Date: Tue, 8 Jan 2002 19:33:25 -0500 (EST) From: Robert Watson <rwatson@FreeBSD.org> To: qa@FreeBSD.org Subject: Re: Reduced reliability due to larger socket queue defaults for TCP Message-ID: <Pine.NEB.3.96L.1020108192957.32228A-100000@fledge.watson.org> In-Reply-To: <Pine.NEB.3.96L.1020106174749.96223A-100000@fledge.watson.org>
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The temptation here is one of two things:
(1) Back out the change increasing the send-q socket buffer size as a
default, and restore tuning(7) to recommend increasing the value, or
(2) To add the following text to the release notes:
In 4.5-RELEASE, default socket buffer sizes are increased to
maximize performance on high speed networks. However, under
some circumstances, this can dramatically increase the memory
requirements of the network system, requiring a manual
bumping of the kernel NMBCLUSTERS setting. This can be
set using kern.ipc.nmbclusters.
My temptation is to bump back (1) a bit, possibly to bump up the keepalive
rate, and stick in this note. Reliability==good.
Robert N M Watson FreeBSD Core Team, TrustedBSD Project
robert@fledge.watson.org NAI Labs, Safeport Network Services
On Sun, 6 Jan 2002, Robert Watson wrote:
>
> Recently ran into the following circumstance on a server with about a 15
> day uptime (and hence about a 15-day old version of -STABLE):
>
> tcp4 0 33090 204.156.12.50.80 213.197.75.52.2378 FIN_WAIT_1
> tcp4 0 33304 204.156.12.50.80 198.54.202.4.24052 FIN_WAIT_1
> tcp4 0 32120 204.156.12.50.80 24.27.14.83.50129 FIN_WAIT_1
> tcp4 0 33089 204.156.12.50.80 213.197.75.52.2381 FIN_WAIT_1
> tcp4 0 33304 204.156.12.50.80 198.54.202.4.23509 FIN_WAIT_1
> tcp4 0 33304 204.156.12.50.80 212.182.63.102.28130 FIN_WAIT_1
> tcp4 0 33304 204.156.12.50.80 62.233.128.65.13712 FIN_WAIT_1
> tcp4 0 33580 204.156.12.50.80 212.182.13.23.3473 LAST_ACK
> tcp4 0 31856 204.156.12.50.80 198.54.202.4.20584 FIN_WAIT_1
> tcp4 0 31856 204.156.12.50.80 212.182.63.102.29962 LAST_ACK
> tcp4 0 33304 204.156.12.50.80 198.54.202.4.23960 FIN_WAIT_1
> tcp4 0 31482 204.156.12.50.80 213.197.75.52.2373 FIN_WAIT_1
> tcp4 0 32551 204.156.12.50.80 213.197.75.52.2374 FIN_WAIT_1
>
> (on the order of hundreds of these), resulting in mbufs getting exhausted.
> maxusers is set to 256, so nmbclusters is 4608, which was previously a
> reasonable default. Presumably the problem I'm experiencing is that dud
> connections have doubled in capacity due to a larger send queue size. I've
> temporarily dropped the send queue max until I can reboot the machine to
> increase nmbclusters, but this failure mode does seem unfortunate. It's
> also worth considering adding a release note entry indicating that while
> this can improve performance, it can also reduce scalability. I suppose
> this shouldn't have caught me by surprise, but it did, since that server
> had previously not had a problem... :-)
>
> I don't suppose the TCP spec allows us to drain send socket queues in
> FIN_WAIT_1 or LAST_ACK? :-) Any other bright suggestions on ways we can
> make this change "safer"?
>
> Robert N M Watson FreeBSD Core Team, TrustedBSD Project
> robert@fledge.watson.org NAI Labs, Safeport Network Services
>
>
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