ixp training part 3 programming tips 2011.04.12. ncku csie cial lab2 outline memory absolute...

IXP Training Part 3Programming Tips

2011.04.12

NCKU CSIE CIAL Lab 2

Outline

Memory Absolute Instruction Selection Task Partition

Memory Relative Reducing Overhead

Reduce the number of memory accesses Reduce average access latency

Hiding Overhead


Memory Absolute Tips

Instruction Selection General Coding Skill Use Hardware Instruction

Task Partition Multi-Processing Context-Pipelining


Coding Skill

Loop Unrolling Shift Operation Inline Function

__inline & __forceinline Branch Prediction

Branch Prediction Penalty


Hardware Instruction

POP_COUNT FFS Multiply CRC Hashing CAM


POP_COUNT--Brief

Population Count Report number of bit set in a 32-bit re

gister Example:

pop_count( 0x3121 ) = ? 0011 0001 0010 0001 Result = 5


POP_COUNT--Naïve Implementationunsigned int pop_count_for (unsigned int x){ unsigned int y=0; unsigned int i;

for(i=0; i<32; i++) { if( (x&1)==1 ) y++; x=x>>1; } return y;}


POP_COUNT--Faster Implementationunsigned int pop_count_agg(unsigned int x){

x -= ((x >> 1) & 0x55555555); x = (((x >> 2) & 0x33333333) + (x & 0x33333333)); x = (((x >> 4) + x) & 0x0f0f0f0f); x += (x >> 8); x += (x >> 16); return(x & 0x0000003f);}}

Reference http://aggregate.org/MAGIC/


POP_COUNT--Hardware Instruction

unsigned int pop_count_hardware(unsigned int x)

{return pop_count (x);

}


POP_COUNT--Additional Information

Bitmap-RFC (Liu, TECS 2008)


FFS Find the first bit set in data and return its po

sition Example:

ffs ( 0x3121 ) = 0 0011 0001 0010 0001

ffs ( 0x3120 ) = 5 0011 0001 0010 0000

ffs ( 0x3100 ) = 8 0011 0001 0000 0000


Multiply

Specific Multiply Instruction Multiply_24x8() Multiply_16x16() Multiply_32x32_hi() Multiply_32x32_lo()


CRC

Example of CRC operationcrc_write( 0x42424242);crc_32_be( source_address, bytes_0_3 );crc_32_be( dest_address, bytes_0_3 );…Cache_index = crc_read();


Hash Hash_48() Hash_64() Hash_128()

Example:SIGNAL sig_hash;Hash48(data_out, data_in, count, sig_done, &sig_

hash);__wait_for_all(&sig_hash);


CAM--Brief Each ME has 16 32-bit CAM entries The CAM is private to other MEs With lookup operation, each entries is

searching in parallel With a success lookup, the index of m

atched entries will be returned Else, the index of entries to be replace

d will be returned


Content Addressable Memory--Structure

cam_lookup_t


CAM--Usagecam_lookup_t cam_result;cam_result = cam_lookup( data );if( cam_result.hit == 1 ) {

Access Entry cam_result.entry_num;…

}else {

……cam_write( cam_result.entry_num, data, 15 );

}


Task Partition

Multi-Processing More Computing Power Easy to implement

Context-Pipelining More Useable Resource Hard to balance


Memory Relative Tips--Reducing Overhead

Reduce the number of memory accesses Wide-word Accesses Result Caches

Reduce average access latency Multi-level Memory Hierarchy Data Cache


Wide-Word Accesses--Brief

Batch Access the needed data Reduce the necessary accesses Useful when the data are linked-list

like structure


Wild-Word Access--Example

MEM_ADDR+0

……

+4 ……

+8 ……

+12 ……

+16 ……

+20 ……

+24 ……

+28 ……


Wide-Word Accesses--Usage (One Node per Access)

__declspec(sram_read_reg) UINT32 A;SIGNAL sig_read;

sram_read( &A, MEM_ADDR+(i*4), 1, sig_done, &sig_read);

__wait_for_all( &sig_read );

Access A ......----------------------------------------------Result: 8 Accesses are needed


Wide-Word Accesses--Usage (Two Node per Access)

__declspec(sram_read_reg) UINT32 A[2];SIGNAL sig_read;





Wide-Word Accesses--Usage (Four Node per Access)

__declspec(sram_read_reg) UINT32 A[4];SIGNAL sig_read;





Wide-Word Accesses--Experiment Platform: IXP2800 Total Accesses: 8 LW (8*4 Byte)

