bug#28149: 26.0.50; Emacs regularly hangs for multiple seconds in e.g. find-file

[Alexander Asteroth <alexander.asteroth@h-brs.de>]

Eli Zaretskii writes:
>> I tried and after bisection after bisection, hangs got less but did not
>> disappear. Then I did one final test with emacs -Q again and actually
>> managed, after several minutes, a hanging as well, after find-file with
>> completion in buffers with lots of files in it (/usr/share/... or
>> similar).
>>
>> I come to the conclusion, that it probably is not an emacs problem at
>> all but rather a hardware or OS problem that manifests only in a singe
>> threaded application like emacs.
>
> Hmm.  This description is not entirely consistent with your original
> report, which said:
>
>> my emacs hangs pretty often anf for 10 secs. ca. when I am prompted for
>> e.g. a file in find-file, or an interactive function in M-x or a
>> shell-command in read-shell-command. It seems to happen only, in these
>> cases, when there is an option to auto-complete.

Not inconsistend but there seems to be a gradual decrease in the
frequency this happens (depending on the size of my init.el). With emacs
-Q it happens so rarely, I have to change folder and complete very
often.

>
> This doesn't sound like the hangs were while Emacs was actually
> auto-completing on file names.  Was that just an inaccuracy in the
> description, and you in fact see the hangs only while Emacs completes
> on file names, after you type TAB etc.?
>
> In any case, 10 sec for completion sounds like a lot to me.
> Especially since yours looks like an optimized build.  How many files
> do you have in the directory where completion is so slow?  And what
> kind of CPU do you have on that system?

I also tried emacs 25.2 with the same result. System is a 1 year old
Lenovo ThinkPad Yoga 260.

As I said I'm not really sure any more it is emacs. What is particularly
surprising is the huge difference between read on file descriptor 3 and 
4. fd 3 take 0.00002 on average, fd 4 takes 0.4 to 0.9 seconds.

-- Alex