# CS 591 K1: Data Stream Processing and Analytics Spring 2020 4/16: Skew mitigation Vasiliki (Vasia) Kalavri vkalavri@bu.edu ## Key partitioning ![Image](/uploads/documents/5/5/9/6/5596d571f02610c key are always processed by the same worker • Popular keys cause imbalance ## Addressing skew - To address skew, the system needs to track the frequencies of the partitioning key values. • We can then

bugs or change business logic • Optimize execution plan • Change operator placement • skew and straggler mitigation • Migrate to a different cluster or software version ## Why is it necessary? Streaming changes: external workload and system performance • Identify bottleneck operators, straggler workers, skew • Enumerate scaling actions, predict their effects, and decide which and when to apply • Allocate changes: external workload and system performance • Identify bottleneck operators, straggler workers, skew • Enumerate scaling actions, predict their effects, and decide which and when to apply • Allocate

0387ee7/p39_1.jpg) split ## Profitability - If it compensates for skew, e.g. when there exist popular keys - if there is skew, throughput is bounded by the instance that receives the highest load Carney et. al. Operator Scheduling in a Data Stream Manager. VLDB 2003. • Load balancing and skew mitigation - Muhammad Anis Uddin Nasir et. al. The power of both choices: Practical load balancing for

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it For more information about nested paging, see chapter 10.7, Nested Paging and VPIDs, page 275. Mitigation options: ##### 13.4.1.1 Disable nested paging By disabling nested paging (EPT), the VMM will HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it For more information about nested paging, see chapter 10.7, Nested Paging and VPIDs, page 276. Mitigation options: ##### 13.4.1.1 Disable nested paging By disabling nested paging (EPT), the VMM will HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it For more information about nested paging, see chapter 10.7, Nested Paging and VPIDs, page 276. Mitigation options: ##### 13.4.1.1 Disable nested paging By disabling nested paging (EPT), the VMM will HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS; the reported CPU usage can be several times higher than what it For more information about nested paging, see chapter 10.7, Nested paging and VPIDs, page 256. Mitigation options: ##### 13.4.1.1 Disable nested paging By disabling nested paging (EPT), the VMM will HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it information about nested paging, see chapter 10.6, Nested Paging and VPIDs, page 306. The following mitigation options are available. ##### 13.4.1.1 Disable Nested Paging By disabling nested paging (EPT) HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it information about nested paging, see chapter 10.6, Nested Paging and VPIDs, page 311. The following mitigation options are available. ##### 13.4.1.1 Disable Nested Paging By disabling nested paging (EPT) HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when

at full speed. Depending on the specific hardware and host OS, this effect can very significantly skew the CPU usage reported by the OS. The reported CPU usage can be several times higher than what it information about nested paging, see chapter 10.6, Nested Paging and VPIDs, page 311. The following mitigation options are available. ##### 13.4.1.1 Disable Nested Paging By disabling nested paging (EPT) HT. In some cases it may be disabled by default, but please verify as the effectiveness of the mitigation depends on it. The default action taken by VirtualBox is to flush the level 1 data cache when a