public Result <IReadOnlyList <ContentEvictionInfo> > GetEffectiveLastAccessTimes(
OperationContext context,
IReadOnlyList <ContentHashWithLastAccessTime> contentHashes)
{
Contract.RequiresNotNull(contentHashes);
Contract.Requires(contentHashes.Count > 0);
// This is required because the code inside could throw.
var effectiveLastAccessTimes = new List <ContentEvictionInfo>();
double logInverseMachineRisk = -Math.Log(_configuration.MachineRisk);
foreach (var contentHash in contentHashes)
{
DateTime lastAccessTime = contentHash.LastAccessTime;
int replicaCount = 1;
ReplicaRank rank = ReplicaRank.None;
var(localInfo, distributedEntry, isDesignatedLocation) = _contentResolver.GetContentInfo(context, contentHash.Hash);
// Getting a size from content directory information first.
long size = localInfo.Size;
if (distributedEntry != null)
{
// Use the latest last access time between LLS and local last access time
DateTime distributedLastAccessTime = distributedEntry.LastAccessTimeUtc.ToDateTime();
lastAccessTime = distributedLastAccessTime > lastAccessTime ? distributedLastAccessTime : lastAccessTime;
rank = GetReplicaRank(contentHash.Hash, distributedEntry, _contentResolver.LocalMachineId, _configuration, _now);
if (isDesignatedLocation)
{
rank = Combine(ReplicaRank.Designated, rank);
}
replicaCount = distributedEntry.Locations.Count;
if (size == 0)
{
size = distributedEntry.ContentSize;
}
}
var localAge = _now - contentHash.LastAccessTime;
var age = _now - lastAccessTime;
var effectiveAge = GetEffectiveLastAccessTime(_configuration, age, replicaCount, size, rank, logInverseMachineRisk);
var info = new ContentEvictionInfo(
contentHash.Hash,
age: age,
localAge: localAge,
effectiveAge: effectiveAge,
replicaCount: replicaCount,
size: size,
rank: rank);
effectiveLastAccessTimes.Add(info);
}
return(Result.Success <IReadOnlyList <ContentEvictionInfo> >(effectiveLastAccessTimes));
}
/// <nodoc />
public ContentEvictionInfo(
ContentHash contentHash,
TimeSpan age,
TimeSpan localAge,
TimeSpan effectiveAge,
int replicaCount,
long size,
ReplicaRank rank)
{
ContentHash = contentHash;
Age = age;
LocalAge = localAge;
EffectiveAge = effectiveAge;
ReplicaCount = replicaCount;
Size = size;
Rank = rank;
}
public static TimeSpan GetEffectiveLastAccessTime(LocalLocationStoreConfiguration configuration, TimeSpan age, int replicaCount, long size, ReplicaRank rank, double logInverseMachineRisk)
{
if (configuration.UseTieredDistributedEviction)
{
if (rank == ReplicaRank.Protected)
{
// Protected replicas should be evicted only as a last resort.
return(TimeSpan.Zero);
}
var ageBucketIndex = FindAgeBucketIndex(configuration, age);
if (rank != ReplicaRank.None)
{
ageBucketIndex = Math.Max(0, ageBucketIndex - configuration.ImportantReplicaBucketOffset);
}
return(configuration.AgeBuckets[ageBucketIndex]);
}
else
{
// Incorporate both replica count and size into an evictability metric.
// It's better to eliminate big content (more bytes freed per eviction) and it's better to eliminate content with more replicas (less chance
// of all replicas being inaccessible).
// A simple model with exponential decay of likelihood-to-use and a fixed probability of each replica being inaccessible shows that the metric
// evictability = age + (time decay parameter) * (-log(risk of content unavailability) * (number of replicas) + log(size of content))
// minimizes the increase in the probability of (content wanted && all replicas inaccessible) / per bytes freed.
// Since this metric is just the age plus a computed quantity, it can be interpreted as an "effective age".
TimeSpan totalReplicaPenalty = TimeSpan.FromMinutes(configuration.ContentLifetime.TotalMinutes * (Math.Max(1, replicaCount) * logInverseMachineRisk + Math.Log(Math.Max(1, size))));
return(age + totalReplicaPenalty);
}
}
private static ReplicaRank Combine(ReplicaRank rank1, ReplicaRank rank2)
{
return((ReplicaRank)Math.Max((byte)rank1, (byte)rank2));
}
