critical path software - Page 673

Latency Matters

- by Frederic P

A lot of interest in low latencies has been expressed within the financial services segment, most especially in the stock trading applications where every millisecond directly influences the profitability of the trader. These days, much of the trading is executed by software applications which are trained to respond to each other almost instantaneously. In fact, you could say that we are in an arms race where traders are using any and all options to cut down on the delay in executing transactions, even by moving physically closer to the trading venue. The Solaris OS network stack has traditionally been engineered for high throughput, at the expense of higher latencies. Knowledge of tuning parameters to redress the imbalance is critical for applications that are latency sensitive. We are presenting in this blog how to configure further a default Oracle Solaris 10 installation to reduce network latency. There are many parameters in fact that can be altered, but the most effective ones are intr_blank_time and intr_blank_packets. These parameters affect on-board network throughput and latency on Solaris systems. If interrupt blanking is disabled, packets are processed by the driver as soon as they arrive, resulting in higher network throughput and lower latency, but with higher CPU utilization. With interrupt blanking disabled, processor utilization can be as high as 80–90% in some high-load web server environments. If interrupt blanking is enabled, packets are processed when the interrupt is issued. Enabling interrupt blanking can result in reduced processor utilization and network throughput, but higher network latency. Both parameters should be set at the same time. You can set these parameters by using the ndd command as follows: # ndd -set /dev/eri intr_blank_time 0 # ndd -set /dev/eri intr_blank_packets 0 You can add them to the /etc/system file as follows: set eri:intr_blank_time 0 set eri:intr_blank_packets 0 The value of the interrupt blanking parameter is a trade-off between network throughput and processor utilization. If higher processor utilization is acceptable for achieving higher network throughput, then disable interrupt blanking. If lower processor utilization is preferred and higher network latency is the penalty, then enable interrupt blanking. Our experience at ISV Engineering is that under controlled experiments the above settings result in reduction of network latency by at least 50%; on a two-socket 3GHz Sun Fire X4170 M2 running Solaris 10 Update 9, the above settings improved ping-pong latency from 60µs to 25-30µs with the on-board NIC.

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