CPU L2緩存初探
探索目的
爲了設計高性能的內存數據庫，往往要考慮CPU緩存的命中率，查閱資料發現，Intel架構的處理器有3級緩存的設置，其中L1，L2較小，L3較大但被三個核心所公用。在我使用的i7 4712mq中，使用dmidecode查看緩存大小：
inszva@inszva-Aspire-E5-572G:~$ sudo dmidecode
[sudo] password for inszva:
# dmidecode 2.12
SMBIOS 2.8 present.
28 structures occupying 1951 bytes.
Table at 0x000E6F30.
Handle 0x0000, DMI type 0, 24 bytes
BIOS Information
Vendor: Insyde Corp.
Version: V1.05
Release Date: 03/17/2015
Address: 0xE0000
Runtime Size: 128 kB
ROM Size: 6144 kB
Characteristics:
PCI is supported
BIOS is upgradeable
BIOS shadowing is allowed
Boot from CD is supported
Selectable boot is supported
EDD is supported
Japanese floppy for NEC 9800 1.2 MB is supported (int 13h)
Japanese floppy for Toshiba 1.2 MB is supported (int 13h)
5.25"/360 kB floppy services are supported (int 13h)
5.25"/1.2 MB floppy services are supported (int 13h)
3.5"/720 kB floppy services are supported (int 13h)
3.5"/2.88 MB floppy services are supported (int 13h)
8042 keyboard services are supported (int 9h)
CGA/mono video services are supported (int 10h)
ACPI is supported
USB legacy is supported
BIOS boot specification is supported
Targeted content distribution is supported
UEFI is supported
BIOS Revision: 1.5
Firmware Revision: 1.5
Handle 0x0001, DMI type 1, 27 bytes
System Information
Manufacturer: Acer
Product Name: Aspire E5-572G
Version: V1.05
Serial Number: NXMV2CN01252001C033400
UUID: F54CECF0-8EF4-E411-85C7-F0761CC0B25C
Wake-up Type: Power Switch
SKU Number: Aspire E5-572G_0922_V1.05
Family: SharkBay System
Handle 0x0002, DMI type 2, 16 bytes
Base Board Information
Manufacturer: Acer
Product Name: EA50_HWS
Version: V1.05
Serial Number: NBMV211001520310DC3400
Asset Tag: Type2 - Board Asset Tag
Features:
Board is a hosting board
Board is replaceable
Location In Chassis: Type2 - Board Chassis Location
Chassis Handle: 0x0003
Type: Motherboard
Contained Object Handles: 0
Handle 0x0003, DMI type 3, 23 bytes
Chassis Information
Manufacturer: Acer
Type: Notebook
Lock: Not Present
Version: Chassis Version
Serial Number: Chassis Serial Number
Asset Tag: Chassis Asset Tag
Boot-up State: Safe
Power Supply State: Safe
Thermal State: Safe
Security Status: None
OEM Information: 0x00000000
Height: Unspecified
Number Of Power Cords: 1
Contained Elements: 0
SKU Number: Not Specified
Handle 0x0004, DMI type 4, 42 bytes
Processor Information
Socket Designation: U3E1
Type: Central Processor
Family: Core i7
Manufacturer: Intel(R) Corporation
ID: C3 06 03 00 FF FB EB BF
Signature: Type 0, Family 6, Model 60, Stepping 3
Flags:
FPU (Floating-point unit on-chip)
VME (Virtual mode extension)
DE (Debugging extension)
PSE (Page size extension)
TSC (Time stamp counter)
MSR (Model specific registers)
PAE (Physical address extension)
MCE (Machine check exception)
CX8 (CMPXCHG8 instruction supported)
APIC (On-chip APIC hardware supported)
SEP (Fast system call)
MTRR (Memory type range registers)
PGE (Page global enable)
MCA (Machine check architecture)
CMOV (Conditional move instruction supported)
PAT (Page attribute table)
PSE-36 (36-bit page size extension)
CLFSH (CLFLUSH instruction supported)
DS (Debug store)
ACPI (ACPI supported)
MMX (MMX technology supported)
FXSR (FXSAVE and FXSTOR instructions supported)
SSE (Streaming SIMD extensions)
SSE2 (Streaming SIMD extensions 2)
SS (Self-snoop)
HTT (Multi-threading)
TM (Thermal monitor supported)
PBE (Pending break enabled)
Version: Intel(R) Core(TM) i7-4712MQ CPU @ 2.30GHz
Voltage: 0.9 V
External Clock: 100 MHz
Max Speed: 2300 MHz
Current Speed: 2900 MHz
Status: Populated, Enabled
Upgrade: <OUT OF SPEC>
L1 Cache Handle: 0x0006
L2 Cache Handle: 0x0007
L3 Cache Handle: 0x0008
Serial Number: To Be Filled By O.E.M.
