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Date:      Tue, 06 May 2008 15:34:23 -0400
From:      Matthew Pope <mpope@oanda.com>
To:        freebsd-ipfw@freebsd.org
Subject:   Re: dummynet queue size relative to bw setting?
Message-ID:  <4820B2BF.6090608@oanda.com>
In-Reply-To: <4820AB8F.2000204@oanda.com>
References:  <4820AB8F.2000204@oanda.com>

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I must correct my test parameters: In one of the two pipes, the bw was 
4K, not 48K as stated. 
When I just now moved it up to 48K to match the other pipe size, my ping 
times plummeted to 129-139ms throughout the Queue sizes listed below, 
again with Q=120 getting total packet loss. 
I thought a ping sent packets slowly, so that the 4K bw on the one pipe 
would not slow things down, but it seems I was wrong.  Still I'm 
wondering why the measured delay is 130, without dummynet its 40, and 
I've set it to 5ms in each direction, so it should be measured as 50, 
not 130.  Thx, Matthew
> Hello,
> I've been reading about dummynet for 2 weeks, including the seminal 
> ACM paper & I'm very impressed.  I've configured and run some 
> preliminary simulations that have my colleagues quite interested too.
>
> However, I'm finding my delay settings are yielding delays of about 
> two orders of magnitude larger that requested.  I believe I don't 
> understand the relationship very well that defines the setting of the 
> queue size to relative to the bandwidth setting (and plr?)  Can 
> someone explain or point me to a source for this?
>
> I recall reading that with lower bandwidths one should use lower queue 
> sizes to avoid long queuing delays.  So I presume that is why my 
> delays are so long. So I've run some tests with various queue sizes.
>
> With Queue sizes of 100, 80, 60, 40, 10 slots on a pipe with a bw of 
> 48Kbits/s, delay of 5ms, and plr 0.025 defined in each direction, I'm 
> consistently getting RT delays of 500-600ms with a ping test, packet 
> loss does come out around 5%.  The target I'm pinging is normally a 40 
> ms latency.  At Queue size 120 I get 100% packet loss (but I can 
> ignore that).
>
> I am not a networking specialist, so I realize my question is ignorant 
> :-) I'm running this in VMWare Server on a dual core 2 GHz with 2 GB 
> RAM using a modification of the dummynet test network design described 
> at codefromthe70s.org
> Thanks,
> Matthew
>
>




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