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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 8:22 PM, Takashi Yano via Cygwin wrote:
> On Mon, 30 Aug 2021 09:13:14 +0900
> Takashi Yano wrote:
>> On Sun, 29 Aug 2021 17:04:56 -0400
>> Ken Brown wrote:
>>> 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?
>>
>> I have just checked this, and the answer is "No". Due to this problem,
>> NtQueryInformationFile() call in select() fails on the write pipe.
>>
>> It seems that we need more consideration...
> 
> We have two easy options:
> 1) Configure the pipe with PIPE_ACCESS_DUPLEX.
> 2) Use nt_create() again and forget C# program issue.

I vote for 2), but let's see what Corinna thinks.

> Even without this problem, select() for writing pipe has a bug
> and does not wrok as expected. The following patch seems to be
> needed.
> 
> diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
> index 83e1c00e0..ac2fd227e 100644
> --- a/winsup/cygwin/select.cc
> +++ b/winsup/cygwin/select.cc
> @@ -612,7 +612,6 @@ pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
>             that.  This means that a pipe could still block since you could
>             be trying to write more to the pipe than is available in the
>             buffer but that is the hazard of select().  */
> -      fpli.WriteQuotaAvailable = fpli.OutboundQuota - fpli.ReadDataAvailable;
>         if (fpli.WriteQuotaAvailable > 0)
>          {
>            paranoid_printf ("fd %d, %s, write: size %u, avail %u", fd,
> 

I agree.

Ken

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