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Index prefetch packs in parallel #1843

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114 changes: 72 additions & 42 deletions GVFS/GVFS.Common/Git/GitObjects.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -676,11 +676,16 @@ private LooseObjectToWrite GetLooseObjectDestination(string sha)
string tempPackFolderPath = Path.Combine(this.Enlistment.GitPackRoot, TempPackFolder);
this.fileSystem.CreateDirectory(tempPackFolderPath);

List<TempPrefetchPackAndIdx> tempPacks = new List<TempPrefetchPackAndIdx>();
foreach (PrefetchPacksDeserializer.PackAndIndex pack in deserializer.EnumeratePacks())
var tempPacksTasks = new List<Task<TempPrefetchPackAndIdx>>();
// Future: We could manage cancellation of index building tasks if one fails (to stop indexing of later
// files if an early one fails), but in practice the first pack file takes the majority of the time and
// all the others will finish long before it so there would be no benefit to doing so.
bool allSucceeded = true;

// Read each pack from the stream to a temp file, and start a task to index it.
foreach (PrefetchPacksDeserializer.PackAndIndex packHandle in deserializer.EnumeratePacks())
{
// The advertised size may not match the actual, on-disk size.
long indexLength = 0;
var pack = packHandle; // Capture packHandle in a new variable to avoid closure issues with async index task
long packLength;

// Write the temp and index to a temp folder to avoid putting corrupt files in the pack folder
Expand All @@ -701,73 +706,98 @@ private LooseObjectToWrite GetLooseObjectDestination(string sha)
if (!this.TryWriteTempFile(activity, pack.PackStream, packTempPath, out packLength, out packFlushTask))
{
bytesDownloaded += packLength;
return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(null, true);
allSucceeded = false;
break;
}

bytesDownloaded += packLength;

// We can't trust the index file from the server, so we will always build our own.
// We still need to consume and handle any exceptions from the index stream though.
var canContinue = true;
GitProcess.Result result;
if (this.TryBuildIndex(activity, packTempPath, out result, gitProcess))
// For performance, we run the index build in the background while we continue downloading the next pack.
var indexTask = Task.Run(async () =>
{
tempPacks.Add(new TempPrefetchPackAndIdx(pack.Timestamp, packName, packTempPath, packFlushTask, idxName, idxTempPath, idxFlushTask: null));
if (pack.IndexStream != null)
// GitProcess only permits one process per instance at a time, so we need to duplicate it to run the index build in parallel.
// This is safe because each process is only accessing the pack file we direct it to which is not yet part
// of the enlistment.
GitProcess gitProcessForIndex = new GitProcess(this.Enlistment);
if (this.TryBuildIndex(activity, packTempPath, out var _, gitProcessForIndex))
{
try
return new TempPrefetchPackAndIdx(pack.Timestamp, packName, packTempPath, packFlushTask, idxName, idxTempPath, idxFlushTask: null);
}
else
{
await packFlushTask;
return null;
}
});
tempPacksTasks.Add(indexTask);

// If the server provided an index stream, we still need to consume and handle any exceptions it even
// though we are otherwise ignoring it.
if (pack.IndexStream != null)
{
try
{
bytesDownloaded += pack.IndexStream.Length;
if (pack.IndexStream.CanSeek)
{
bytesDownloaded += pack.IndexStream.Length;
if (pack.IndexStream.CanSeek)
{
pack.IndexStream.Seek(0, SeekOrigin.End);
}
else
{
pack.IndexStream.CopyTo(Stream.Null);
}
pack.IndexStream.Seek(0, SeekOrigin.End);
}
catch (Exception e)
else
{
canContinue = false;
EventMetadata metadata = CreateEventMetadata(e);
activity.RelatedWarning(metadata, "Failed to read to end of index stream");
pack.IndexStream.CopyTo(Stream.Null);
}
}
catch (Exception e)
{
EventMetadata metadata = CreateEventMetadata(e);
activity.RelatedWarning(metadata, "Failed to read to end of index stream");
allSucceeded = false;
break;
}
}
else
}

// Wait for the index tasks to complete. If any fail, we still copy the prior successful ones
// to the pack folder so that the retry will be incremental from where the failure occurred.
var tempPacks = new List<TempPrefetchPackAndIdx>();
bool indexTasksSucceededSoFar = true;
foreach (var task in tempPacksTasks)
{
TempPrefetchPackAndIdx tempPack = task.Result;
if (tempPack != null && indexTasksSucceededSoFar)
{
canContinue = false;
tempPacks.Add(tempPack);
}

if (!canContinue)
else
{
if (packFlushTask != null)
{
packFlushTask.Wait();
}

// Move whatever has been successfully downloaded so far
Exception moveException;
this.TryFlushAndMoveTempPacks(tempPacks, ref latestTimestamp, out moveException);

return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(null, true);
indexTasksSucceededSoFar = false;
tempPack?.PackFlushTask.Wait();
break;
}
}
allSucceeded = allSucceeded && indexTasksSucceededSoFar;

Exception exception = null;
if (!this.TryFlushAndMoveTempPacks(tempPacks, ref latestTimestamp, out exception))
{
return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(exception, true);
allSucceeded = false;
}

foreach (TempPrefetchPackAndIdx tempPack in tempPacks)
{
packIndexes.Add(tempPack.IdxName);
}

return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(
new GitObjectsHttpRequestor.GitObjectTaskResult(success: true));
if (allSucceeded)
{
return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(
new GitObjectsHttpRequestor.GitObjectTaskResult(success: true));
}
else
{
return new RetryWrapper<GitObjectsHttpRequestor.GitObjectTaskResult>.CallbackResult(exception, shouldRetry: true);
}
}
}

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6 changes: 5 additions & 1 deletion GVFS/GVFS.Common/Git/GitProcess.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -580,7 +580,11 @@ public Result VerifyCommitGraph(string objectDir)

public Result IndexPack(string packfilePath, string idxOutputPath)
{
return this.InvokeGitAgainstDotGitFolder($"index-pack -o \"{idxOutputPath}\" \"{packfilePath}\"");
/* Git's default thread count is Environment.ProcessorCount / 2, with a maximum of 20.
* Testing shows that we can get a 5% decrease in gvfs clone time for large repositories by using more threads, but
* we won't go over ProcessorCount or 20. */
var threadCount = Math.Min(Environment.ProcessorCount, 20);
return this.InvokeGitAgainstDotGitFolder($"index-pack --threads={threadCount} -o \"{idxOutputPath}\" \"{packfilePath}\"");
}

/// <summary>
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