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Implement core garbage collection logic for the disk cache.
The garbage collection policy is defined by a maximum target size and a maximum age of individual cache entries, both of which may be simultaneously provided. I/O operations are parallelized to improve performance for large caches or slow filesystems. PiperOrigin-RevId: 677860078 Change-Id: Ib342ad5e80ef4ef4af237aae243a300d13caaa06
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src/main/java/com/google/devtools/build/lib/remote/disk/DiskCacheGarbageCollector.java
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// Copyright 2024 The Bazel Authors. All rights reserved. | ||
// | ||
// Licensed under the Apache License, Version 2.0 (the "License"); | ||
// you may not use this file except in compliance with the License. | ||
// You may obtain a copy of the License at | ||
// | ||
// http://www.apache.org/licenses/LICENSE-2.0 | ||
// | ||
// Unless required by applicable law or agreed to in writing, software | ||
// distributed under the License is distributed on an "AS IS" BASIS, | ||
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
// See the License for the specific language governing permissions and | ||
// limitations under the License. | ||
package com.google.devtools.build.lib.remote.disk; | ||
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import static com.google.common.collect.ImmutableSet.toImmutableSet; | ||
import static java.util.Comparator.comparing; | ||
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import com.google.common.collect.ImmutableSet; | ||
import com.google.devtools.build.lib.concurrent.AbstractQueueVisitor; | ||
import com.google.devtools.build.lib.concurrent.ErrorClassifier; | ||
import com.google.devtools.build.lib.vfs.Dirent; | ||
import com.google.devtools.build.lib.vfs.FileStatus; | ||
import com.google.devtools.build.lib.vfs.IORuntimeException; | ||
import com.google.devtools.build.lib.vfs.Path; | ||
import com.google.devtools.build.lib.vfs.Symlinks; | ||
import java.io.IOException; | ||
import java.time.Duration; | ||
import java.time.Instant; | ||
import java.util.ArrayList; | ||
import java.util.List; | ||
import java.util.Optional; | ||
import java.util.concurrent.ExecutorService; | ||
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/** | ||
* A garbage collector for the disk cache. | ||
* | ||
* <p>Garbage collection works by enumerating the entire contents of the disk cache, identifying | ||
* candidates for deletion according to a {@link CollectionPolicy}, and deleting them. This process | ||
* may take a significant amount of time on large disk caches and slow filesystems, and may be | ||
* interrupted at any time. | ||
*/ | ||
public final class DiskCacheGarbageCollector { | ||
private static final ImmutableSet<String> EXCLUDED_DIRS = ImmutableSet.of("tmp", "gc"); | ||
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/** | ||
* Describes a disk cache entry. | ||
* | ||
* @param path path relative to the root directory of the disk cache | ||
* @param size file size in bytes | ||
* @param mtime file modification time | ||
*/ | ||
private record Entry(String path, long size, long mtime) {} | ||
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/** Determines which entries should be collected. */ | ||
public static final class CollectionPolicy { | ||
private final Optional<Long> maxSizeBytes; | ||
private final Optional<Duration> maxAge; | ||
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/** | ||
* Creates a new policy. | ||
* | ||
* @param maxSizeBytes the maximum total size in bytes, or empty for no size limit | ||
* @param maxAge the maximum age of cache entries, or empty for no age limit | ||
*/ | ||
CollectionPolicy(Optional<Long> maxSizeBytes, Optional<Duration> maxAge) { | ||
this.maxSizeBytes = maxSizeBytes; | ||
this.maxAge = maxAge; | ||
} | ||
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/** | ||
* Returns the entries to be deleted. | ||
* | ||
* @param entries the full list of entries | ||
*/ | ||
List<Entry> getEntriesToDelete(List<Entry> entries) { | ||
entries.sort(comparing(Entry::mtime)); | ||
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long excessSizeBytes = getExcessSizeBytes(entries); | ||
long timeCutoff = getTimeCutoff(); | ||
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int i = 0; | ||
for (; i < entries.size(); i++) { | ||
if (excessSizeBytes <= 0 && entries.get(i).mtime() >= timeCutoff) { | ||
break; | ||
} | ||
excessSizeBytes -= entries.get(i).size(); | ||
} | ||
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return entries.subList(0, i); | ||
} | ||
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private long getExcessSizeBytes(List<Entry> entries) { | ||
if (maxSizeBytes.isEmpty()) { | ||
