[mythtv-users] pdflush is killing me

Nick Tan nick at wehi.EDU.AU
Mon Feb 5 21:20:18 UTC 2007

Mark Lehrer wrote:
> Hello there, I'm having a problem with frequent pauses and I'm wondering
> if anyone here is familiar with pdflush and how to make it a little
> less painful.
> I use NFS for my storage, XFS file system.  The problem only seems to
> happen when I am both recording and playing back a hi def stream at
> the same time.  Every minute or so on the server, pdflush will wake up
> and freeze all I/O for a couple of seconds.  The server is running
> Ubuntu 6.06.1 LTS, with this kernel: 2.6.15-26-686 #1 SMP PREEMPT
> Any idea how to fix the problem?  It is disturbing that the ring
> buffer size doesn't seem to help at all - 90MB should be enough to
> handle a 30 second I/O pause but the myth playback is pausing the
> instant that pdflush wakes up on the server.  Is the ring buffer not
> being filled in a separate thread?  Does MythTV have an option that
> works the same way as mplayer's -cache option?  That seemed to always
> work pretty well.

Hi Mark,

According to the kernel documentation, pdflush periodically wakes up and 
flushes old data to disk.  On one of my servers it was set to 5 seconds. 
  Here's what I found in the kernel docs:


The pdflush writeback daemons will periodically wake up and write `old' 
data out to disk.  This tunable expresses the interval between those 
wakeups, in 100'ths of a second.

Setting this to zero disables periodic writeback altogether.


Try setting dirty_writeback_centisecs to zero by doing:

echo "0" > /proc/sys/vm/dirty_writeback_centisecs

and see if that helps

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