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A regular TX WQE execution involves two or more DMA reads - one to fetch the WQE, and another one per WQE gather entry. These DMA reads obviously increase the TX latency. There are two mlx5 mechanisms to bypass these DMA reads: 1) Inline WQE 2) Blue Flame (BF) An inline WQE contains a whole packet, thus saves the DMA read/s of the regular WQE gather entry/s. Inline WQE support was already added in the previous commit. A BF WQE is written directly to the device I/O mapped memory, thus enables saving the DMA read that fetches the WQE. The BF WQE I/O write must be in cache line granularity, thus uses the CPU write combining mechanism. A BF WQE I/O write acts also as a TX doorbell for notifying the device of new TX WQEs. A BF WQE is written to the same I/O mapped address as the regular TX doorbell, thus this address is being mapped twice - once by ioremap() and once by io_mapping_map_wc(). While both mechanisms reduce the TX latency, they both consume more CPU cycles than a regular WQE: - A BF WQE must still be written to host memory, in addition to being written directly to the device I/O mapped memory. - An inline WQE involves copying the SKB data into it. To handle this tradeoff, we introduce here a heuristic algorithm that strives to avoid using these two mechanisms in case the TX queue is being back-pressured by the device, and limit their usage rate otherwise. An inline WQE will always be "Blue Flamed" (written directly to the device I/O mapped memory) while a BF WQE may not be inlined (may contain gather entries). Preliminary testing using netperf UDP_RR shows that the latency goes down from 17.5us to 16.9us, while the message rate (tested with pktgen) stays the same. Signed-off-by: Achiad Shochat <achiad@mellanox.com> Signed-off-by: Amir Vadai <amirv@mellanox.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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| acpi | ||
| asm-generic | ||
| clocksource | ||
| crypto | ||
| drm | ||
| dt-bindings | ||
| keys | ||
| kvm | ||
| linux | ||
| math-emu | ||
| media | ||
| memory | ||
| misc | ||
| net | ||
| pcmcia | ||
| ras | ||
| rdma | ||
| rxrpc | ||
| scsi | ||
| soc | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| video | ||
| xen | ||
| Kbuild | ||