Asset Tag: To Be Filled By O.E.M.
Part Number: To Be Filled By O.E.M.
Core Count: 4
Core Enabled: 4
Thread Count: 8
Characteristics:
64-bit capable
Multi-Core
Hardware Thread
Execute Protection
Enhanced Virtualization
Power/Performance Control
Handle 0x0005, DMI type 7, 19 bytes
Cache Information
Socket Designation: L1 Cache
Configuration: Enabled, Not Socketed, Level 1
Operational Mode: Write Back
Location: Internal
Installed Size: 32 kB
Maximum Size: 32 kB
Supported SRAM Types:
Synchronous
Installed SRAM Type: Synchronous
Speed: Unknown
Error Correction Type: Single-bit ECC
System Type: Data
Associativity: 8-way Set-associative
Handle 0x0006, DMI type 7, 19 bytes
Cache Information
Socket Designation: L1 Cache
Configuration: Enabled, Not Socketed, Level 1
Operational Mode: Write Back
Location: Internal
Installed Size: 32 kB
Maximum Size: 32 kB
Supported SRAM Types:
Synchronous
Installed SRAM Type: Synchronous
Speed: Unknown
Error Correction Type: Single-bit ECC
System Type: Instruction
Associativity: 8-way Set-associative
Handle 0x0007, DMI type 7, 19 bytes
Cache Information
Socket Designation: L2 Cache
Configuration: Enabled, Not Socketed, Level 2
Operational Mode: Write Back
Location: Internal
Installed Size: 256 kB
Maximum Size: 256 kB
Supported SRAM Types:
Synchronous
Installed SRAM Type: Synchronous
Speed: Unknown
Error Correction Type: Single-bit ECC
System Type: Unified
Associativity: 8-way Set-associative
Handle 0x0008, DMI type 7, 19 bytes
Cache Information
Socket Designation: L3 Cache
Configuration: Enabled, Not Socketed, Level 3
Operational Mode: Write Back
Location: Internal
Installed Size: 6144 kB
Maximum Size: 6144 kB
Supported SRAM Types:
Synchronous
Installed SRAM Type: Synchronous
Speed: Unknown
Error Correction Type: Single-bit ECC
System Type: Unified
Associativity: 12-way Set-associative
Handle 0x0009, DMI type 10, 6 bytes
On Board Device Information
Type: Video
Status: Enabled
Description: Video Graphics Controller
Handle 0x000A, DMI type 10, 6 bytes
On Board Device Information
Type: Ethernet
Status: Enabled
Description: Realtek Lan Controller
Handle 0x000B, DMI type 11, 5 bytes
OEM Strings
String 1: Acer System
String 2: String2 for Original Equipment Manufacturer
String 3: String3 for Original Equipment Manufacturer
String 4: String4 for Original Equipment Manufacturer
String 5: String5 for Original Equipment Manufacturer
Handle 0x000C, DMI type 12, 5 bytes
System Configuration Options
Option 1: String1 for Type12 Equipment Manufacturer
Option 2: String2 for Type12 Equipment Manufacturer
Option 3: String3 for Type12 Equipment Manufacturer
Option 4: String4 for Type12 Equipment Manufacturer
Option 5: String5 for Type12 Equipment Manufacturer
Option 6: String6 for Type12 Equipment Manufacturer
Option 7: String7 for Type12 Equipment Manufacturer
