag && slot_guard.file.is_some() {
// Found a cached file descriptor.
slot.recently_used.store(true, Ordering::Relaxed);
return Ok(FileGuard { slot_guard });
}
}
// The slot didn't contain our File. We will have to open it ourselves,
// but before that, grab a write lock on handle in the VirtualFile, so
// that no other thread will try to concurrently open the same file.
let handle_guard = self.handle.write().unwrap();
let handle_guard = self.handle.write().await;
// If another thread changed the handle while we were not holding the lock,
// then the handle might now be valid again. Loop back to retry.
@@ -390,25 +402,22 @@ impl VirtualFile {
// We need to open the file ourselves. The handle in the VirtualFile is
// now locked in write-mode. Find a free slot to put it in.
let (handle, mut slot_guard) = open_files.find_victim_slot();
let (handle, mut slot_guard) = open_files.find_victim_slot().await;
// Open the physical file
let file = STORAGE_IO_TIME_METRIC
.get(StorageIoOperation::Open)
.observe_closure_duration(|| self.open_options.open(&self.path))?;
// Perform the requested operation on it
let result = STORAGE_IO_TIME_METRIC
.get(op)
.observe_closure_duration(|| func(&file));
// Store the File in the slot and update the handle in the VirtualFile
// to point to it.
slot_guard.file.replace(file);
*handle_guard = handle;
Ok(result)
return Ok(FileGuard {
slot_guard: slot_guard.downgrade(),
});
}
pub fn remove(self) {
@@ -423,11 +432,9 @@ impl VirtualFile {
self.pos = offset;
}
SeekFrom::End(offset) => {
self.pos = self
.with_file(StorageIoOperation::Seek, |mut file| {
file.seek(SeekFrom::End(offset))
})
.await??
self.pos = with_file!(self, StorageIoOperation::Seek, |file| file
.as_ref()
.seek(SeekFrom::End(offset)))?
}
SeekFrom::Current(offset) => {
let pos = self.pos as i128 + offset as i128;
@@ -515,9 +522,9 @@ impl VirtualFile {
}
pub async fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize, Error> {
let result = self
.with_file(StorageIoOperation::Read, |file| file.read_at(buf, offset))
.await?;
let result = with_file!(self, StorageIoOperation::Read, |file| file
.as_ref()
.read_at(buf, offset));
if let Ok(size) = result {
STORAGE_IO_SIZE
.with_label_values(&["read", &self.tenant_id, &self.timeline_id])
@@ -527,9 +534,9 @@ impl VirtualFile {
}
async fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize, Error> {
let result = self
.with_file(StorageIoOperation::Write, |file| file.write_at(buf, offset))
.await?;
let result = with_file!(self, StorageIoOperation::Write, |file| file
.as_ref()
.write_at(buf, offset));
if let Ok(size) = result {
STORAGE_IO_SIZE
.with_label_values(&["write", &self.tenant_id, &self.timeline_id])
@@ -539,6 +546,18 @@ impl VirtualFile {
}
}
struct FileGuard<'a> {
slot_guard: RwLockReadGuard<'a, SlotInner>,
}
impl<'a> AsRef<File> for FileGuard<'a> {
fn as_ref(&self) -> &File {
// This unwrap is safe because we only create `FileGuard`s
// if we know that the file is Some.
self.slot_guard.file.as_ref().unwrap()
}
}
#[cfg(test)]
impl VirtualFile {
pub(crate) async fn read_blk(
@@ -571,20 +590,39 @@ impl VirtualFile {
impl Drop for VirtualFile {
/// If a VirtualFile is dropped, close the underlying file if it was open.
fn drop(&mut self) {
let handle = self.handle.get_mut().unwrap();
let handle = self.handle.get_mut();
// We could check with a read-lock first, to avoid waiting on an
// unrelated I/O.
let slot = &get_open_files().slots[handle.index];
let mut slot_guard = slot.inner.write().unwrap();
if slot_guard.tag == handle.tag {
slot.recently_used.store(false, Ordering::Relaxed);
// there is also operation "close-by-replace" for closes done on eviction for
// comparison.
STORAGE_IO_TIME_METRIC
.get(StorageIoOperation::Close)
.observe_closure_duration(|| drop(slot_guard.file.take()));
fn clean_slot(slot: &Slot, mut slot_guard: RwLockWriteGuard<'_, SlotInner>, tag: u64) {
if slot_guard.tag == tag {
slot.recently_used.store(false, Ordering::Relaxed);
// there is also operation "close-by-replace" for closes done on eviction for
// comparison.
STORAGE_IO_TIME_METRIC
.get(StorageIoOperation::Close)
.observe_closure_duration(|| drop(slot_guard.file.take()));
}
}
// We don't have async drop so we cannot directly await the lock here.
// Instead, first do a best-effort attempt at closing the underlying
// file descriptor by using `try_write`, and if that fails, spawn
// a tokio task to do it asynchronously: we just want it to be
// cleaned up eventually.
// Most of the time, the `try_lock` should succeed though,
// as we have `&mut self` access. In other words, if the slot
// is still occupied by our file, there should be no access from
// other I/O operations; the only other possible place to lock
// the slot is the lock algorithm looking for free slots.
let slot = &get_open_files().slots[handle.index];
if let Ok(slot_guard) = slot.inner.try_write() {
clean_slot(slot, slot_guard, handle.tag);
} else {
let tag = handle.tag;
tokio::spawn(async move {
let slot_guard = slot.inner.write().await;
clean_slot(slot, slot_guard, tag);
});
};
}
}