Date: Tue, 3 Sep 2013 13:56:07 -0700 From: Adrian Chadd <adrian@freebsd.org> To: David Wolfskill <david@catwhisker.org> Cc: FreeBSD Net <net@freebsd.org> Subject: Re: TSO and FreeBSD vs Linux Message-ID: <CAJ-Vmo=97-EAC8ELV=VhdHPXmMa%2BGE5FxHy2ik7kj-S0S7VKcg@mail.gmail.com> In-Reply-To: <20130903192734.GA19406@albert.catwhisker.org> References: <520A6D07.5080106@freebsd.org> <5214F506.3070706@freebsd.org> <20130903192734.GA19406@albert.catwhisker.org>
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... this is bad behaviour. So yes, it needs to be chased up and repaired. Thanks for finding it out! On 3 September 2013 12:27, David Wolfskill <david@catwhisker.org> wrote: > On Wed, Aug 21, 2013 at 07:12:38PM +0200, Andre Oppermann wrote: > > On 13.08.2013 19:29, Julian Elischer wrote: > > > I have been tracking down a performance embarrassment on AMAZON EC2 > and have found it I think. > > > Our OS cousins over at Linux land have implemented some interesting > behaviour when TSO is in use. > > > > There used to be a different problem with EC2 and FreeBSD TSO. The Xen > hypervisor > > doesn't like large 64K TSO bursts we generate, the drivers drops the > whole TSO chain, > > TCP gets upset and turns off TSO alltogether leaving the connection > going at one > > packet a time as in the old days. > > ... > > My apologies for jumping in so late -- I'm not subscribed to -net@. > > At work, I received a new desktop machine a few months ago; here's a > recent history of what it has been running: > > FreeBSD 9.2-PRERELEASE #4 r254801M/254827:902501: Sun Aug 25 05:15:29 PDT > 2013 root@dwolf-fbsd:/usr/obj/usr/src/sys/DWOLF amd64 > FreeBSD 9.2-PRERELEASE #5 r255066M/255091:902503: Sat Aug 31 11:58:53 PDT > 2013 root@dwolf-fbsd:/usr/obj/usr/src/sys/DWOLF amd64 > FreeBSD 9.2-PRERELEASE #5 r255104M/255115:902503: Sun Sep 1 05:02:12 PDT > 2013 root@dwolf-fbsd:/usr/obj/usr/src/sys/DWOLF amd64 > > Now, I like to have a "private playground" for doing things with > machines, so I make use of both em(4) NICs on the machine: em0 connects > to the rest of the work network; em1 is connected to a switch I brought > in from home, and to which I connect "other things" (such as my laptop). > And because I'm fairly comfortable with them, I use IPFW & natd. For > some folks here, none of that should come as a surprise. :-}) > > For reference, the em(4) devices in question are: > > em0@pci0:0:25:0: class=0x020000 card=0x060d15d9 chip=0x10ef8086 > rev=0x06 hdr=0x00 > vendor = 'Intel Corporation' > device = '82578DM Gigabit Network Connection' > > and > > em1@pci0:3:0:0: class=0x020000 card=0x060d15d9 chip=0x10d38086 rev=0x00 > hdr=0x00 > vendor = 'Intel Corporation' > device = '82574L Gigabit Network Connection' > > > > I noticed that when I tried to write files to NFS, I could write small > files OK, but larger ones seemed to ... hang. > > Note: We don't use jumbo frames. (Work IT is convinced that they > don't help. I'm trying to better-understand their reasoning.) > > Further poking around showed that (under the above conditions): > * natd CPU% was climbing as more of the file was copied, up to 2^21 > bytes. (At that point, nothing further was saved on NFS.) > * dhcpd CPU% was also climbing. I tried killing that, but doing so > didn't affect the other results. (Killing natd made connectivity > cease, given the IPFW rules in effect.) > * Performing a tcpdump while trying to copy a file of length 117709618 > showed lots of TCP retransmissions. In fact, I'd hazard that every TCP > packet was getting retransmitted. > * "ifconfig -v em0" showed flags TSO4 & VLAN_HWTSO turned on. > * "sysctl net.inet.tcp.tso" showed "1" -- enabled. > > As soon as I issued "sudo net.inet.tcp.tso=0" ... the copy worked without > a hitch or a whine. And I was able to copy all 117709618 bytes, not just > 2097152 (2^21). > > Is the above expected? It came rather as a surprise to me. > > Peace, > david > -- > David H. Wolfskill david@catwhisker.org > Taliban: Evil cowards with guns afraid of truth from a 14-year old girl. > > See http://www.catwhisker.org/~david/publickey.gpg for my public key. >
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