When the driver needs to send new packets to the device, it always
queues the new sk_buffs into an intermediate queue (send_pkt_queue)
and schedules a worker (send_pkt_work) to then queue them into the
virtqueue exposed to the device.
This increases the chance of batching, but also introduces a lot of
latency into the communication. So we can optimize this path by
adding a fast path to be taken when there is no element in the
intermediate queue, there is space available in the virtqueue,
and no other process that is sending packets (tx_lock held).
The following benchmarks were run to check improvements in latency and
throughput. The test bed is a host with Intel i7-10700KF CPU @ 3.80GHz
and L1 guest running on QEMU/KVM with vhost process and all vCPUs
pinned individually to pCPUs.
- Latency
Tool: Fio version 3.37-56
Mode: pingpong (h-g-h)
Test runs: 50
Runtime-per-test: 50s
Type: SOCK_STREAM
In the following fio benchmark (pingpong mode) the host sends
a payload to the guest and waits for the same payload back.
fio process pinned both inside the host and the guest system.
Before: Linux 6.9.8
Payload 64B:
1st perc. overall 99th perc.
Before 12.91 16.78 42.24 us
After 9.77 13.57 39.17 us
Payload 512B:
1st perc. overall 99th perc.
Before 13.35 17.35 41.52 us
After 10.25 14.11 39.58 us
Payload 4K:
1st perc. overall 99th perc.
Before 14.71 19.87 41.52 us
After 10.51 14.96 40.81 us
- Throughput
Tool: iperf-vsock
The size represents the buffer length (-l) to read/write
P represents the number of parallel streams
P=1
4K 64K 128K
Before 6.87 29.3 29.5 Gb/s
After 10.5 39.4 39.9 Gb/s
P=2
4K 64K 128K
Before 10.5 32.8 33.2 Gb/s
After 17.8 47.7 48.5 Gb/s
P=4
4K 64K 128K
Before 12.7 33.6 34.2 Gb/s
After 16.9 48.1 50.5 Gb/s
The performance improvement is related to this optimization,
I used a ebpf kretprobe on virtio_transport_send_skb to check
that each packet was sent directly to the virtqueue
Co-developed-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Message-Id: <20240730-pinna-v4-2-5c9179164db5@outlook.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
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