cygrunsrv + sshd + rsync = 20 times too slow -- throttled?

[Ken Brown]

On 9/12/2021 7:04 AM, Takashi Yano wrote:
> On Sun, 12 Sep 2021 17:48:49 +0900
> Takashi Yano wrote:
>> On Sat, 11 Sep 2021 09:12:02 -0400
>> Ken Brown wrote:
>>> On 9/10/2021 10:35 PM, Takashi Yano wrote:
>>>> On Fri, 10 Sep 2021 22:17:21 -0400
>>>> Ken Brown wrote:
>>>>> On 9/10/2021 6:57 PM, Takashi Yano wrote:
>>>>>> On Fri, 10 Sep 2021 11:17:58 -0400
>>>>>> Ken Brown wrote:
>>>>>>> I've rerun your test with the latest version, and the test results are similar.
>>>>>>>      I've also run a suite of fifo tests that I've accumulated, and they all pass
>>>>>>> also, so I pushed your patch.
>>>>>>>
>>>>>>> I think we're in pretty good shape now.  The only detail remaining, AFAIK, is
>>>>>>> how to best avoid a deadlock if the pipe has been created by a non-Cygwin
>>>>>>> process.  I've proposed a timeout, but maybe there's a better idea.
>>>>>>
>>>>>> I am not pretty sure what is the problem, but is not the following
>>>>>> patch enough?
>>>>>>
>>>>>> diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
>>>>>> index d309be2f7..13fba9a14 100644
>>>>>> --- a/winsup/cygwin/fhandler.h
>>>>>> +++ b/winsup/cygwin/fhandler.h
>>>>>> @@ -1205,6 +1205,7 @@ public:
>>>>>>       select_record *select_except (select_stuff *);
>>>>>>       char *get_proc_fd_name (char *buf);
>>>>>>       int open (int flags, mode_t mode = 0);
>>>>>> +  void open_setup (int flags);
>>>>>>       void fixup_after_fork (HANDLE);
>>>>>>       int dup (fhandler_base *child, int);
>>>>>>       int close ();
>>>>>> diff --git a/winsup/cygwin/fhandler_pipe.cc b/winsup/cygwin/fhandler_pipe.cc
>>>>>> index 6994a5dce..d84e6ad84 100644
>>>>>> --- a/winsup/cygwin/fhandler_pipe.cc
>>>>>> +++ b/winsup/cygwin/fhandler_pipe.cc
>>>>>> @@ -191,6 +191,17 @@ out:
>>>>>>       return 0;
>>>>>>     }
>>>>>>
>>>>>> +void
>>>>>> +fhandler_pipe::open_setup (int flags)
>>>>>> +{
>>>>>> +  fhandler_base::open_setup (flags);
>>>>>> +  if (get_dev () == FH_PIPER && !read_mtx)
>>>>>> +    {
>>>>>> +      SECURITY_ATTRIBUTES *sa = sec_none_cloexec (flags);
>>>>>> +      read_mtx = CreateMutexW (sa, FALSE, NULL);
>>>>>> +    }
>>>>>> +}
>>>>>> +
>>>>>>     off_t
>>>>>>     fhandler_pipe::lseek (off_t offset, int whence)
>>>>>>     {
>>>>>>
>>>>>>
>>>>>> AFAIK, another problem remaining is:
>>>>>>
>>>>>> On Mon, 6 Sep 2021 14:49:55 +0200
>>>>>> Corinna Vinschen wrote:
>>>>>>> - What about calling select for writing on pipes read by non-Cygwin
>>>>>>>      processes?  In that case, we still can't rely on WriteQuotaAvailable,
>>>>>>>      just as before.
>>>>>
>>>>> This is the problem I was talking about.  In this case the non-Cygwin process
>>>>> might have a large pending read, so that the Cygwin process calling select on
>>>>> the write side will see WriteQuotaAvailable == 0.  This could lead to a deadlock
>>>>> with the Cygwin process waiting for write ready while the non-Cygwin process is
>>>>> blocked trying to read.
>>>>
>>>> Then, the above patch is for another issue.
>>>> The problem happes when:
>>>> 1) Start command prompt.
>>>> 2) Run 'echo AAAAAAAAAAAA | \cygwin64\bin\cat
>>>> This causes hang up in cat. In this case, pipe is created by cmd.exe.
>>>> Therefore, read_mtx is not created.
>>>
>>> Confirmed, and your patch fixes it.  Maybe you should check for error in the
>>> call to CreateMutexW and print a debug message in that case.
>>>
>>>>> My suggestion is that we impose a timeout in this situation, after which select
>>>>> reports write ready.
>>>>
>>>> Keeping read handle in write pipe (Corinna's query_hdl) causes problem
>>>> that write side cannot detect close on read side.
>>>> Is it possible to open read handle temporally when pipe_data_available()
>>>> is called?
>>>
>>> That would be nice, but I have no idea how you could do that.
>>
>> Hmm. Then, what about PoC code attached? This returns to Corinna's
>> query_hdl, and counts read/write handles to detect closing reader side.
>>
>> If the number of read handles is equal to number of write handles,
>> only the pairs of write handle and query_hdl are alive. So, read pipe
>> supposed to be closed.
>>
>> This patch depends another patch I posted a few hours ago.
> 
> Revised a bit.

I don't see how this solves the problem.  In the case we were worried about 
where we have a non-Cygwin reader, the writer has no query_hdl, and you're just 
always reporting write ready, aren't you?  Or am I missing something?

Ken