Re: port-xen/50290 (Regression: xennet very slow in netbsd-7)

[Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>]

The following reply was made to PR port-xen/50290; it has been noted by GNATS.

From: =?UTF-8?B?SmFyb23DrXIgRG9sZcSNZWs=?= <jaromir.dolecek%gmail.com@localhost>
To: "gnats-bugs%NetBSD.org@localhost" <gnats-bugs%netbsd.org@localhost>
Cc: 
Subject: Re: port-xen/50290 (Regression: xennet very slow in netbsd-7)
Date: Fri, 24 Apr 2020 15:50:17 +0200

 FYI - contrary to xennet(4) most network adapters don't call MCLAIM()
 so don't actually support MBUFTRACE and are not hit by eventual
 performance problems.
 
 mowner_claim(), invoked via MCLAIM()->m_claim() manipulates SPL,
 disables/enables preemption via percpu_getref(), and I believe also
 does IPIs. All of these are relatively expensive on XenPV, that's why
 it's somewhat disproportionally hit.
 
 I've updated the documentation in options(4) to make it reflect more
 accurately that it does, in fact, affect the network performance
 significantly.