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Modern datacenter applications are concurrent, so they require synchronization to control access to shared data. Requests can contend for different combinations of locks, depending on application and request state. In this paper, we show that locks, especially blocking synchronization, can squander throughput and harm tail latency, even when the CPU is underutilized. Moreover, the presence of a large number of contention points, and the unpredictability in knowing which locks a request will require, make it difficult to prevent contention through overload control using traditional signals such as queueing delay and CPU utilization. We present Protego, a system that resolves these problems with two key ideas. First, it contributes a new admission control strategy that prevents compute congestion in the presence of lock contention. The key idea is to use marginal improvements in observed throughput, rather than CPU load or latency measurements, within a credit-based admission control algorithm that regulates the rate of incoming requests to a server. Second, it introduces a new latency-aware synchronization abstraction called Active Synchronization Queue Management (ASQM) that allows applications to abort requests if delays exceed latency objectives. We apply Protego to two real-world applications, Lucene and Memcached, and show that it achieves up to 3.3x more goodput and 12.2x lower 99th percentile latency than the state-of-the-art overload control systems while avoiding congestion collapse.
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Backdraft: a Lossless Virtual Switch that Prevents the Slow Receiver Problem
Virtual switches, used for end-host networking, drop packets when the receiving application is not fast enough to consume them. This is called the slow receiver problem, and it is important because packet loss hurts tail communication latency and wastes CPU cycles, resulting in application-level performance degradation. Further, solving this problem is challenging because application throughput is highly variable over short timescales as it depends on workload, memory contention, and OS thread scheduling. This paper presents Backdraft, a new lossless virtual switch that addresses the slow receiver problem by combining three new components: (1) Dynamic Per-Flow Queuing (DPFQ) to prevent HOL blocking and provide on-demand memory usage; (2) Doorbell queues to reduce CPU overheads; (3) A new overlay network to avoid congestion spreading. We implemented Backdraft on top of BESS and conducted experiments with real applications on a 100 Gbps cluster with both DCTCP and Homa, a state-of-the-art congestion control scheme. We show that an application with Backdraft can achieve up to 20x lower tail latency at the 99th percentile.
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- PAR ID:
- 10344915
- Date Published:
- Journal Name:
- USENIX Symposium on Networked Systems Design and Implementation (NSDI '22)
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Balakrishnan, Mahesh; Ghobadi, Manya (Ed.)Modern datacenter applications are concurrent, so they require synchronization to control access to shared data. Requests can contend for different combinations of locks, depending on application and request state. In this paper, we show that locks, especially blocking synchronization, can squander throughput and harm tail latency, even when the CPU is underutilized. Moreover, the presence of a large number of contention points, and the unpredictability in knowing which locks a request will require, make it difficult to prevent contention through overload control using traditional signals such as queueing delay and CPU utilization. We present Protego, a system that resolves these problems with two key ideas. First, it contributes a new admission control strategy that prevents compute congestion in the presence of lock contention. The key idea is to use marginal improvements in observed throughput, rather than CPU load or latency measurements, within a credit-based admission control algorithm that regulates the rate of incoming requests to a server. Second, it introduces a new latency-aware synchronization abstraction called Active Synchronization Queue Management (ASQM) that allows applications to abort requests if delays exceed latency objectives. We apply Protego to two real-world applications, Lucene and Memcached, and show that it achieves up to 3.3x more goodput and 12.2x lower 99th percentile latency than the state-of-the-art overload control systems while avoiding congestion collapse.more » « less
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