Date: Thu, 18 Feb 2010 13:57:28 +0100 From: Harald Schmalzbauer <h.schmalzbauer@omnilan.de> To: Chuck Swiger <cswiger@mac.com> Cc: freebsd-stable@freebsd.org Subject: Help for TCP understanding wanted, ACK-MSS-Window [Was: Re: best practice to watch TCP parms of established sockets] Message-ID: <4B7D3938.1000309@omnilan.de> In-Reply-To: <4B7C4066.5040006@omnilan.de> References: <4B7C1365.9070806@omnilan.de> <70CD649D-7659-4CE2-A16C-49B8C891CB5B@mac.com> <4B7C4066.5040006@omnilan.de>
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[-- Attachment #1 --] Harald Schmalzbauer schrieb am 17.02.2010 20:15 (localtime): ... >>> Now my first idea is to compare MSS and windows sizes before and after the performance drop. >>> How do I best capture them? tdpcump? It's GbE linkspeed... >> It seems more likely that ZFS is running into slowdowns from resource contention, memory fragmentation, etc than your network would suddenly drop out, but tcpdump -w outfile.pcap is a good method of looking.... > > Thanks, but fisrt tests showed that ZFS is not causing the slowdown. Hello, I got exactly the same limitations when using tmpfs. So for now I'll concentrate on that, back to ZFS later. Please clarify my TCP understanding. If I have the window set to 65535 in the header and a MSS of 1460, how often should the receiver send ACK segments? window/MSS, right? Now I see every two segments acknowledged in my dump (rsync between two em0 interaces). I'd like to understand a) why disabling net.inet.tcp.rfc1323 gives slightly better rsync throughput than enabled b) why I can't transfer more than 50MB/s over my direct linked GbE boxes. But right now I even don't understand the dump I see. As far as I understand I should only see every 45 data segments one ACK segment. That would clearly explain to me why I can't saturate my GbE link. But I can't imagine this is a uncovered faulty behaviour, so I guess I haven't understood TCP. Please help. Thanks in advance, -Harry [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.13 (FreeBSD) iEYEARECAAYFAkt9OUAACgkQLDqVQ9VXb8hkMgCgj1MefInItFaZNUR0MYGKy0yh MesAoIFExZul/0EiKCDnIIf60SMC+xVW =yH9D -----END PGP SIGNATURE-----
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