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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 8/29/2021 5:07 AM, Takashi Yano via Cygwin wrote:
> On Sat, 28 Aug 2021 18:41:02 +0900
> Takashi Yano 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.
> 
> I have found that set_pipe_non_blocking() succeeds for both read and
> write pipes if the write pipe is created by CreateNamedPipe() and the
> read pipe is created by CreateFile() contrary to the current create()
> code. Therefore, not only nt_create() but also PeekNamedPipe() become
> unnecessary.
> 
> Please see the revised patch attached.

I haven't had a chance to test this myself yet, but occurs to me that we might 
have a different problem after this patch: Does the write handle that we get 
from CreateNamedPipe() have FILE_READ_ATTRIBUTES access?

Ken

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