Uncore Manual

Reference Number: 329468-002 25
Uncore Performance Monitoring
Cacheing Agent (Cbo) Performance Monitoring
RACU_REQUESTS
• Title: RACU Request
• Category: RACU Events
• Event Code: 0x46
• Max. Inc/Cyc:. 1, Register Restrictions: 0-1
• Definition:
• NOTE: This will be dropped because PHOLD is not implemented this way.
2.3 CACHEING AGENT (CBO) PERFORMANCE MONITORING
2.3.1 Overview of the CBo
The LLC coherence engine (CBo) manages the interface between the core and the last level cache
(LLC). All core transactions that access the LLC are directed from the core to a CBo via the ring inter-
connect. The CBo is responsible for managing data delivery from the LLC to the requesting core. It is
also responsible for maintaining coherence between the cores within the socket that share the LLC;
generating snoops and collecting snoop responses from the local cores when the MESIF protocol
requires it.
So, if the CBo fielding the core request indicates that a core within the socket owns the line (for a
coherent read), the request is snooped to that local core. That same CBo will then snoop all peers
which might have the address cached (other cores, remote sockets, etc) and send the request to the
appropriate Home Agent for conflict checking, memory requests and writebacks.
In the process of maintaining cache coherency within the socket, the CBo is the gate keeper for all
Intel
®
QuickPath Interconnect (Intel
®
QPI) messages that originate in the core and is responsible for
ensuring that all Intel
®
QPI messages that pass through the socket’s LLC remain coherent.
The CBo manages local conflicts by ensuring that only one request is issued to the system for a
specific cacheline.
The uncore of Intel Xeon Processors based on the Ivy Bridge-EP microarchitecture contains multiple
instances of the CBo, each assigned to manage a distinct 2.5MBslice of the processor’s total LLC
capacity. A slice that can be up to 20-way set associative. For processors with fewer than fully popu-
lated 2.5MB LLC slices, the CBo Boxes or missing slices will still be active and track ring traffic caused
by their co-located core even if they have no LLC related traffic to track (i.e. hits/misses/snoops).
Every physical memory address in the system is uniquely associated with a single CBo instance via a
proprietary hashing algorithm that is designed to keep the distribution of traffic across the CBo
instances relatively uniform for a wide range of possible address patterns. This enables the individual
CBo instances to operate independently, each managing its slice of the physical address space
without any CBo in a given socket ever needing to communicate with the other CBos in that same
socket.
Table 2-11. Unit Masks for PHOLD_CYCLES
Extension
umask
[15:8]
Description
ASSERT_TO_ACK bxxxxxxx1 Assert to ACK