DELL 2 x Intel Xeon E7-4860 338-BFMX User Manual

Datasheet Volume 2 of 2

27

TID Assignment Restrictions have been updated for modes 4, 5, 6, and 7.

DAS stands for Directory Assisted Snoopy. It enables early data return from Home 
(BBox) to Requestor (Caching agent), without waiting for peer snoop responses if 
directory state for the requested line indicates that no peer socket has the line. DAS is 
enabled only for local socket request. For enabling DAS (for local socket) we have 
introduced a new directory state known as  Remote State (or R state). Intel Xeon 
processor 7500 series only had two directory states namely  I state (Idle state) or E 
state (owned by IOH).  In Intel Xeon Processor E7-8800/4800/2800 Product Families, R 
state is also introduced. These 3 state are defined in Intel Xeon Processor E7-8800/
4800/2800 Product Families as:

• I state:  Line is either not present in any caching agent or is owned by local socket 

caching agent, but definitely not present in any remote socket/IOH.

• R state: Line may be present in a remote socket.
• E (or D)  state: Line is owned by IOH.

If DAS is enabled and local socket sends a read request (RdData/RdInvown), BBox 
does a MemRead (basically a prefetch) and directory bits (received with prefetch ack 
from Mbox) indicates I state, then data can be send to the requesting  local socket even 
before all the peer snoop responses have been received. DAS is a very significant 
performance feature for 8 socket systems and NC based topologies which are limited 
by snoop bandwidth (that is, where idle read latency is snoop limited and not Memory 
latency limited).

DAS is only supported in hemisphere mode and not in non-hemisphere  mode.

DAS is not supported with mirroring.

IO Directory Cache (IODC) is a new feature for Intel Xeon Processor E7-8800/4800/
2800 Product Families. The aim of this feature is to improve Streams throughput by 
eliminating directory reads for InvItoE from snoopy caching agents.

We Implement a 64 entry directory cache to complement directory in memory. 

This feature  leverages 2 IOH properties:

• IOH can only get ownership of a line via InvItoE.
• IOH can only own as many lines as it has RTIDs. 

IOH takes ownership of the line by issuing an InvItoE. This exclusive ownership phase 
(started by IOH issuing an InvItoE and ends in BBox sending GntE_CMP) is almost 
immediately followed by WriteBack Phase. 

5

8S HemiSphere + 4 IOH/XNC

TID  [0...31],TID[0..15]

TID  [0...15]

6

4S Hemisphere + 2 IOH/XNC

TID [0…63], TID [0…31]

TID  [0...31]

7

4S Non-Hemisphere + 2 IOH/XNC

TID [0…31], TID [0…15]

TID  [0...31]