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Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 8/28/2021 4:43 AM, Corinna Vinschen via Cygwin wrote:
> On Aug 28 02:21, Takashi Yano via Cygwin wrote:
>> On Fri, 27 Aug 2021 12:00:50 -0400
>> Ken Brown wrote:
>>> Two years ago I thought I needed nt_create to avoid problems when calling
>>> set_pipe_non_blocking.  Are you saying that's not an issue?  Is
>>> set_pipe_non_blocking unnecessary?  Is that the point of your modification to
>>> raw_read?
>>
>> Yes. Instead of making windows read function itself non-blocking,
>> it is possible to check if the pipe can be read before read using
>> PeekNamedPipe(). If the pipe cannot be read right now, EAGAIN is
>> returned.
> 
> The problem is this:
> 
>    if (PeekNamedPipe())
>      ReadFile(blocking);
> 
> is not atomic.  I. e., if PeekNamedPipe succeeds, nothing keeps another
> thread from draining the pipe between the PeekNamedPipe and the ReadFile
> call.  And as soon as ReadFile runs, it hangs indefinitely and we can't
> stop it via a signal.
> 
> Is a blocking ReadFile actually faster than a non-blocking read?  Or
> does it mainly depend on BYTE vs. MESSAGE mode?
> 
> What if the pipe is created non-blocking and stays non-blocking all the
> time and uses BYTE mode all the time?  Just as sockets, it would always
> only emulate blocking mode.  Wouldn't that drop code size a lot and fix
> most problems?

It would certainly reduce the code size.  I think it's worth a try, and we can 
see if any problems remain.

Ken

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