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Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Ken Brown via Cygwin <cygwin@cygwin.com>
Reply-To: Ken Brown <kbrown@cornell.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.

That's a great idea.

I've applied your two patches to the topic/pipe branch.  I also rebased it and 
did a forced push in order to bring in Corinna's loader script fix.  So you'll 
have to do 'git fetch' and 'git rebase --hard origin/topic/pipe'.

Does this now fix all known problems with pipes?

Corinna, do you still see any benefit to switching to PIPE_NOWAIT?  AFAICT, it 
wouldn't decrease the code size at this point, so the only question is whether 
it might improve performance.

If you think it's worth trying, I'd be glad to code it up on a new branch, and 
we could compare the two.

Aside from that, I'm wondering how and when to merge the new pipe implementation 
to master.  It obviously needs much more widespread testing than it's gotten so 
far.  I'm a little nervous about it because I haven't thought about the details 
for two years, and no one other than me has tested it until a few days ago.

Ken

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