CentOS - Oct 2010 - pdflush kernel thread pops up every 10 seconds or so and video decoding grinds to a halt for 1/2 a second

Hi. A friend of mine was doing real-time video decoding on
Fedora Core 13 and he had a performance glitch (1/2 a second
freeze) every 5-10 seconds.  "top" showed flush-253:0
process at the moment of the freeze.

Major device number 253 corresponds to device-mapper.  I advised my
friend to re-install his FC13 without LVM, to see if the glitch
is related to LVM.

After re-installing FC13 without LVM, he is seeing the glitch
every 10 seconds, and it shows flush-8:16 where before it said
flush-253:0.  8 is scsi disk driver. So it's not an LVM thing...
maybe a kernel thing?

I suggested he try Fedora 14 beta, as it has a newer kernel.
Maybe this kernel thing is fixed in the newer kernel.

He also tried CentOS 5.5, and saw a "pdflush" process popping
up with the same frequency, and resulting in a similar glitch.

Any other suggestions?

Best,
-at

On Mon, 2010-10-18 at 18:25 -0700, Aleksey Tsalolikhin
wrote:
> Hi. A friend of mine was doing real-time video decoding on
> Fedora Core 13 and he had a performance glitch (1/2 a second
> freeze) every 5-10 seconds.  "top" showed flush-253:0
> process at the moment of the freeze.
And what is the Priority of it running at?  How many Cores also?
> He also tried CentOS 5.5, and saw a "pdflush" process popping
> up with the same frequency, and resulting in a similar glitch.
> 
> Any other suggestions?
I would not even be concerned. ATM I am seeing pdflush on a server pop
every second.
load average: 4.51, 3.41, 3.51

I would only be concerned with the Freeze.  What's uname -a on the
CentOS machine?

John

On Monday, October 18, 2010 09:25:41 pm Aleksey Tsalolikhin
wrote:
> Hi. A friend of mine was doing real-time video decoding on
> Fedora Core 13 and he had a performance glitch (1/2 a second
> freeze) every 5-10 seconds.  "top" showed flush-253:0
> process at the moment of the freeze.
[snip]
> He also tried CentOS 5.5, and saw a "pdflush" process popping
> up with the same frequency, and resulting in a similar glitch.
> 
> Any other suggestions?
What kind of hard drive is this?  How is/are your drive(s) set up?

You need the iostat program (in the sysstat package, I think) to give you more
detail; there are some pointers to its use in this list's archives.