Assume a quad-core computer system can process database queries at a steady stat

Question

Assume a quad-core computer system can process database queries at a steady state rate of requests per second. Also assume that each transacon takes, on average, a fixed amount of time to process. The following table shows pairs of transaction latenct and processing rate. for each of the pairs in the table, answer the following questions: on average, how many requiests are being processed at any given instant? If move to an 8-core system, ideally, what will happen to the system throughput(i.e, how many queriess/second will the computer process)/ Discuss why we rarely obtain this kind of speedup by simply increasing the number of cores.

Explanation / Answer

6.20.1

Since the given system is a quad-core computer system, number of cores= 4.

At any instant (in 1 ms) Average number of requests processed by the quad-core system= (Maximum transaction processing rate X Average transaction latency) / (Number of cores).

If average transaction latency 1ms and maximum processing rate 5,000/sec,

Average number of requests processed= (5000X1)/4=1.25

Similarly,

Average Transaction

latency

Maximum transaction

processing rate

Avg.# requests

per core

1ms

5,000/sec

=1.25

2ms

5,000/sec

=2.5

1ms

10,000/sec

=2.5

2ms

10,000/sec

=5

6.20.2

For example, in the first case, average transaction latency = 1ms, maximum transaction processing rate = 5,000 /sec.

Since, on an average (at an instant) one core processing 1.25 queries, 8 cores processes 8X1.25=10 queries / 1 ms

Thus, in 1 second 8-core system can processes, 1000X10= 10,000 queries/second.

Average Transaction

latency

Maximum transaction

processing rate

Avg.# requests

per core

1ms

10,000/sec

=1.25

2ms

10,000/sec

=2.5

1ms

20,000/sec

=2.5

2ms

20,000/sec

=5

Thus, If moved from quad-core system to an 8-core system then the maximum transaction rate will be doubled.

6.20.3

Each time we double the number of processors(or cores) the speedup ratio will be diminished.

Bottleneck problem:

On increasing the cores, each core shares the available memory accessing bandwidth (That is accessing memory slows down), thus this effects the performance.

Hence, although the number of cores are increased, obtaining this kind of speedup is rare.

Average Transaction

latency

Maximum transaction

processing rate

Avg.# requests

per core

1ms

5,000/sec

=1.25

2ms

5,000/sec

=2.5

1ms

10,000/sec

=2.5

2ms

10,000/sec

=5

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