[SPDK] Why two perf processes can't scale out?

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Hi guys,
 The nvmf_tgt is running in one node and perf is running in another node.
I first get the peak performance of nvmf_tgt by setting the io depth (-q) o=
f perf to 64.
And the peak performance is 4GB/s.
Then I set the io depth of perf to 1 and found that the performance becomes=
 1GB/s.
This is understandable to me.
But when I run two perf with io depth equal to 1, the performance is shared=
 by two perf processes (i.e., each has 500mb/s).
so, what resources are they competing for so that performance cannot scale?
Thanks.
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