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PCI-E NIC Cards provide redundant connectivity to ensure an uninterrupted network connection.
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PCI-E Card Knowledge
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Cheating: Non DMA Method
In this page, we will talk about the Non DMA Method. If you don’t know why, please see the previous page.
The simplest way is having the driver software write the data to the hardware in a loop, with something like strcpy (). This is indeed very inefficient, but given the fast PCIe hardware available, it’s possible to reach satisfactory results with this plain method: For example, in a worst-case scenario, each DW (4 bytes) of payload requires 16 bytes of TLP headers (64-bit addressing) + 6 bytes for data link layer overhead, so there are 26 bytes transmitted on the data link layer for each 4 bytes of payload.
On a Gen1 link with x8 lanes, this raw link layer runs at 2.5 Gbps x (8/10) x 8 = 16 Gbps = 2 GB/s (the 8/10 factor accounts for the error correction code). The payload link rate is hence limited to 2000 MB/s x 4 / 26 = 307 MB/s. In reality, the rate is somewhat lower due to data link layer overhead, so 280-300MB/s is a more realistic figure.
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Gigabit EF Dual Port Server Adapter
In some cases, the processor may send more than 4 bytes on each TLP packet, in particular if a 64-bit write is supported by its instruction set. This significantly improves the throughput. Not all hosts support Gen2 links, but if it does, the potential throughput is doubled. So all in all, impressive data rates are possible using this primitive method.
The obvious downside of this non-DMA method is that the CPU is busy writing. This may be less of a concern when the processor has multiple cores, in particular with hyper threading support. For example, with a quad core processor supporting hyper threading, the data copy routine occupies one core out of the 8 virtual.
And still, the host’s hardware wasn’t designed to support high bandwidth traffic being carried out this way. Even though the bridge between the processor’s internal bus and PCIe should take the load, other peripherals may suffer from a significant performance degradation due to the flood of requests.
Complicated bus scenarios, which the processor wasn’t designed to cope with, may cause a significant performance hit on the entire system. For example, if the Ethernet hardware driver attempts to access its registers, the TLPs requesting these operations may be significantly delayed, causing a slowdown of apparently unrelated tasks.
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