CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 1 CSE 598c Virtual Machines “Diagnosing Performance Overheads in the Xen Virtual Machine Environment” Aravind Menon, Jose Renato Santos, Yoshio Turner, G. Janakiraman, Willy Zwaenepoel Lisa Johansen March 13, 2006

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 2Motivation Performance of an application in VM environments are affected by: –Operating System –Other Processes –Underlying VMM –Other VMs We want a way to measure the elements which effect performance in a Virtual Machine environment (Xen)

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 3Outline Overview Statistical Analysis in VMs Xenoprof Performance Debugging Performance Overhead Analysis in Xen

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 4 Issues in VM Statistical Analysis Hardware VMM Kernel P1 P2 Kernel P1 P2 P3 P4 Kernel P1 Kernel P1 P2 P3 P4 Hardware OSVirtual Machine Distributed computing –Distributed profiling VM’s don’t have access to hardware events –VMM does VM

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 5Xenoprof In order to handle distributed profiling, each VM runs an OProfile for individual profiling In order to monitor hardware, Xenoprof accepts hypercalls from OProfile and returns samples through interrupts Hardware VMM Kernel P1 P2 Kernel P1 P2 P3 P4 Dom0 P1 OProfile Xenoprof

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 6 How it works Each profiling domain queries the Xenoprof to find out if it should be the initiator –If there are multiple domains, Dom0 must be the initiator The initiator collects profiling requirements from the participants and forwards this information to the Xenoprof Xenoprof collects program counter samples in accordance to the instructions These samples are then given to the OProfilers where they are mapped to the correct process Individual or system wide performance can then be determined

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 7 Performance Debugging - Networking The motivating example was looking at the comparison of receiver throughput between Linux and XenoLinux –Varying the size of the user-level buffer greatly effects XenoLinux. Why? Using Xenoprof they found: –XenoLinux kernel was the source of the increase in execution time –skb_copy_bits, skbuff_ctor, and tcp_collapse were the culprit functions This is all due to time spent defragmenting memory taken up by empty socket buffer contents

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 8 Performance Overhead Evaluation Given this cool new tool, let’s apply it and determine performance overheads –Namely in network communication because it is an important element of VMs Evaluate: –Receiver workload –Sender workload –Web server workload In three configurations: –Xen-domain0 –Xen-guest0 (same CPU) –Xen-guest1 (different CPUs)

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 9 Receiver workload Domain0 –Degraded performance when compared to Linux –Found that instruction TLB misses and data TLB misses are much greater than in Linux (primary cause) May be TLB flushing or increase in working set size –Instruction cost is greater in XenoLinux due to overheads that exist within Xen Guest0 & Guest1 –Degraded performance when compared to Dom0 –Significant increase in instructions Page remapping and transfer from Dom0 to DomUs –Increased L2 cache misses caused by increased working set size

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 10 Sender workload Domain0 –No throughput differences when compared to Linux Guest0 –Huge throughput degradation based on the high instruction cost (max 706 Mb/s compared to 3764 Mb/s) –The TCP stack processes a larger number of packets than Dom0 to transfer the same amount of data Due to the lack of TCP segmentation offload support Also computes large checksums Driver domain model prevents these instructions to be offloaded into physical interface If similar abilities are taken away from Dom0, we see similar results

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 11 Webserver workload Overall, very similar to the receive and send Domain0 –Higher TLB miss rate than that of Linux Guest0 –Higher instruction costs –Highest L2 cache miss rates –Highest computational overhead –TSO offload don’t matter due to the small payloads Guest1 –Higher instruction costs –Higher L2 cache miss rates –TSO offload don’t matter due to the small payloads

CSE598c - Virtual Machines - Spring Diagnosing Performance Overheads in the Xen Virtual Machine EnvironmentPage 12Conclusion Xenoprof is a tool to examine performance within Xen Xenoprof has been used to examine the different performance elements of network communication in Xen It can be used to evaluate other performance within Xen