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:
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 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:

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:

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:

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.

P.S. Due to popular demand, here's how the last graph looks like after adjusting for inflation:

Tuesday, February 7, 2023

Five Decades of Database Research

Since 1975, over 24 thousand articles have have been published in major database venues (SIGMOD, VLDB/PVLDB, ICDE, EDBT, CIDR, TODS, VLDB Journal, TKDE). The number of papers per year is rising:

Over time, the topics change. Looking at the percentage of keywords appearing in paper titles (in that particular year), we can see interesting trends:

Monday, January 23, 2023

For systems, research is development and development is research

The Conference on Innovative Data Systems Research (CIDR) 2023 is over, and as usual both the official program and the informal discussions have been great. CIDR encourages innovative, risky, and controversial ideas as well as honest exchanges. One intensely-discussed talk was the keynote by Hannes Mühleisen, who together with Mark Raasveldt is the brain behind DuckDB.

In the keynote, Hannes lamented the incentives of systems researchers in academia (e.g., papers over running code). He also criticized the often obscure topics database systems researchers work on while neglecting many practical and pressing problems (e.g., top-k algorithms rather than practically-important issues like strings). Michael Stonebraker has similar thoughts on the database systems community. I share many of these criticisms, but I'm more optimistic regarding what systems research in academia can do, and would therefore like to share my perspective.

Software is different: copying it is free, which has two implications: (1) Most systems are somewhat unique -- otherwise one could have used an existing one. (2) The cost of software is dominated by development effort. I argue that, together, these two observations mean that systems research and system development are two sides of the same coin.

Because developing complex systems is difficult, reinventing the wheel is not a good idea -- it's much better to stand on the proverbial shoulders of giants. Thus, developers should look at the existing literature to find out what others have done, and should experimentally compare existing approaches. Often there are no good solutions for some problems, requiring new inventions, which need to be written up to communicate them to others. Writing will not just allow communication, it will also improve conceptual clarity and understanding, leading to better software. Of course, all these activities (literature review, experiments, invention, writing) are indistinguishable from systems research.

On the other hand, doing systems research without integrating the new techniques into real systems can also lead to problems. Without being grounded by real systems, researchers risk wasting their time on intellectually-difficult, but practically-pointless problems. (And indeed much of what is published at the major database conferences falls into this trap.) Building real systems leads to a treasure trove of open problems. Publishing solutions to these often directly results in technological progress, better systems, and adoption by other systems.

To summarize: systems research is (or should be) indistinguishable from systems development. In principle, this methodology could work in both industry and academia. Both places have problematic incentives, but different ones. Industry often has a very short time horizon, which can lead to very incremental developments. Academic paper-counting incentives can lead to lots of papers without any impact on real systems.

Building systems in academia may not be the best strategy to publish the maximum number of papers or citations, but can lead to real-world impact, technological progress, and (in the long run even) academic accolades. The key is therefore to work with people who have shown how to overcome these systemic pathologies, and build systems over a long time horizon. There are many examples such academic projects (e.g., PostgreSQL, C-Store/Vertica, H-Store/VoltDB, ShoreMT, Proteus, Quickstep, Peloton, KÙZU, AsterixDB, MonetDB, Vectorwise, DuckDB, Hyper, LeanStore, and Umbra).