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Counting binary vs decimal powers in the STREAM benchmark

Posted by John D. McCalpin, Ph.D. on January 5, 2013

A question came up recently about my choice of definitions for “MB” used in the computation of memory bandwidth (in “MB/s”) in the STREAM benchmark.

According to this reference from NIST, the convention is:

Binary Powers Value abbreviation full name
2^10 1,024 KiB kibibyte
2^20 1,048,576 MiB mebibyte
2^30 1,073,741,824 GiB gibibyte
Decimal Powers Value abbreviation full name
10^3 1,000 kB kilobyte
10^6 1,000,000 MB megabyte
10^9 1,000,000,000 GB gigabyte

Since its inception in 1991, the STREAM benchmark has reported the amount of memory used in MiB (2^20) and (more recently) GiB (2^30), but always reports the transfer rates in MB/s (10^6).

An example may make my motivation more clear.

Suppose a computer system reads 524,288 Bytes and writes 524,288 Bytes in 1.000 seconds, for a total of 1,048,576 Bytes transferred in 1.000 seconds.
The corresponding performance could be reported in a variety of ways:

  • Option 1: report as 1,048,576 Bytes/s
  • Option 2: report as 1.000000 MiB/s
  • Option 3: report as 1.049 MB/s

From my perspective:

  • Option 1 gives inconveniently large numbers.
  • Option 2 is consistent with typical units for memory storage, but:
    • it is not consistent with typical units for counting arithmetic operations (more on that below), and
    • it would allow unscrupulous parties (or simply parties with different opinions about how to “properly” count) to change the definition of “MB” from 2^20 to 10^6, allowing them to report values that were almost 4.9% higher than the *same* performance on other systems.
  • Option 3 is what I chose. It is consistent with how FLOPS are counted and it preempts the potential “performance inflation” from abusing Option 2.

Note that if floating-point arithmetic operation counts define “MFLOPS” as 10^6 FP Ops/s (as is typical), then “balance” ratios of (MB/s)/MFLOPS require that (MB/s) also be defined using a decimal base.
These “balance” ratios are an important output of the STREAM benchmark project.
(Aside: I would not encourage anyone to consider a 5% difference in “balance” to mean very much — these are intended as relatively coarse scaling estimates.)

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