Handle 0x000D, DMI type 16, 23 bytes
Physical Memory Array
Location: System Board Or Motherboard
Use: System Memory
Error Correction Type: None
Maximum Capacity: 32 GB
Error Information Handle: Not Provided
Number Of Devices: 4
Handle 0x000E, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x000D
Error Information Handle: Not Provided
Total Width: Unknown
Data Width: Unknown
Size: No Module Installed
Form Factor: DIMM
Set: None
Locator: DIMM0
Bank Locator: BANK 0
Type: Unknown
Type Detail: Unknown
Speed: Unknown
Manufacturer: Empty
Serial Number: Empty
Asset Tag: Unknown
Part Number: Empty
Rank: Unknown
Configured Clock Speed: Unknown
Minimum voltage: Unknown
Maximum voltage: Unknown
Configured voltage: Unknown
Handle 0x000F, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x000D
Error Information Handle: Not Provided
Total Width: Unknown
Data Width: Unknown
Size: No Module Installed
Form Factor: DIMM
Set: None
Locator: DIMM1
Bank Locator: BANK 1
Type: Unknown
Type Detail: Unknown
Speed: Unknown
Manufacturer: Empty
Serial Number: Empty
Asset Tag: Unknown
Part Number: Empty
Rank: Unknown
Configured Clock Speed: Unknown
Minimum voltage: Unknown
Maximum voltage: Unknown
Configured voltage: Unknown
Handle 0x0010, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x000D
Error Information Handle: Not Provided
Total Width: 64 bits
Data Width: 64 bits
Size: 8192 MB
Form Factor: SODIMM
Set: None
Locator: DIMM2
Bank Locator: BANK 2
Type: DDR3
Type Detail: Synchronous
Speed: 1600 MHz
Manufacturer: Hynix
Serial Number: 282C5AB7
Asset Tag: Unknown
Part Number: HMT41GS6BFR8A-PB
Rank: 2
Configured Clock Speed: 1600 MHz
Minimum voltage: Unknown
Maximum voltage: Unknown
Configured voltage: Unknown
Handle 0x0011, DMI type 17, 40 bytes
Memory Device
Array Handle: 0x000D
Error Information Handle: Not Provided
Total Width: Unknown
Data Width: Unknown
Size: No Module Installed
Form Factor: DIMM
Set: None
Locator: DIMM3
Bank Locator: BANK 3
Type: Unknown
Type Detail: Unknown
Speed: Unknown
Manufacturer: Empty
Serial Number: Empty
Asset Tag: Unknown
Part Number: Empty
Rank: Unknown
Configured Clock Speed: Unknown
Minimum voltage: Unknown
Maximum voltage: Unknown
Configured voltage: Unknown
Handle 0x0012, DMI type 19, 31 bytes
Memory Array Mapped Address
Starting Address: 0x00000000000
Ending Address: 0x001FFFFFFFF
Range Size: 8 GB
Physical Array Handle: 0x000D
Partition Width: 4
Handle 0x0013, DMI type 20, 35 bytes
Memory Device Mapped Address
Starting Address: 0x00000000000
Ending Address: 0x001FFFFFFFF
Range Size: 8 GB
Physical Device Handle: 0x0010
Memory Array Mapped Address Handle: 0x0012
Partition Row Position: Unknown
Interleave Position: 2
Interleaved Data Depth: 1
Handle 0x0014, DMI type 24, 5 bytes
Hardware Security
Power-On Password Status: Disabled
