9a4510ac32
Add ICL_NOCOPY flag to icl_pdu_append_data(), specifying that the method can just reference the data buffer instead of immediately copying it. Extend the offload KPI with optional PDU queue method, allowing to specify completion callback, called when all the data referenced by above has been transferred and won't be accessed any more (the buffers can be freed). Implement the above functionality in software iSCSI driver using mbufs with external storage and reference counter. Note that some NICs (ixl(4)) may keep the mbuf in TX queue for a long time, so CTL has to be ready. Add optional method to struct ctl_scsiio for buffer reference counting. Implement it for CTL block backend, allowing to delay free of the struct ctl_be_block_io and memory it references as needed. In first reincarnation of the patch I tried to delay whole I/O as it is done for FibreChannel, that was cleaner, but due to the above callback delays I had to rewrite it this way to not leave LUN referenced potentially for hours or more. All together on sequential read from ZFS ARC this saves about 30% of CPU time and memory bandwidth by avoiding one of 3 memory copies (the other two are from ZFS ARC to DMU cache and then from DMU cache to CTL buffers). On tests with 2x Xeon Silver 4114 this allows to reach full line rate of 100GigE NIC. Tests with Gold CPUs and two 100GigE NICs are stil TBD, but expectations to saturate them are pretty high. ;) Discussed with: Chelsio Sponsored by: iXsystems, Inc.
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