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Date:      Mon, 31 Aug 2020 22:05:59 +0000
From:      bugzilla-noreply@freebsd.org
To:        net@FreeBSD.org
Subject:   [Bug 248652] netmap: pkt-gen TX huge pps difference between 11-STABLE and 12-STABLE/CURRENT on ix & ixl NIC
Message-ID:  <bug-248652-7501-ka9xQdZs8s@https.bugs.freebsd.org/bugzilla/>
In-Reply-To: <bug-248652-7501@https.bugs.freebsd.org/bugzilla/>
References:  <bug-248652-7501@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D248652

--- Comment #9 from Vincenzo Maffione <vmaffione@FreeBSD.org> ---
Thanks a lot for the tests.
I think the way netmap tx is handled right now needs improvement.

As far as I can tell, in your setup TX interrupts are simply not used (ix a=
nd
ixl seem to use softirq for TX interrupt processing).
Your experiments with increasing kern.hz cause the interrupt rate of the OS
timer to increase, and therefore causing the iflib_timer() routine to be ca=
lled
more often. Being called more often, the TX ring is cleaned up (TX credits
update) more often and therefore the application can submit new TX packets =
more
often, hence the improved pps.

However, clearly increasing kern.hz is not a viable approach.
I think we should try to use a separate timer for netmap TX credits update,
using higher resolution (e.g. callout_reset_sbt_on()), and maybe try to
dynamically adjust the timer period to become smaller when transmitting at =
high
rate, and lower when transmitting ad low rate.
I'll try to come up with an experimental patch in the next days.

--=20
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