Reintroduce garbage pack coalescing when ttl > 0.

Disabling the garbage pack coalescing when garbageTtl > 0 can result in
lot of garbage packs if they are created within the garbageTtl time.

To avoid a large number of garbage packs, re-introducing garbage pack
coalescing for the packs that are created within a single calendar day
when the garbageTtl is more than one day or one third of the garbageTtl.

Change-Id: If969716aeb55fb4fd0ff71d75f41a07638cd5a69
Signed-off-by: Thirumala Reddy Mutchukota <thirumala@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java

@@ -16,12 +16,15 @@ import java.io.IOException;
 import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource;
+import org.eclipse.jgit.junit.MockSystemReader;
 import org.eclipse.jgit.junit.TestRepository;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.util.SystemReader;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -29,6 +32,7 @@ public class DfsGarbageCollectorTest {
 	private TestRepository<InMemoryRepository> git;
 	private InMemoryRepository repo;
 	private DfsObjDatabase odb;
+	private MockSystemReader mockSystemReader;
 
 	@Before
 	public void setUp() throws IOException {
@@ -36,6 +40,13 @@ public class DfsGarbageCollectorTest {
 		git = new TestRepository<>(new InMemoryRepository(desc));
 		repo = git.getRepository();
 		odb = repo.getObjectDatabase();
+		mockSystemReader = new MockSystemReader();
+		SystemReader.setInstance(mockSystemReader);
+	}
+
+	@After
+	public void tearDown() {
+		SystemReader.setInstance(null);
 	}
 
 	@Test
@@ -171,6 +182,58 @@ public class DfsGarbageCollectorTest {
 		}
 	}
 
+	@Test
+	public void testCollectionWithGarbageCoalescenceWithShortTtl()
+			throws Exception {
+		RevCommit commit0 = commit().message("0").create();
+		RevCommit commit1 = commit().message("1").parent(commit0).create();
+		git.update("master", commit0);
+
+		// Create commits at 1 minute intervals with 1 hour ttl.
+		for (int i = 0; i < 100; i++) {
+			mockSystemReader.tick(60);
+			commit1 = commit().message("g" + i).parent(commit1).create();
+
+			DfsGarbageCollector gc = new DfsGarbageCollector(repo);
+			gc.setGarbageTtl(1, TimeUnit.HOURS);
+			run(gc);
+
+			// Make sure we don't have more than 4 UNREACHABLE_GARBAGE packs
+			// because all the packs that are created in a 20 minutes interval
+			// should be coalesced and the packs older than 60 minutes should be
+			// removed due to ttl.
+			int count = countPacks(UNREACHABLE_GARBAGE);
+			assertTrue("Garbage pack count should not exceed 4, but found "
+					+ count, count <= 4);
+		}
+	}
+
+	@Test
+	public void testCollectionWithGarbageCoalescenceWithLongTtl()
+			throws Exception {
+		RevCommit commit0 = commit().message("0").create();
+		RevCommit commit1 = commit().message("1").parent(commit0).create();
+		git.update("master", commit0);
+
+		// Create commits at 1 hour intervals with 2 days ttl.
+		for (int i = 0; i < 100; i++) {
+			mockSystemReader.tick(3600);
+			commit1 = commit().message("g" + i).parent(commit1).create();
+
+			DfsGarbageCollector gc = new DfsGarbageCollector(repo);
+			gc.setGarbageTtl(2, TimeUnit.DAYS);
+			run(gc);
+
+			// Make sure we don't have more than 3 UNREACHABLE_GARBAGE packs
+			// because all the packs that are created in a single day should
+			// be coalesced and the packs older than 2 days should be
+			// removed due to ttl.
+			int count = countPacks(UNREACHABLE_GARBAGE);
+			assertTrue("Garbage pack count should not exceed 3, but found "
+					+ count, count <= 3);
+		}
+	}
+
 	@Test
 	public void testEstimateGcPackSizeInNewRepo() throws Exception {
 		RevCommit commit0 = commit().message("0").create();
@@ -420,20 +483,17 @@ public class DfsGarbageCollectorTest {
 		run(gc);
 	}
 
-	private void gcWithTtl() throws InterruptedException, IOException {
-		// Wait for the system clock to move by at least 1 millisecond.
-		// This allows the DfsGarbageCollector to recognize the boundary.
-		long start = System.currentTimeMillis();
-		do {
-			Thread.sleep(10);
-		} while (System.currentTimeMillis() <= start);
-
+	private void gcWithTtl() throws IOException {
+		// Move the clock forward by 1 minute and use the same as ttl.
+		mockSystemReader.tick(60);
 		DfsGarbageCollector gc = new DfsGarbageCollector(repo);
-		gc.setGarbageTtl(1, TimeUnit.MILLISECONDS);
+		gc.setGarbageTtl(1, TimeUnit.MINUTES);
 		run(gc);
 	}
 
 	private void run(DfsGarbageCollector gc) throws IOException {
+		// adjust the current time that will be used by the gc operation.
+		mockSystemReader.tick(1);
 		assertTrue("gc repacked", gc.pack(null));
 		odb.clearCache();
 	}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java

@@ -56,9 +56,11 @@ import static org.eclipse.jgit.internal.storage.pack.PackExt.PACK;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Calendar;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
+import java.util.GregorianCalendar;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
@@ -82,6 +84,7 @@ import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.storage.pack.PackStatistics;
+import org.eclipse.jgit.util.SystemReader;
 import org.eclipse.jgit.util.io.CountingOutputStream;
 
