Martin Maas
I am a Research Scientist in the Google Brain team. Before joining Google, I completed my PhD in the Electrical Engineering and Computer Sciences department at UC Berkeley, working with Krste Asanović and John Kubiatowicz. My primary research interests are in machine learning for systems, managed languages, operating systems and computer architecture. I am interested in the entire stack from the hardware to the programming systems layer.
Authored Publications
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Thesios: Synthesizing Accurate Counterfactual I/O Traces from I/O Samples
Mangpo Phothilimthana
Soroush Ghodrati
Selene Moon
ASPLOS 2024, Association for Computing Machinery
Preview abstract
Representative modeling of I/O activity is crucial when designing large-scale distributed storage systems. Particularly important use cases are counterfactual “what-if” analyses that assess the impact of anticipated or hypothetical new storage policies or hardware prior to deployment. We propose Thesios, a methodology to accurately synthesize such hypothetical full-resolution I/O traces by carefully combining down-sampled I/O traces collected from multiple disks attached to multiple storage servers. Applying this approach to real-world traces that a real ready routinely sampled at Google, we show that our synthesized traces achieve 95–99.5% accuracy in read/write request numbers, 90–97% accuracy in utilization, and 80–99.8% accuracy in read latency compared to metrics collected from actual disks. We demonstrate how The-sios enables diverse counterfactual I/O trace synthesis and analyses of hypothetical policy, hardware, and server changes through four case studies: (1) studying the effects of changing disk’s utilization, fullness, and capacity, (2) evaluating new data placement policy, (3) analyzing the impact on power and performance of deploying disks with reduced rotations-per-minute (RPM), and (4) understanding the impact of increased buffer cache size on a storage server. Without Thesios, such counterfactual analyses would require costly and potentially risky A/B experiments in production.
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Predicting Dynamic Properties of Heap Allocations Using Neural Networks Trained on Static Code
Christian Navasca
Guoqing Harry Xu
2023 ACM SIGPLAN International Symposium on Memory Management (ISMM 2023)
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Memory allocators and runtime systems can leverage dynamic properties of heap allocations – such as object lifetimes, hotness or access correlations – to improve performance and resource consumption. A significant amount of work has focused on approaches that collect this information in performance profiles and then use it in new memory allocator or runtime designs, both offline (in ahead-of-time compilers) and online (in JIT compilers). This is a special instance of profile-guided optimization.
This approach has significant disadvantages: 1) The profiling oftentimes introduces substantial overheads, which are prohibitive in many production scenarios, 2) Creating a representative profiling run adds significant engineering complexity and reduces deployment velocity, and 3) Profiles gathered ahead of time or during the warm-up phase of a server are often not representative of all workload behavior and may miss important corner cases.
In this paper, we investigate a fundamentally different approach. Instead of deriving heap allocation properties from profiles, we explore the ability of neural network models to predict them from the statically available code. As an intellectual abstract, we do not offer a conclusive answer but describe the trade-off space of this approach, investigate promising directions, motivate these directions with data analysis and experiments, and highlight challenges that future work needs to overcome.
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Adaptive Hugepage Subrelease for Non-moving Memory Allocators in Warehouse-Scale Computers
Khanh Nguyen
International Symposium on Memory Management (ISMM) 2021 (to appear)
Preview abstract
Modern C++ server workloads rely on 2 MB huge pages to improve memory system performance via higher TLB hit rates. Huge pages have traditionally been supported at the kernel level, but recent work has shown that user-level, huge page-aware memory allocators can achieve higher huge page coverage and thus performance. These memory allocators deal with a trade-off: 1) allocate memory from the operating system (OS) at the granularity of a huge page, achieve high performance, but potentially waste memory due to fragmentation, or 2) limit fragmentation by breaking up huge pages into smaller 4 KB pages and returning them to the OS, but reduce performance due to lower huge page coverage. For example, the state-of-the-art TCMalloc allocator handles this trade-off by releasing memory to the OS at a configurable release rate, breaking up huge pages as necessary.
This approach balances performance and fragmentation well for machines running one workload. For multiple applications on the same machine however, the reduction in memory usage is only useful to overall performance if another workload uses this memory. In warehouse-scale computers, when an application releases and then reacquires the same amount or more memory quickly, but no other application uses the memory in the meantime, the release causes poorer huge page coverage without any system-wide benefit.
We introduce a metric, realized fragmentation, to capture this effect. We then present an adaptive release policy that dynamically determines when to break up huge pages and return them to the OS to optimize system-wide performance. We built this policy into TCMalloc and deployed it fleet-wide in our data centers, leading to an estimated 1% fleet-wide throughput improvement at negligible memory overhead.
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Preview abstract
Storage services in data centers continuously make decisions, such as for cache admission, prefetching, and block allocation. These decisions are typically driven by heuristics based on statistical properties like temporal locality or common file sizes. The quality of decisions can be improved through application-level information such as the database operation a request belongs to. While such features can be exploited through application hints (e.g., explicit prefetches), this process requires manual work and is thus only viable for the most tuned workloads.
In this work, we show how to leverage application-level information automatically, by building on distributed traces that are already available in warehouse-scale computers. As these traces are used for diagnostics and accounting, they contain information about requests, including those to storage services. However, this information is mostly unstructured (e.g., arbitrary text) and thus difficult to use. We demonstrate how to do so automatically using machine learning, by applying ideas from natural language processing.
We show that different storage-related decisions can be learned from distributed traces, using models ranging from simple clustering techniques to neural networks. Instead of designing specific models for different storage-related tasks, we show that the same models can be used as building blocks for different tasks. Our models improve prediction accuracy by 11-33% over non-ML baselines, which translates to significantly improving the hit rate of a caching task, as well as improvements to an SSD/HDD tiering task, on production data center storage traces.
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Learning-based Memory Allocation for C++ Server Workloads
David G. Andersen
Mohammad Mahdi Javanmard
Colin Raffel
25th ACM International Conference on Architectural Support for Programming Languages and Operating Systems (ASPLOS) (2020) (to appear)
Preview abstract
Modern C++ servers have memory footprints that vary widely over time, causing persistent heap fragmentation of up to 2x from long-lived objects allocated during peak memory usage. This fragmentation is exacerbated by the use of huge (2MB) pages, a requirement for high performance on large heap sizes. Reducing fragmentation automatically is challenging because C++ memory managers cannot move objects.
This paper presents a new approach to huge page fragmentation. It combines modern machine learning techniques with a novel memory manager (LLAMA) that manages the heap based on object lifetimes and huge pages (divided into blocks and lines). A neural network-based language model predicts lifetime classes using symbolized calling contexts. The model learns context-sensitive per-allocation site lifetimes from previous runs, generalizes over different binary versions, and extrapolates from samples to unobserved calling contexts. Instead of size classes, LLAMA's heap is organized by lifetime classes that are dynamically adjusted based on observed behavior at a block granularity.
LLAMA reduces memory fragmentation by up to 78% while only using huge pages on several production servers. We address ML-specific questions such as tolerating mispredictions and amortizing expensive predictions across application execution. Although our results focus on memory allocation, the questions we identify apply to other system-level problems with strict latency and resource requirements where machine learning could be applied.
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A Taxonomy of ML for Systems Problems
IEEE Micro (2020)
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Machine learning has the potential to significantly improve systems, but only under certain conditions. We describe a taxonomy to help identify whether or not machine learning should be applied to particular systems problems, and which approaches are most promising. We believe that this taxonomy can help practitioners and researchers decide how to most effectively use machine learning in their systems, and provide the community with a framework and vocabulary to discuss different approaches for applying machine learning in systems.
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