TSO and FreeBSD vs Linux

[David Wolfskill]

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 .

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 at 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 at 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 at 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 at pci0:0:25:0:        class=0x020000 card=0x060d15d9 chip=0x10ef8086 rev=0x06 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = '82578DM Gigabit Network Connection'

and

em1 at 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 at 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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