cygwin performance problem

[Chris Rapier]

Could you tell me a bit more about your network setup including the RTT 
between the hosts? I'm going to try this under OS X on 100bT and between 
two linux hosts at 1Gb. The results will mean more if the differences in 
the network environment are accounted for.

Corinna Vinschen wrote:
> On Apr 18 22:47, Damien Miller wrote:
>> On Tue, 4 Apr 2006, Corinna Vinschen wrote:
>>> Not sure if it's ok to discuss this here. However, we have some
>>> performance problems on Cygwin with the vanilla version of OpenSSH.  The
>>> main problem is the size of the read buffers in the client loop, resp.
>>> client_process_net_input/client_process_input.  The buffer size is a
>>> fixed 8K.  For some reason this degrades performance on Windows
>>> enormously
>> Is it necessary to adjust both these buffers? Or does increasing only
>> the network buffer in client_process_net_input() help?
> 
> Yes, it's apparently enough to change the buffer in client_process_net_input.
>>> By raising the aforementioned buffers in clientloop.c from 8192 to, say,
>>> 87380 (default tcp recv buffer size), the latter copy is way faster:
>> Have you tried smaller numbers than 87380? It isn't much effort to
>> crank these buffers up if smaller reads have a performance penalty
>> on Windows, but I'd prefer to make changes based on some measurements.
>> Could you try a range of buffer sizes from 8192 up (doubling perhaps)
>> and see if there is a sweet spot of size vs. performance?
> 
> I must admit that I have only tried 65536 and bigger sizes.  I made some
> tests now with smaller buffer sizes, and the results are interesting.
> There's a performance maximum, which is at buffer sizes between 17408
> and 21504 bytes.  Every other buffer size below or above these values
> results in a decrease in performance.
> 
>   Buffersize   Throughput
>     Bytes          MB/s
> 
>      8192          1.3
>     12288          3.5
>     16384          6.6
>     17408          9.1
>     18432          9.1
>     19456          9.1
>     20480          9.1
>     21504          9.1
>     22528          5.9
>     24576          5.9
>     32768          5.9 
>     49152          5.9
>     65536          5.6
>     87380          5.6
>    131072          5.6
>    524288          5.6
> 
> I don't see a reason for the maximum at this 17K to 21K area, but it's
> definitely not a problem of intermediate buffering in Cygwin.  The user
> buffer is directly used in the call to the underlying Windows sockets
> call.
> 
> So, well, maybe setting the buffer size to 20480 would be a useful
> compromise for all OSes?!?
> 
> 
> Corinna
>