[Bug 198168] tail a FiFo: high IVCSW after first echo: 100% load

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198168

Mark Johnston  changed:

   What|Removed |Added

 CC||ma...@freebsd.org
 Status|New |Closed
 Resolution|--- |DUPLICATE

--- Comment #1 from Mark Johnston  ---


*** This bug has been marked as a duplicate of bug 203366 ***

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[Bug 198168] tail a FiFo: high IVCSW after first echo: 100% load

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198168

Bug ID: 198168
   Summary: tail a FiFo: high IVCSW after first echo: 100% load
   Product: Base System
   Version: 10.0-RELEASE
  Hardware: Any
OS: Any
Status: New
  Severity: Affects Only Me
  Priority: ---
 Component: kern
  Assignee: freebsd-bugs@FreeBSD.org
  Reporter: kal...@gmx.at

Created attachment 153665
  -- https://bugs.freebsd.org/bugzilla/attachment.cgi?id=153665action=edit
truss tail -f TestFiFo

Greetings!

I have stumbled over one very interesting problem on amd64 10.0-p18 as well
as on amd64 10.1-p6.

The shortest reconstruction I can present to you is as follows:

In one terminal:
$ mkfifo TestFiFo
$ tail -f TestFiFo

(and the manual page of tail(1) allows »tail -f« of a fifo)

In a second terminal let us start good old top(1). Use a-key to toggle
the displaying of process parameters (that way, we can find our tail more
easily). Our tail process should not do much.

Now in a third terminal:
$ echo I am some input  TestFiFo

We can see the text in our first terminal displayed by tail.

But: the load of that tail rises up to 100%; switching to io mode by
pressing the m-key in top shows us, that IVCSW rises to the sky;
furthermore the used cpu changes all the time.

The manual page does not state what IVCSW stands for, but in this article
http://lists.freebsd.org/pipermail/freebsd-questions/2007-September/158479.html
we are told: “IVCSW = 'Involuntary context switches'”.

I had a look at the output of truss(1). You can find the output of
»truss tail -f TestFiFo« in the attachment. (I did hit C-c at some point)
After writing our test text to the screen, the following happens:
---8---
madvise(0x80100d000,0x1000,0x5,0xaaab,0x7fffd650,0x800bc8be0) =
0 (0x0)
madvise(0x80101d000,0x1000,0x5,0xaaab,0x7fffd650,0x800bc8be0) =
0 (0x0)
kqueue(0x800b9fbc0,0x7fffc,0x0,0x0,0x9,0x3a8) = 4 (0x4)
fstatfs(0x3,0x7fffdfb8,0x1,0x800624330,0x2,0x80100d010) = 0 (0x0)
kevent(4,{0x3,EVFILT_READ,EV_ADD|EV_ENABLE|EV_CLEAR,0,0x0,0x0},1,0x0,0,{0.0
}) = 0 (0x0)
kevent(4,0x0,0,{0x3,EVFILT_READ,EV_CLEAR|EV_EOF,0,0x0,0x0},1,0x0) = 1 (0x1)
read(3,0x801009000,16384) = 0 (0x0)
kevent(4,0x0,0,{0x3,EVFILT_READ,EV_CLEAR|EV_EOF,0,0x0,0x0},1,0x0) = 1 (0x1)
read(3,0x801009000,16384) = 0 (0x0)
kevent(4,0x0,0,{0x3,EVFILT_READ,EV_CLEAR|EV_EOF,0,0x0,0x0},1,0x0) = 1 (0x1)
read(3,0x801009000,16384) = 0 (0x0)
---8---
repeating those last two lines over and over again.

I think, that is quite some interesting bug ;-)

(Could it be related to the closed Bug 23099 Comment 2 form the year 2000
in some way? As »cat TestFiFo« closes after the echo-command, I assume,
that EOF is sent?)

Regards,
 Kalten

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[Bug 198168] tail a FiFo: high IVCSW after first echo: 100% load

[bugzilla-noreply]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198168

Kalten kal...@gmx.at changed:

   What|Removed |Added

   See Also||https://bugs.freebsd.org/bu
   ||gzilla/show_bug.cgi?id=2309
   ||9
Version|10.0-RELEASE|10.1-RELEASE
   Hardware|Any |amd64
   Severity|Affects Only Me |Affects Many People

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