 /** Repack and garbage collect a repository. */
@@ -100,7 +103,8 @@ public class DfsGarbageCollector {
 
 	private PackConfig packConfig;
 
-	// See pack(), below, for how these two variables interact.
+	// See packIsCoalesceableGarbage(), below, for how these two variables
+	// interact.
 	private long coalesceGarbageLimit = 50 << 20;
 	private long garbageTtlMillis = TimeUnit.DAYS.toMillis(1);
 
@@ -228,14 +232,8 @@ public class DfsGarbageCollector {
 		if (packConfig.getIndexVersion() != 2)
 			throw new IllegalStateException(
 					JGitText.get().supportOnlyPackIndexVersion2);
-		if (garbageTtlMillis > 0) {
-			// We disable coalescing because the coalescing step will keep
-			// refreshing the UNREACHABLE_GARBAGE pack and we wouldn't
-			// actually prune anything.
-			coalesceGarbageLimit = 0;
-		}
 
-		startTimeMillis = System.currentTimeMillis();
+		startTimeMillis = SystemReader.getInstance().getCurrentTime();
 		ctx = (DfsReader) objdb.newReader();
 		try {
 			refdb.refresh();
@@ -310,14 +308,14 @@ public class DfsGarbageCollector {
 		expiredGarbagePacks = new ArrayList<DfsPackFile>(packs.length);
 
 		long mostRecentGC = mostRecentGC(packs);
-		long now = System.currentTimeMillis();
+		long now = SystemReader.getInstance().getCurrentTime();
 		for (DfsPackFile p : packs) {
 			DfsPackDescription d = p.getPackDescription();
 			if (d.getPackSource() != UNREACHABLE_GARBAGE) {
 				packsBefore.add(p);
 			} else if (packIsExpiredGarbage(d, mostRecentGC, now)) {
 				expiredGarbagePacks.add(p);
-			} else if (d.getFileSize(PackExt.PACK) < coalesceGarbageLimit) {
+			} else if (packIsCoalesceableGarbage(d, now)) {
 				packsBefore.add(p);
 			}
 		}
@@ -360,6 +358,68 @@ public class DfsGarbageCollector {
 				&& now - d.getLastModified() >= garbageTtlMillis;
 	}
 
+	private boolean packIsCoalesceableGarbage(DfsPackDescription d, long now) {
+		// An UNREACHABLE_GARBAGE pack can be coalesced if its size is less than
+		// the coalesceGarbageLimit and either garbageTtl is zero or if the pack
+		// is created in a close time interval (on a single calendar day when
+		// the garbageTtl is more than one day or one third of the garbageTtl).
+		//
+		// When the garbageTtl is more than 24 hours, garbage packs that are
+		// created within a single calendar day are coalesced together. This
+		// would make the effective ttl of the garbage pack as garbageTtl+23:59
+		// and limit the number of garbage to a maximum number of
+		// garbageTtl_in_days + 1 (assuming all of them are less than the size
+		// of coalesceGarbageLimit).
+		//
+		// When the garbageTtl is less than or equal to 24 hours, garbage packs
+		// that are created within a one third of garbageTtl are coalesced
+		// together. This would make the effective ttl of the garbage packs as
+		// garbageTtl + (garbageTtl / 3) and would limit the number of garbage
+		// packs to a maximum number of 4 (assuming all of them are less than
+		// the size of coalesceGarbageLimit).
+
+		if (d.getPackSource() != UNREACHABLE_GARBAGE
+				|| d.getFileSize(PackExt.PACK) >= coalesceGarbageLimit) {
+			return false;
+		}
+
+		if (garbageTtlMillis == 0) {
+			return true;
+		}
+
+		long lastModified = d.getLastModified();
+		long dayStartLastModified = dayStartInMillis(lastModified);
+		long dayStartToday = dayStartInMillis(now);
+
+		if (dayStartLastModified != dayStartToday) {
+			return false; // this pack is not created today.
+		}
+
+		if (garbageTtlMillis > TimeUnit.DAYS.toMillis(1)) {
+			return true; // ttl is more than one day and pack is created today.
+		}
+
+		long timeInterval = garbageTtlMillis / 3;
+		if (timeInterval == 0) {
+			return false; // ttl is too small, don't try to coalesce.
+		}
+
+		long modifiedTimeSlot = (lastModified - dayStartLastModified) / timeInterval;
+		long presentTimeSlot = (now - dayStartToday) / timeInterval;
+		return modifiedTimeSlot == presentTimeSlot;
+	}
+
+	private static long dayStartInMillis(long timeInMillis) {
+		Calendar cal = new GregorianCalendar(
+				SystemReader.getInstance().getTimeZone());
+		cal.setTimeInMillis(timeInMillis);
+		cal.set(Calendar.HOUR_OF_DAY, 0);
+		cal.set(Calendar.MINUTE, 0);
+		cal.set(Calendar.SECOND, 0);
+		cal.set(Calendar.MILLISECOND, 0);
+		return cal.getTimeInMillis();
+	}
+
 	/** @return all of the source packs that fed into this compaction. */
 	public List<DfsPackDescription> getSourcePacks() {
 		return toPrune();