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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 11:43 AM, Takashi Yano via Cygwin wrote:
> On Sat, 28 Aug 2021 13:58:08 +0200
> Corinna Vinschen wrote:
>> On Aug 28 18:41, Takashi Yano via Cygwin wrote:
>>> On Sat, 28 Aug 2021 10:43:27 +0200
>>> Corinna Vinschen 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.
>>>
>>> Hmm, you are right. Mutex guard seems to be necessary like pty code
>>> if we go this way.
>>>
>>>> Is a blocking ReadFile actually faster than a non-blocking read?  Or
>>>> does it mainly depend on BYTE vs. MESSAGE mode?
>>>
>>> Actually, I don't think so. Perhaps it is not essential problem of
>>> overlapped I/O but something is wrong with current pipe code.
>>>
>>>> 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?
>>>
>>> If 'non-blocking' means overlapped I/O, only the problem will be:
>>> https://cygwin.com/pipermail/cygwin/2021-March/247987.html
>>
>> Sorry if that wasn't clear, but I was not talking about overlapped I/O,
>> which we should get rid off, but of real non-blocking mode, which
>> Windows pipes are fortunately capable of.
> 
> Do you mean, PIPE_NOWAIT flag? If this flags is specified in
> the read pipe, non-cygwin apps cannot read the pipe correctly.

While waiting for Corinna's response to this, I have one more question.  Do you 
understand why nt_create() failed and you had to revert to create()?  Was it an 
access problem because nt_create requested FILE_WRITE_ATTRIBUTES?  Or did I make 
some careless mistake in writing nt_create?

Ken

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