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https://github.com/AuxXxilium/linux_dsm_epyc7002.git
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3d9a0d2f82
While doing high throughput test on a BQL enabled NIC, I found a very high cost in ndo_start_xmit() when accessing BQL data. It turned out the problem was caused by compiler trying to be smart, but involving a bad MESI transaction : 0.05 │ mov 0xc0(%rax),%edi // LOAD dql->num_queued 0.48 │ mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count 58.23 │ add %edx,%edi 0.58 │ cmp %edi,0xc4(%rax) 0.76 │ mov %edi,0xc0(%rax) // STORE dql->num_queued += count 0.72 │ js bd8 I got an incredible 10 % gain [1] by making sure cpu do not attempt to get the cache line in Shared mode, but directly requests for ownership. New code : mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count add %edx,0xc0(%rax) // RMW dql->num_queued += count mov 0xc4(%rax),%ecx // LOAD dql->adj_limit mov 0xc0(%rax),%edx // LOAD dql->num_queued cmp %edx,%ecx The TX completion was running from another cpu, with high interrupts rate. Note that I am using barrier() as a soft hint, as mb() here could be too heavy cost. [1] This was a netperf TCP_STREAM with TSO disabled, but GSO enabled. Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Jesper Dangaard Brouer <brouer@redhat.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/tegra | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| video | ||
| xen | ||
| Kbuild | ||