Case Total Cycle

Average Cycle/ LW

1LW * 8 Time

1211 151.38

2LW * 4 Time

725 90.63

4LW * 2 Time

460 57.50

8LW * 1 Time

387 48.38


Wide-Word Accesses--Limitation

Data must be contiguous Suitable for linear search Not support random accesses

Number of Transfer Registers are fixed Each thread has 16 read / write registers The Tx-Regs may be reserved by others


Resulting Cache--Brief

Caching the result of application If same fields appear again, the

cached result is return Memory accesses are reduced

when cache hit. Depends on time locality of the

traffic


Result Cache--IXP2400

No hardware cache is supported in IXP2400 ME

Not easy to implement set-associative cache

Replacement policy will also be an overhead


Result Cache--Design Consideration

Shared or Private Cache ? Size of Cache ? Works with specific Hardware ? Miss penalty handling ?


Result Cache--Experiment


Multi-Level Memory Hierarchy--Brief

Reduce the average access latency Number of accesses remained

unchanged If data can fit in faster memory,

then do it


Multi-Level Memory Hierarchy--Data Placement Size smaller while read-only

Hard Code Size smaller while need updating

Local Memory Size larger

Scratchpad Size largest

SRAM


Multi-Level Memory Hierarchy--Packet Data Type Packet related data

Temporary Data Valid with specific packet Local Memory

Flow related data Related to specific flow Spatial Locality Wide-Word Access

Application related data Valid with specific application Temporal Locality Result Cache


Split-Cache (Z. Liu, IET-COM 2007) Two separate hardware for application data and flow data


Data Cache--Brief

Hardware Cache Mechanism that cached the data for packet processing App-Cache Flow-Cache

However, not supported by IXP2400


Data Cache--CAM + Local Memory

CAM works with Local Memory acts like hardware cache

However, number of CAM entries is less

Each CAM entry may co-worked with several Local Memory Cache entry


Memory Relative Tips--Hiding Overhead

Not really reduce the overhead, but overlapped it Hardware Multi-Threading Asynchronous Memory


Hardware Multi-Threading

Swap out itself and let another thread to execute while access memory

Each thread kept its own set of registers, thus no stack are needed for thread swapping

Round Robin Scheduling No thread preemptive


Asynchronous Memory--Brief

Thread will not be blocked when issue a memory request

Thus, thread can issues multiple memory requests at a time


Asynchronous Memory--Example (1 Issue)

Read X__wait_for_all ( &sig_x )Read Y__wait_for_all ( &sig_y )

// Use X and Y …


Asynchronous Memory--Example (2 Issue)

Read XRead Y__wait_for_all ( &sig_x, &sig_y )

// Use X and Y …


Wild-Word Access +Multiple Issues

MEM_ADDR+0

……

+4 ……

+8 ……

+12 ……

+16 ……

+20 ……

+24 ……

+28 ……


Wild-Word Access +Multiple Issues (1LW, 2 Issue)

MEM_ADDR+0

……

+4 ……

+8 ……

+12 ……

+16 ……

+20 ……

+24 ……

+28 ……



MEM_ADDR+0

……

+4 ……

+8 ……

+12 ……

+16 ……

+20 ……

+24 ……

+28 ……


Wild-Word Access +Multiple Issues (Experiment)Scheme Total Cycles Average Cycles / LW

1 LW * 1 Issue 1211 151.382 LW * 1 Issue 725 90.63

4 LW * 1 Issue 460 57.50

8 LW * 1 Issue 387 48.38

1 LW * 2 Issue 716 89.50

2 LW * 2 Issue 445 55.63

4 LW * 2 Issue 364 45.50

1 LW * 4 Issue 396 49.50

2 LW * 4 Issue 320 40.00

1 LW * 8 Issue 318 39.75


Reference (1) Jayaram Mudigonda, Harrick M. Vin, Raj Yav

atkar, “Overcoming the memory wall in packet processing: hammers or ladders?”, ANCS 2005

Duo Liu, Zheng Chen, Bei Hua, Nenghai Yu, Xinan Tang, “High-Performance Packet Classification Algorithm for Multireaded IXP Network Processor”, ACM TECS 2008.


Reference (2)

Z. Liu, K. Zheng, B. Liu, “Hybrid cache architecture for high-speed packet processing”, IET-COM 2007

ixp training part 3 programming tips 2011.04.12. ncku csie cial lab2 outline memory absolute...

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