FICON Express2 Channel Performance Version 1.0 - IBM

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FICON Express2 Channel Performance Version 1.0 Page 7 simple benchmark and in general, we usually recommend that you keep FPU below 50% to achieve good online transaction response times. The 50% FPU point occurs between 6000 and 7000 I/O’s per second when running this very simple 4K read hit benchmark workload. FICON Express2(FEx2) 4K read hit response times & FICON processor utilization(FPU) resp time in millisec 5 100 90 4 80 70 3 60 50 2 40 30 1 20 10 0 0 0 2 4 6 8 10 12 14 io/sec Thousands Figure 5 Figure 6 shows the breakdown of response time components for this simple 4k read hit benchmark from 10% FICON Processor Utilization(FPU) through 70% FPU or just before the knee of the response time curve. Both total response times and the response time components of IOSQ, PEND, DISC (disconnect) and CONN (connect) time can be found on the RMF Device Activity report. IOSQ time is the time that an I/O is delayed due to the fact that another I/O from this system is already using the target device for this I/O. PEND time starts when the CP sends the I/O request to the SAP and includes the amount of time it takes for the channel to process the first few CCWs (Channel Command Words) in the channel program and send the commands to the Control Unit and does not end until the Control Unit sends a CMR or Command Response back to the channel. Disconnect time is the amount of time it takes the Control Unit to service a CU cache miss and retrieve the data from the device. CONNECT time is basically the data transfer time for the I/O. So, in this 4k read hit benchmark, there is no DISC time since there are no CU cache misses and there is no IOSQ time since the I/O driver program we use is designed to wait for an I/O to an individual device to finish before it issues another I/O to that same device. So, all we have is PEND FICON Processor Util% FEx2 resp time FEx2 FPU%
FICON Express2 Channel Performance Version 1.0 Page 8 time + CONN time. Since each I/O only transfers 4K bytes of data, CONN time is the smaller of the two components and remains relatively constant from low I/O rates to high I/O rates. On the other hand, PEND time grows as the FPU% grows. If we had used a point-to-point configuration to do this measurement and if the CU port had less capability than the new FICON Express2 channel, then the PEND time would have grown faster as a function of the CU port processor utilization and we would not have been able to push the channel to its maximum capability. response time components for 4K read hits...4K bytes/io, 100% cu cache hit ratio FICON processor utilization 10% FPU 20% FPU 30% FPU 40% FPU 50% FPU 60% FPU 70% FPU 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 response times in ms Figure 6 pend conn Figure 7 shows the response time components for a more realistic version of an online transaction processing workload with a mix of reads and writes and a 70 to 80% CU cache hit ratio. In this case disconnect time is the largest component of the total response time and the component that grows the most as the activity rate increases. PEND and CONNECT times are about equal up to the 60% FPU point. There is a more significant increase in total response time beyond the 50% FPU point than there was with the simpler 4K read benchmark.
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FICON Express2 Channel Performance Version 1.0 - IBM

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