Date: Wed, 9 Apr 2003 16:07:33 -0700 From: Sean Chittenden <sean@chittenden.org> To: "Jin Guojun [DSD]" <j_guojun@lbl.gov> Cc: freebsd-performance@freebsd.org Subject: Re: tcp_output starving -- is due to mbuf get delay? Message-ID: <20030409230733.GX79923@perrin.int.nxad.com> In-Reply-To: <3E94A22D.174321F0@lbl.gov> References: <3E94A22D.174321F0@lbl.gov>
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> When testing GigE path that has 67 ms RTT, the maximum TCP throughput is > limited at 250 Mb/s. By tracing the problem, I found that tcp_output() is > starving > where snd_wnd and snd_cwnd are fully open. The snd_cc is never filled beyond > the 4.05MB even though the snd_hiwat is 10MB and snd_sbmax is 8MB. That is, > sosend never stopped at sbwait. So only place can slow down is the mbuf > allocation > in sosend(). The attached trace file shows that each MGET and MCLGET takes > significant time -- around 8 us at slow start time, and gradually increasing > after that > in an range 18 to 648. > Each packet Tx on GigE takes 12 us. It average mbuf allocation takes 18 us, then > > the performance will be reduced to 40%, in fact it is down to 25%, which means > higher average delay. > > I have change NMBCLUSTER from 2446 to 6566 to 10240, and nothing is improved. > > Any one can tell what factors would cause MGET / MCLGET to wait? > Is there any way to make MGET/MCLGET not to wait? Luigi posted a patch about this a while back (last summer sometime, iirc). http://people.freebsd.org/~seanc/patches/#o1_mbuf_lookup I updated his patch but haven't had a chance to test it. If you're feeling brave, see if applying this patch fixes this bottle neck. -sc -- Sean Chittenden
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