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Re: [Xen-users] strangeness with high BDP tests

  • To: Nivedita Singhvi <niv@xxxxxxxxxx>
  • From: Diwaker Gupta <diwakergupta@xxxxxxxxx>
  • Date: Mon, 4 Apr 2005 12:18:43 -0700
  • Cc: xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 04 Apr 2005 19:18:44 +0000
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  • List-id: Xen user discussion <xen-users.lists.xensource.com>

> > I don't trust the 10 second tests either, especially for such a high
> > RTT. Thats why I ran the tests for 80 seconds (thats 1000 RTTs, and
> > should give TCP enough time to stabilize). I'll get some numbers using
> > these options in any case.
> Cool :). Thanks for offering to test, too.

No problemo :) So I tried the -i and -I options... not too much of a
difference. Slight improvement in the numbers, but the difference is
still stark.

> > Yeah, but in my experience it usually picks up the "default" value as
> > set by the sysctl. I'll check the code.
> In your netperf output, it's listing the socket size as
> the default system 64K. If you invoke netperf with -s 131762 -S 131762
> it should at least use 128K (local and remote). Bumping that
> up by 3 times usually gives good gain on netperf stream type
> tests bymmv..

I looked at the netperf source. If the -s/-S values are not specified,
it seems it sticks to the default values. Also, setsockopt only
changes the maximum buffer size, the default is still governed by the
sysctl values. Further, AFAIK, even the max value (in Linux) is just a
hint to the TCP stack -- the actual size of the buffer is determined
by the TCP auto-buffer tuning  code. In any case, since both dom0 and
the VM are using the same buffer sizes, I'm not too concerned about
setting the "right" buffer sizes. Right now, I want to figure out the
discrepancy in performance.

Diwaker Gupta

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