return 0; | ||
} | ||
long currentSizeBytes = entries.stream().mapToLong(Entry::size).sum(); | ||
return currentSizeBytes - maxSizeBytes.get(); | ||
} | ||
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private long getTimeCutoff() { | ||
if (maxAge.isEmpty()) { | ||
return 0; | ||
} | ||
return Instant.now().minus(maxAge.get()).toEpochMilli(); | ||
} | ||
} | ||
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private final Path root; | ||
private final CollectionPolicy policy; | ||
private final ExecutorService executorService; | ||
private final ImmutableSet<Path> excludedDirs; | ||
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/** | ||
* Creates a new garbage collector. | ||
* | ||
* @param root the root directory of the disk cache | ||
* @param executorService the executor service to schedule I/O operations onto | ||
* @param policy the garbage collection policy to use | ||
*/ | ||
public DiskCacheGarbageCollector( | ||
Path root, ExecutorService executorService, CollectionPolicy policy) { | ||
this.root = root; | ||
this.policy = policy; | ||
this.executorService = executorService; | ||
this.excludedDirs = EXCLUDED_DIRS.stream().map(root::getChild).collect(toImmutableSet()); | ||
} | ||
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/** | ||
* Runs garbage collection. | ||
* | ||
* @throws IOException if an I/O error occurred | ||
* @throws InterruptedException if the thread was interrupted | ||
*/ | ||
public void run() throws IOException, InterruptedException { | ||
EntryScanner scanner = new EntryScanner(); | ||
EntryDeleter deleter = new EntryDeleter(); | ||
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List<Entry> allEntries = scanner.scan(); | ||
List<Entry> entriesToDelete = policy.getEntriesToDelete(allEntries); | ||
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for (Entry entry : entriesToDelete) { | ||
deleter.delete(root.getRelative(entry.path())); | ||
} | ||
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deleter.await(); | ||
} | ||
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/** Lists all disk cache entries, performing I/O in parallel. */ | ||
private final class EntryScanner extends AbstractQueueVisitor { | ||
private final ArrayList<Entry> entries = new ArrayList<>(); | ||
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EntryScanner() { | ||
super( | ||
executorService, | ||
ExecutorOwnership.SHARED, | ||
ExceptionHandlingMode.FAIL_FAST, | ||
ErrorClassifier.DEFAULT); | ||
} | ||
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/** Lists all disk cache entries. */ | ||
List<Entry> scan() throws IOException, InterruptedException { | ||
execute(() -> visitDirectory(root)); | ||
try { | ||
awaitQuiescence(true); | ||
} catch (IORuntimeException e) { | ||
throw e.getCauseIOException(); | ||
} | ||
return entries; | ||
} | ||
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private void visitDirectory(Path path) { | ||
try { | ||
for (Dirent dirent : path.readdir(Symlinks.NOFOLLOW)) { | ||
Path childPath = path.getChild(dirent.getName()); | ||
if (dirent.getType().equals(Dirent.Type.FILE)) { | ||
// The file may be gone by the time we open it. | ||
FileStatus status = childPath.statIfFound(); | ||
if (status != null) { | ||
Entry entry = | ||
new Entry( | ||
childPath.relativeTo(root).getPathString(), | ||
status.getSize(), | ||
status.getLastModifiedTime()); | ||
synchronized (entries) { | ||
entries.add(entry); | ||
} | ||
} | ||
} else if (dirent.getType().equals(Dirent.Type.DIRECTORY) | ||
&& !excludedDirs.contains(childPath)) { | ||
execute(() -> visitDirectory(childPath)); | ||
} | ||
// Deliberately ignore other file types, which should never occur in a well-formed cache. | ||
} | ||
} catch (IOException e) { | ||
throw new IORuntimeException(e); | ||
} | ||
} | ||
} | ||
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/** Deletes disk cache entries, performing I/O in parallel. */ | ||
private final class EntryDeleter extends AbstractQueueVisitor { | ||
EntryDeleter() { | ||
super( | ||
executorService, | ||
ExecutorOwnership.SHARED, | ||
ExceptionHandlingMode.FAIL_FAST, | ||
ErrorClassifier.DEFAULT); | ||
} | ||
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/** Enqueues an entry to be deleted. */ | ||
void delete(Path path) { | ||
execute( | ||
() -> { | ||
try { | ||
path.delete(); | ||
} catch (IOException e) { | ||
throw new IORuntimeException(e); | ||
} | ||
}); | ||
} | ||
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/** Waits for all enqueued deletions to complete. */ | ||
void await() throws IOException, InterruptedException { | ||
try { | ||
awaitQuiescence(true); | ||
} catch (IORuntimeException e) { | ||
throw e.getCauseIOException(); | ||
} | ||
} | ||
} | ||
} |
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