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From: Ken Brown <kbrown AT cornell DOT edu>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject: Re: The timerfd functions slow down emacs
Date: Sat, 23 Feb 2019 16:05:03 +0000
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On 2/21/2019 6:52 PM, Ken Brown wrote:
> When emacs is built, it detects the timerfd functions and uses them if they're
> found.  Now that Cygwin has these functions, the resulting build of emacs is
> very slow to respond to user input.  If I press a key, there is a 1-2 second
> delay before emacs responds.
> 
> I can work around this by configuring emacs with emacs_cv_have_timerfd=no, but
> it would be nice to find the cause.  I'll try to make an STC, but I thought I'd
> report it here in the meantime, in case anyone has ideas or debugging suggestions.

I don't have an STC yet, but if I start emacs and then attach gdb to it at 
random times, the backtrace always look like this:

#0  0x00007ffbb192b4f4 in ntdll!ZwWaitForMultipleObjects ()
    from /c/WINDOWS/SYSTEM32/ntdll.dll
#1  0x00007ffbadf56099 in WaitForMultipleObjectsEx ()
    from /c/WINDOWS/System32/KERNELBASE.dll''
#2  0x00007ffbadf55f8e in WaitForMultipleObjects ()
    from /c/WINDOWS/System32/KERNELBASE.dll
#3  0x0000000180159f9d in timerfd_tracker::wait (this=this AT entry=0x1803a22c0,
     nonblocking=false)
     at /usr/src/debug/cygwin-3.0.1-1/winsup/cygwin/timerfd.cc:448
#4  0x00000001800a7b0e in fhandler_timerfd::read (this=<optimized out>,
     ptr=0xffffba30, len=@0xffffb9b0: 8)
     at /usr/src/debug/cygwin-3.0.1-1/winsup/cygwin/fhandler_timerfd.cc:134
[...]

This suggests to me that the slowdown comes from timerfd_tracker::wait.

The context is that emacs creates a timerfd timer that expires every 2 seconds, 
'select' waits for the timer's file descriptor to be ready for reading, and then 
emacs calls 'read'.

Ken

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