// kick off a prefetch
public override async Task RunPrefetchSession(IAsyncEnumerable <TrackedObjectKey> keys)
{
int maxConcurrency = 500;
using SemaphoreSlim prefetchSemaphore = new SemaphoreSlim(maxConcurrency);
Guid sessionId = Guid.NewGuid();
this.blobManager.TraceHelper.FasterProgress($"PrefetchSession {sessionId} started (maxConcurrency={maxConcurrency})");
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
long numberIssued = 0;
long numberMisses = 0;
long numberHits = 0;
long lastReport = 0;
void ReportProgress(int elapsedMillisecondsThreshold)
{
if (stopwatch.ElapsedMilliseconds - lastReport >= elapsedMillisecondsThreshold)
{
this.blobManager.TraceHelper.FasterProgress(
$"FasterKV PrefetchSession {sessionId} elapsed={stopwatch.Elapsed.TotalSeconds:F2}s issued={numberIssued} pending={maxConcurrency-prefetchSemaphore.CurrentCount} hits={numberHits} misses={numberMisses}");
lastReport = stopwatch.ElapsedMilliseconds;
}
}
try
{
// these are disposed after the prefetch thread is done
using var prefetchSession = this.CreateASession();
// for each key, issue a prefetch
await foreach (TrackedObjectKey key in keys)
{
// wait for an available prefetch semaphore token
while (!await prefetchSemaphore.WaitAsync(50, this.terminationToken))
{
prefetchSession.CompletePending();
ReportProgress(1000);
}
FasterKV.Key k = key;
EffectTracker noInput = null;
TrackedObject ignoredOutput = null;
var status = prefetchSession.Read(ref k, ref noInput, ref ignoredOutput, userContext: prefetchSemaphore, 0);
numberIssued++;
switch (status)
{
case Status.NOTFOUND:
case Status.OK:
// fast path: we hit in the cache and complete the read
numberHits++;
prefetchSemaphore.Release();
break;
case Status.PENDING:
// slow path: upon completion
numberMisses++;
break;
case Status.ERROR:
this.partition.ErrorHandler.HandleError(nameof(RunPrefetchSession), "FASTER reported ERROR status", null, true, this.partition.ErrorHandler.IsTerminated);
break;
}
this.terminationToken.ThrowIfCancellationRequested();
prefetchSession.CompletePending();
ReportProgress(1000);
}
ReportProgress(0);
this.blobManager.TraceHelper.FasterProgress($"PrefetchSession {sessionId} is waiting for completion");
// all prefetches were issued; now we wait for them all to complete
// by acquiring ALL the semaphore tokens
for (int i = 0; i < maxConcurrency; i++)
{
while (!await prefetchSemaphore.WaitAsync(50, this.terminationToken))
{
prefetchSession.CompletePending();
ReportProgress(1000);
}
}
ReportProgress(0);
this.blobManager.TraceHelper.FasterProgress($"PrefetchSession {sessionId} completed");
}
catch (OperationCanceledException) when(this.terminationToken.IsCancellationRequested)
{
// partition is terminating
}
catch (Exception e) when(!Utils.IsFatal(e))
{
this.partition.ErrorHandler.HandleError(nameof(RunPrefetchSession), "PrefetchSession {sessionId} encountered exception", e, false, this.partition.ErrorHandler.IsTerminated);
}
}
// create a tracked object on the main session (only one of these is executing at a time) public abstract ValueTask <TrackedObject> CreateAsync(FasterKV.Key key);
public abstract ValueTask ProcessEffectOnTrackedObject(FasterKV.Key k, EffectTracker tracker);
// read a tracked object on the main session and wait for the response (only one of these is executing at a time) public abstract ValueTask <TrackedObject> ReadAsync(FasterKV.Key key, EffectTracker effectTracker);
