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Sunday, April 9, 2023

The Great CPU Stagnation

For at least five decades, Moore's law consistently delivered
increasing numbers of transistors. Equally significant, Dennard
scaling led to each transistor using less energy, enabling higher
clock frequencies. This was great, as higher clock frequencies
enhanced existing software performance automatically, without
necessitating any code rewrite. However, around 2005, Dennard scaling
began to falter, and clock frequencies have largely plateaued since
then.

Despite this, Moore's law continued to advance, with the additional
available transistors being channeled into creating more cores per
chip. The following graph displays the number of cores for the
largest available x86 CPU at the time: [multicore]
Notice the logarithmic scale: this represents the exponential trend
we had become accustomed to, with core counts doubling roughly every
three years. Regrettably, when considering cost per core, this
impressive trend appears to have stalled, ushering in an era of CPU
stagnation.

To demonstrate this stagnation, I gathered data from wikichip.org on
AMD's Epyc single-socket CPU lineup, introduced in 2017 and now in
its fourth generation (Naples, Rome, Milan, Genoa):
Model  Gen   Launch  Cores GHz IPC   Price
7351P Naples 06/2017    16 2.4 1.00    $750
7401P Naples 06/2017    24 2.0 1.00  $1,075
7551P Naples 06/2017    32 2.0 1.00  $2,100
7302P Rome   08/2019    16 3.0 1.15    $825
7402P Rome   08/2019    24 2.8 1.15  $1,250
7502P Rome   08/2019    32 2.5 1.15  $2,300
7702P Rome   08/2019    64 2.0 1.15  $4,425
7313P Milan  03/2021    16 3.0 1.37    $913
7443P Milan  03/2021    24 2.9 1.37  $1,337
7543P Milan  03/2021    32 2.8 1.37  $2,730
7713P Milan  03/2021    64 2.0 1.37  $5,010
9354P Genoa  11/2022    32 3.3 1.57  $2,730
9454P Genoa  11/2022    48 2.8 1.57  $4,598
9554P Genoa  11/2022    64 3.1 1.57  $7,104
9654P Genoa  11/2022    96 2.4 1.57 $10,625

Over these past six years, AMD has emerged as the x86 performance per
dollar leader. Examining these numbers should provide insight into
the state of server CPUs. Let's first observe CPU cores per dollar:
[coresperdo]

This deviates significantly from the expected exponential improvement
graphs. In fact, CPU cores are becoming slightly more expensive over
time! Admittedly, newer cores outperform their predecessors. When
accounting for both clock frequency and higher IPC, we obtain the
following image:

[instrperdo]

This isn't much better. The performance improvement over a 6-year
period is underwhelming when normalized for cost. Similar results can
also be observed for Intel CPUs in EC2.

Lastly, let's examine transistor counts, only taking into account the
logic transistors. Despite improved production nodes from 14nm
(Naples) over 7nm (Rome/Milan) to 5nm (Genoa), cost-adjusted figures
reveal stagnation: [transistor]

In conclusion, the results are disheartening. Rapid and exponential
improvements in CPU speed seem to be relics of the past. We now find
ourselves in a markedly different landscape compared to the
historical norm in computing. The implications could be far-reaching.
For example, most software is extremely inefficient when compared to
what hardware can theoretically achieve, and maybe this needs to
change. Furthermore, historically specialized chips enjoyed only
limited success due to the rapid advancement of commodity CPUs.
Perhaps, custom chips will have a much bigger role in the future.

Posted by Viktor Leis at 2:08 PM #
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