kern/57160: NFS (over TCP) very slow in 10_BETA

[bouyer%antioche.eu.org@localhost]

	Note: There was a bad value `' for the field `Class'.
	It was set to the default value of `sw-bug'.

>Number:         57160
>Category:       kern
>Synopsis:       NFS (over TCP) very slow in 10_BETA
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jan 04 19:45:00 +0000 2023
>Originator:     Manuel Bouyer
>Release:        NetBSD 10.0_BETA
>Organization:
>Environment:
System: NetBSD rochebonne.antioche.eu.org 10.0_BETA NetBSD 10.0_BETA (GENERIC_CAN) #0: Wed Jan 4 16:41:59 MET 2023 bouyer%armandeche.soc.lip6.fr@localhost:/local/armandeche1/tmp/build/amd64/obj/local/armandeche2/netbsd-10/src/sys/arch/amd64/compile/GENERIC_CAN amd64
Architecture: x86_64
Machine: amd64
>Description:
	I have my mailbox (in mbox format) stored on a NFS server (which
	is a cubieboard 2 with a SATA SSD). The NFS mount uses:
	type nfs (nodev, nosuid, fsid: 0xb06/0x70b, reads: sync 0 async 0, writes: sync 0 async 0)
	The client is connected to a gigabit switch (using a alc(4) interface),
	the server is connected at 100Mbs.
	Since upgrading the client to 10.0_BETA loading the mailbox
	with mutt is very slow, and the transfers seems to pause
	several times.
	Reading the mailbox with dd gives expected speed:
	(I made sure there isn't local cache in the way)
	rochebonne:~> dd if=Mail/Inbox of=/dev/null
	132+1 records in
	132+1 records out
	138625298 bytes transferred in 17.273 secs (8025548 bytes/sec)
	I can't reproduce the slow read with dd.

	I have a similar setup at work (but with a much faster NFS server)
	and I didn't notice change in mutt performance on upgrade.

	tcpdump pcap file while loading the mailbox with mutt available
	on request in private mail.

>How-To-Repeat:
	store mailbox on NFS server and read it with mutt on a 10.0 system;
	it may depend on the network hardware.
>Fix: