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244 • Linux Symposium 2004 • Volume One gsyprf3:~# pfmon -e uc_loads_retired -k --system-wide \ -- /usr/src/pktgen-testing/pktgen-single-tg3 Adding devices to run. Configuring devices Running... ctrl^C to stop 57: 918505 0 IO-SAPIC-level eth1 Result: OK: 7613693(c7613006+d687) usec, 5000000 (64byte) 656771pps 320Mb/sec (336266752bps) errors: 0 57: 1834959 0 IO-SAPIC-level eth1 CPU0 2749675 UC_LOADS_RETIRED CPU1 1175 UC_LOADS_RETIRED } Figure 1: tg3 v3.6 MMIO reads with pktgen/IRQ on same CPU saw. Using Message Signaled Interrupts (MSI) instead of Line based IRQs would guarantee the most recent status block update (transferred via DMA writes) would be visible to the CPU before tg3_interrupt() gets called. The protocol would allow correct operation without using MSI, too. 2.5 Benchmarking, pfmon, and CPU bindings The purpose of binding pktgen to CPU1 is to verify the transmit code path is NOT doing any MMIO reads. We split the transmit code path and interrupt handler across CPUs to narrow down which code path is performing the MMIO reads. This change is not obvious from Figure 2 output since tg3 only performs MMIO reads from CPU 0 (tg3_interrupt()). But in Figure 2, performance goes up 30%! Offhand, I don’t know if this is due to CPU utilization (pktgen and tg3_interrupt() contending for CPU cycles) or if DMA is more efficient because of cache-line flows. When I don’t have any deadlines looming, I’d like to determine the difference. 2.6 e1000 Memory Reads e1000 version 5.2.52-k4 has a more efficient implementation than tg3 driver. In a nut shell, MMIO reads are pretty much irrelevant to the pktgen workload with e1000 driver using default values. Figure 3 shows e1000 performs 173315/(703829 − 622143) ≈ 2 MMIO reads per interrupt and 5000000/(703829 − 622143) ≈ 61 packets per interrupt. Being the curious soul I am, I tracked down the two MMIO reads anyway. One is in the interrupt handler and the second when interrupts are re-enabled. It looks like e1000 will always need at least 2 MMIO reads per interrupt. 3 Measuring MMIO Writes 3.1 Why worry about MMIO Writes? MMIO writes are clearly not as significant as MMIO reads. Nonetheless, every time a driver writes to MMIO space, some subtle things happen. There are four minor issues to think about: memory ordering, PCI bus utilization, filling outbound write queues, and stalling MMIO reads longer than necessary.
Linux Symposium 2004 • Volume One • 245 gsyprf3:~# pfmon -e uc_loads_retired -k --system-wide \ -- /usr/src/pktgen-testing/pktgen-single-tg3 Adding devices to run. Configuring devices Running... ctrl^C to stop 57: 5809687 0 IO-SAPIC-level eth1 Result: OK: 5914889(c5843865+d71024) usec, 5000000 (64byte) 845451pps 412Mb/se c (432870912bps) errors: 0 57: 6427969 0 IO-SAPIC-level eth1 CPU0 1855253 UC_LOADS_RETIRED CPU1 950 UC_LOADS_RETIRED Figure 2: tg3 v3.6 MMIO reads with pktgen/IRQ on diff CPU gsyprf3:~# pfmon -e uc_loads_retired -k --system-wide \ -- /usr/src/pktgen-testing/pktgen-single-e1000 Configuring devices Running... ctrl^C to stop 59: 622143 0 IO-SAPIC-level eth3 Result: OK: 10228738(c9990105+d238633) usec, 5000000 (64byte) 488854pps 238Mb/ sec (250293248bps) errors: 81669 59: 703829 0 IO-SAPIC-level eth3 CPU0 173315 UC_LOADS_RETIRED CPU1 1422 UC_LOADS_RETIRED Figure 3: MMIO reads for e1000 v5.2.52-k4
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Proceedings of the Linux Symposium
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The State of ACPI in the Linux Kern
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Conference Organizers Andrew J. Hut
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TCP Connection Passing Werner Almes
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Linux Symposium 2004 • Volume One
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Cooperative Linux Dan Aloni da-x@co
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Build your own Wireless Access Poin
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Run-time testing of LSB Application
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Linux Block IO—present and future
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Linux AIO Performance and Robustnes
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Methods to Improve Bootup Time in L
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Linux on NUMA Systems Martin J. Bli
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Improving Kernel Performance by Unm
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Linux Virtualization on IBM POWER5
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The State of ACPI in the Linux Kern
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Scaling Linux® to the Extreme From
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Get More Device Drivers out of the
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Big Servers—2.6 compared to 2.4 W
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Multi-processor and Frequency Scali
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Dynamic Kernel Module Support: From
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