Keyboard Password Status: Disabled
Administrator Password Status: Disabled
Front Panel Reset Status: Disabled
Handle 0x0015, DMI type 170, 98 bytes
OEM-specific Type
Header and Data:
AA 62 15 00 01 08 00 00 7F 00 0F 00 06 00 03 02
01 08 21 02 00 00 23 02 00 00 25 02 10 00 41 02
04 00 42 02 20 00 43 02 40 00 44 02 08 00 45 02
10 00 48 02 01 00 49 02 02 00 61 02 08 00 62 02
01 00 63 02 02 00 64 02 04 00 81 02 04 00 83 02
02 00 85 02 00 00 86 02 00 00 87 02 00 00 88 02
00 04
Handle 0x0016, DMI type 171, 44 bytes
OEM-specific Type
Header and Data:
AB 2C 16 00 01 86 80 16 04 02 EC 10 68 81 03 DE
10 47 13 04 F2 04 8A B4 05 86 80 20 8C 07 8C 16
36 00 08 CA 04 0B 30 11 CB 06 70 29
Handle 0x0017, DMI type 172, 24 bytes
OEM-specific Type
Header and Data:
AC 18 17 00 02 1E 01 FF 00 02 01 00 03 FF 00 04
01 00 05 0F 00 06 FF 00
Handle 0x0018, DMI type 173, 9 bytes
OEM-specific Type
Header and Data:
AD 09 18 00 00 00 00 00 00
Handle 0x0019, DMI type 221, 12 bytes
OEM-specific Type
Header and Data:
DD 0C 19 00 01 01 00 01 07 00 00 00
Strings:
Reference Code - ACPI
Handle 0x001A, DMI type 221, 12 bytes
OEM-specific Type
Header and Data:
DD 0C 1A 00 01 01 00 01 07 00 00 00
Strings:
Reference Code - Intel Rapid Start
Handle 0x001B, DMI type 127, 4 bytes
End Of Table
可以看到，Handle 0x0004 是描述CPU的，裏面有以下幾行內容：
L1 Cache Handle: 0x0006
L2 Cache Handle: 0x0007
L3 Cache Handle: 0x0008分別在對應handle查看三級緩存大小，得到L1 32kb,L2 256kb,L3 6144kb，有資料表明L1緩存中儲存大多是L2與L3的索引，CPU讀取時從L1中獲得地址，再從L2或L3中讀取數據，而大容量的L3相對L2的速度是極爲緩慢的。在設計數據庫的過程中要儘量將索引放入L2中。[1]
實驗
爲了證明[1]的正確性，設計瞭如下程序：
//test.go
package main
import "fmt"
var num [60000]int32
var t int32
func find(n int32) (int32) {
c := int32(15000)
pos := int32(30000)
for {
if num[pos] == n{break;}
if num[pos] > n{
pos -= c
}else {
pos += c
}
c /= 2
if c==0{c=1}
}
return pos
}
func main() {
var sum int64
for i := int32(0);i < 60000;i++{
num[i] = i
}
for i := int32(0);i < 60000;i++{
sum += int64(find(i))
}
fmt.Println(sum)
}//test2.go
package main
import "fmt"
var num [70000]int32
var t int32
func find(n int32) (int32) {
c := int32(15000)
pos := int32(30000)
for {
if num[pos] == n{break;}
if num[pos] > n{
pos -= c
}else {
pos += c
}
c /= 2
if c==0{c=1}
}
return pos
}
func main() {
var sum int64
for i := int32(0);i < 70000;i++{
num[i] = i
}
for i := int32(0);i < 70000;i++{
sum += int64(find(i))
}
fmt.Println(sum)
}程序先創建了一個遞增數組，然後對這個數組進行了多次二分查找，在第一份代碼中，數組大小爲60000，佔用空間60000*4=240000<256K L2緩存，而程序二中佔用 70000*4=280000>>256K L2緩存，相對於耗時較高的二分查詢，建立初始化數組的時間可以忽略(O(n))，而n次二分查詢的理論時間複雜度爲(O(nlogn))，按理說程序二運行時間應該是程序一的7/6*log7/log6=1.183012861倍，然而實驗結果：
inszva@inszva-Aspire-E5-572G:~/cache$ time ./test
1799970000
real 0m0.016s
user 0m0.017s
sys 0m0.000s
inszva@inszva-Aspire-E5-572G:~/cache$ time ./test2
2449965000
real 0m0.097s
user 0m0.098s
sys 0m0.000s0.097/0.016=6.0625倍，程序二由於L2緩存失配，到底查詢進入L3進行，大大降低了程序運行效率。