/// <summary>Pull a slot out of a preexisting shared memory segment.</summary>
/// <remarks>
/// Pull a slot out of a preexisting shared memory segment.
/// Must be called with the manager lock held.
/// </remarks>
/// <param name="blockId">The blockId to put inside the Slot object.</param>
/// <returns>
/// null if none of our shared memory segments contain a
/// free slot; the slot object otherwise.
/// </returns>
private ShortCircuitShm.Slot AllocSlotFromExistingShm(ExtendedBlockId blockId)
{
if (this.notFull.IsEmpty())
{
return(null);
}
KeyValuePair <ShortCircuitShm.ShmId, DfsClientShm> entry = this.notFull.FirstEntry
();
DfsClientShm shm = entry.Value;
ShortCircuitShm.ShmId shmId = shm.GetShmId();
ShortCircuitShm.Slot slot = shm.AllocAndRegisterSlot(blockId);
if (shm.IsFull())
{
if (DfsClientShmManager.Log.IsTraceEnabled())
{
DfsClientShmManager.Log.Trace(this + ": pulled the last slot " + slot.GetSlotIdx(
) + " out of " + shm);
}
DfsClientShm removedShm = Sharpen.Collections.Remove(this.notFull, shmId);
Preconditions.CheckState(removedShm == shm);
this.full[shmId] = shm;
}
else
{
if (DfsClientShmManager.Log.IsTraceEnabled())
{
DfsClientShmManager.Log.Trace(this + ": pulled slot " + slot.GetSlotIdx() + " out of "
+ shm);
}
}
return(slot);
}
/// <summary>Stop tracking a slot.</summary>
/// <remarks>
/// Stop tracking a slot.
/// Must be called with the EndpointShmManager lock held.
/// </remarks>
/// <param name="slot">The slot to release.</param>
internal virtual void FreeSlot(ShortCircuitShm.Slot slot)
{
DfsClientShm shm = (DfsClientShm)slot.GetShm();
shm.UnregisterSlot(slot.GetSlotIdx());
if (shm.IsDisconnected())
{
// Stale shared memory segments should not be tracked here.
Preconditions.CheckState(!this.full.Contains(shm.GetShmId()));
Preconditions.CheckState(!this.notFull.Contains(shm.GetShmId()));
if (shm.IsEmpty())
{
if (DfsClientShmManager.Log.IsTraceEnabled())
{
DfsClientShmManager.Log.Trace(this + ": freeing empty stale " + shm);
}
shm.Free();
}
}
else
{
ShortCircuitShm.ShmId shmId = shm.GetShmId();
Sharpen.Collections.Remove(this.full, shmId);
// The shm can't be full if we just freed a slot.
if (shm.IsEmpty())
{
Sharpen.Collections.Remove(this.notFull, shmId);
// If the shared memory segment is now empty, we call shutdown(2) on
// the UNIX domain socket associated with it. The DomainSocketWatcher,
// which is watching this socket, will call DfsClientShm#handle,
// cleaning up this shared memory segment.
//
// See #{DfsClientShmManager#domainSocketWatcher} for details about why
// we don't want to call DomainSocketWatcher#remove directly here.
//
// Note that we could experience 'fragmentation' here, where the
// DFSClient allocates a bunch of slots in different shared memory
// segments, and then frees most of them, but never fully empties out
// any segment. We make some attempt to avoid this fragmentation by
// always allocating new slots out of the shared memory segment with the
// lowest ID, but it could still occur. In most workloads,
// fragmentation should not be a major concern, since it doesn't impact
// peak file descriptor usage or the speed of allocation.
if (DfsClientShmManager.Log.IsTraceEnabled())
{
DfsClientShmManager.Log.Trace(this + ": shutting down UNIX domain socket for " +
"empty " + shm);
}
this.Shutdown(shm);
}
else
{
this.notFull[shmId] = shm;
}
}
}
