[mythtv-users] pdflush is killing me

Tony Lill ajlill at ajlc.waterloo.on.ca
Tue Feb 6 03:47:26 UTC 2007

Nick Tan <nick at wehi.EDU.AU> writes:

> 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:
> dirty_writeback_centisecs
> -------------------------
> 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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THis worked for me (basicly by making pdflush run more often, but
with less work to do).

# % of physical memory after which a process doing a write will be paused
# untill dirty pages have been written
# default 40
echo 50 > /proc/sys/vm/dirty_ratio

# % of physical memory at which pdflush will be woken. Hopefull this will limit
# system pauses for flushes
# default 10
echo 5 > /proc/sys/vm/dirty_background_ratio

Tony Lill,                         Tony.Lill at AJLC.Waterloo.ON.CA
President, A. J. Lill Consultants        fax/data (519) 650 3571
539 Grand Valley Dr., Cambridge, Ont. N3H 2S2     (519) 241 2461
--------------- http://www.ajlc.waterloo.on.ca/ ----------------

Understatement of the century:
"Hello everybody out there using minix - I'm doing a (free) operating
 system (just a hobby, won't be big and professional like gnu) for
 386(486) AT clones"

	- Linus Torvalds, August 1991

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