Dependable Memory - Laboratoire Interface Capteurs ...

70 CHAPTER 3. DESIGN METHODOLOGY AND MODEL SPECIFICATIONS
CPI*/CPI
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
Permutation Benchmark
(Random Errors Injection)
CPI*/CPI (seq_duration=10) CPI*/CPI (seq_duration=50)
CPI*/CPI (seq_duration=100)
0 2.00E-04 4.00E-04 8.00E-04 1.60E-03 3.20E-03 6.40E-03 1.28E-02 2.56E-02
Error Injection Rate (EIR)
Figure 3.18: Model-I: additional CPI for benchmark group II
From the simulation results of graphs 3.24, 3.25, 3.26 the CPI*/CPI ratio is smaller than model-I.
In graph 3.24 with SD of 10 when EIR varies from 2e-4 to 2e-2, the additional CPI increases only by
50% (on y-axis CPI*/CPI is 1.5) which means the execution time increases by only 50% even when
the EIR becomes 100 times higher. This shows a good performance for the proposed architecture.
For example, if we accept increasing by 50% the CPI, with SD of 10 there can be 20 errors per 1000
instructions whereas with SD of 50 there will be only 6 errors per 1000 instructions. Furthermore, the
SD has a direct impact on the size of journal memory in architecture and subsequently on its area.
3.7.3 Comparison
A comparison between model-I and model-II is summarized up in table 3.1. In both models, the
effect of rollback is more dominant in higher SDs like 100 and 50. For example, as VP occurs after
every hundred instructions in the SD= 100 there is more chances of provoking errors which rises the
CPI*/CPI ratio more rapidly as compared to SD = 10 and 50. Since there is a large interval between
the two consecutive VPs there is more chances for error occurrence which on the other hand increases
the rate of re-execution of instructions.
From the performance point of view in model-II, due to parallel access to the memory and journal
in read operation the overall efficiency of the system is increased resulting in lower CPI ratios at
higher EIRs as compared to model-I. Therefore, no clock-cycles are wasted if data is not found in the
Journal. It has a better performance than our previous model as shown in the graphs 3.24, 3.25, 3.26.