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Date:	Wed, 4 Aug 2010 02:00:13 -0600
From:	Jack Zhang <>
To:	Bill Fink <>
Subject: Re: can TCP socket send buffer be over used?

Hi Bill,

Thanks a lot for your help.

It does make sense!

As I'm writing this part into my master thesis, do you happen to know
which part in the source code I can maybe use as a proof in the thesis
that Linux uses 1/4 of the doubled buffer size for metadata?


On 4 August 2010 01:20, Bill Fink <> wrote:
> On Tue, 3 Aug 2010, Jack Zhang wrote:
>> Hi there,
>> I'm doing experiments with (modified*) software iSCSI over a link with
>> an emulated Round-Trip Time (RTT) of 100 ms by netem.
>> For example, when I set the send buffer size to 128 KB, i could get a
>> throughput up to 43 Mbps, which seems to be impossible as the (buffer
>> size) / RTT is only 10 Mbps.
> I'm not sure what's going on with this first case.
>> And When I set the send buffer size to 512 KB, i can get a throughput
>> up to 60 Mbps, which also seems to be impossible as the (buffer size)
>> / RTT is only 40 Mbps.
> But this case seems just about right.  Linux doubles the requested
> buffer size, then uses one quarter of that for overhead (not half),
> so you effectively get 50% more than requested (2X * 3/4 = 1.5X).
> Plugging your case into bc:
> wizin% bc
> scale=10
> 512*1024*8/0.100/10^6*3/2
> 62.9145600000
>                                                -Bill
>> I understand that when the buffer size is set to 128 KB, I actually
>> got a buffer of 256 KB as the kernel doubles the buffer size. I also
>> understand that half the doubled buffer size is used for meta data
>> instead of the actual data to be transferred. So basically the
>> effective buffer sizes for the two examples  are just 128 KB and 512
>> KB respectively.
>> So I was confused because, theoretically, send buffers of 128 KB and
>> 512 KB should achieve no more than 10 Mbps and 40 Mbps respectively
>> but I was able to get way much more than the theoretical limit. So
>> I was wondering is there any chance the send buffer can be "overused"?
>> or there is some other mechanism inside TCP is doing some
>> optimization?
>> * the modification is to disable "TCP_NODELAY" , enable
>> "use_clustering" for SCSI, and set different send buffer sizes for the
>> TCP socket buffer.
>> Any idea will be highly appreciated.
>> Thanks a lot!
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