Re: Cygwin TCP slow

[Sam Habiel <sam.habiel@gmail.com>]

"Then we made a private build of Windows that ignores SO_RCVBUF and
SO_SNDBUF and just always uses
autotuning no matter what the app does."

Out of curiosity, how did you do that? Do you work for Microsoft, or
is there something fantastic I missed about building your own modified
DLLs?

On Tue, Nov 29, 2016 at 8:08 PM, Daniel Havey <dhavey@gmail.com> wrote:
> Okay, I will find some time to produce a patch.  It might take a while
> though because I have a day job :).  BTW, what the heck is an STC?
> Here is an experiment with three machines like this: O----O----O
> The one in the middle has a 50ms of delay (25ms in each direction).
>
> Here are the results from Cygwin on top of normal Windows:
> [send side perf]
> f:\home>iperf3 -c 10.178.204.101
> [ ID] Interval           Transfer     Bandwidth
> [  4]   0.00-10.00  sec  47.5 MBytes  39.8 Mbits/sec                  sender
> [  4]   0.00-10.00  sec  47.5 MBytes  39.8 Mbits/sec                  receiver
>
> [receive side perf]
> f:\home>iperf3 -c 10.178.204.101 -R
> [ ID] Interval           Transfer     Bandwidth       Retr
> [  4]   0.00-10.00  sec  41.2 MBytes  34.5 Mbits/sec    0             sender
> [  4]   0.00-10.00  sec  39.7 MBytes  33.3 Mbits/sec                  receiver
>
> This matches the calculated performance.  Then we made a private build
> of Windows that ignores SO_RCVBUF and SO_SNDBUF and just always uses
> autotuning no matter what the app does.
> [send side perf]
> C:\testbox\tests>iperf3 -c 10.178.204.101
> [ ID] Interval           Transfer     Bandwidth
> [  4]   0.00-10.00  sec   540 MBytes   453 Mbits/sec                  sender
> [  4]   0.00-10.00  sec   540 MBytes   453 Mbits/sec                  receiver
>
> [receive side perf]
> C:\testbox\tests>iperf3 -c 10.178.204.101 -R
> [ ID] Interval           Transfer     Bandwidth       Retr
> [  4]   0.00-10.00  sec   553 MBytes   464 Mbits/sec    0             sender
> [  4]   0.00-10.00  sec   553 MBytes   464 Mbits/sec                  receiver
>
> If you set SO_RCVBUF and SO_SNDBUF then performance will be limited
> according to the calculated values.  If you don't set those values and
> let Windows autotuning do its thing then you will always get the
> maximum available throughput.
>
> I'll email again when I have the patch.  If you would like more
> testing let me know and we can have our test people run some more
> experiments.
>
> thanxs :)
> ...Daniel
>
>
> On Mon, Nov 28, 2016 at 12:51 PM, Brian Inglis
> <Brian.Inglis@systematicsw.ab.ca> wrote:
>> On 2016-11-28 12:54, Daniel Havey wrote:
>>> We have had complaints from several large hardware vendors that
>>> Windows networking is slow for apps like iperf that are used to
>>> measure throughput.  Iperf on Windows is compiled against the
>>> cygwin1.dll.  We have root caused the problem to a couple of lines of
>>> code in net.cc that set SO_RCVBUF and SO_SNDBUF to about 200KB.
>>>
>>> The theoretical window/RTT plot for the buffer size set by Cygwin
>>> (0x34000 = 200KB) gives us:
>>> 1ms -> 1703Mbps
>>> 2ms -> 851Mbps
>>> 3ms -> 567Mbps
>>> 4ms -> 425Mbps
>>> 5ms -> 340Mbps
>>> 6ms -> 283Mbps
>>> 7ms -> 243Mbps
>>> 8ms -> 212Mbps
>>> 9ms -> 189Mbps
>>> 10ms -> 170Mbps
>>> 20ms -> 85Mbps
>>> 40ms -> 42Mbps
>>> 60ms -> 28Mbps
>>> 80ms -> 21Mbps
>>>
>>> We have confirmed this by experiment and also confirmed that the
>>> limitation goes away if the buffers are not manually set.  Windows has
>>> autotuning and when the buffers are set manually the autotuning is
>>> disabled.  This is causing the throughput limitation.  So we would
>>> like to formally ask that you please not manually set SO_RCVBUF or
>>> SO_SNDBUF.
>>
>> See problem reports: http://cygwin.com/problems.html
>> Provide STC, patches, attach cygcheck -svr output?
>> Links to downstream bug reports, testing, results?
>> Note that Cygwin iperf is year old 2.0.5.
>>
>> --
>> Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
>>
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