Sandy bridge performance degradation compare to Westmere

[amk21]

i created a simple memtest that allocate large vector that get random number and update the vector data.
pseudo code 

DataCell* dataCells = new DataCell[VECTOR_SIZE]
for(int cycles = 0; cycles < gCycles; cycles++){    u64 randVal = random()
    DataCell* dataCell = dataCells[randVal % VECTOR_SIZE]

    dataCell->m_count = cycles

    dataCell->m_random = randVal

    dataCell->m_flag = 1

}

i'm using perf util to gather performance counter info.
the most interesting results are when the vector size is larger then the cache size tix8 20MB tix2 12MB 

hardware specification

tix2 - cpu X5680 3.33GHz, mother board - Supermicro X8DTU , memory - 64GB divided 32GB to each bank at 1.33GHz

tix8 - cpu E5-2690 2.90GHz, mother board - Intel S2600GZ, memory - 64GB divided 32GB to each bank at 1.60GHz

compiled with gcc 4.6.1 -O3 -mtune=native -march=native

amk@tix2:~/amir/memtest$ perf stat -e cycles:u -e instructions:u -e l1-dcache-loads:u -e l1-dcache-load-misses:u -e L1-dcache-stores:u -e L1-dcache-store-misses:u ./memtest -v 10000019 -c 100000000
total Time (rdtsc) 21800971556 nano time 6542908630 vector size 240000456

Performance counter stats for './memtest -v 10000019 -c 100000000':

21842742688 cycles # 0.000 M/sec
5869556879 instructions # 0.269 IPC
1700665337 L1-dcache-loads # 0.000 M/sec
221870903 L1-dcache-load-misses # 0.000 M/sec
1130278738 L1-dcache-stores # 0.000 M/sec
0 L1-dcache-store-misses # 0.000 M/sec

6.628680493 seconds time elapsed

amk@tix8:~/amir/memtest$ perf stat -e cycles:u -e instructions:u -e l1-dcache-loads:u -e l1-dcache-load-misses:u -e L1-dcache-stores:u -e L1-dcache-store-misses:u ./memtest -v 10000019 -c 100000000
total Time (rdtsc) 24362574412 nano time 8424126698 vector size 240000456

Performance counter stats for './memtest -v 10000019 -c 100000000':

24409499958 cycles # 0.000 M/sec
5869656821 instructions # 0.240 IPC
1192635035 L1-dcache-loads # 0.000 M/sec
94702716 L1-dcache-load-misses # 0.000 M/sec
1373779283 L1-dcache-stores # 0.000 M/sec
306775598 L1-dcache-store-misses # 0.000 M/sec

8.525456817 seconds time elapsed

what am is missing is Sandy bridge slower then Westmere ???????

Amir.

[amk21]

attached memtest