Messages in this thread | | | Date | Thu, 11 Jan 2007 12:08:10 +1100 | From | Nick Piggin <> | Subject | Re: [REGRESSION] 2.6.19/2.6.20-rc3 buffered write slowdown |
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David Chinner wrote: > On Thu, Jan 11, 2007 at 10:13:55AM +1100, Nick Piggin wrote: > >>David Chinner wrote: >> >>>On Wed, Jan 10, 2007 at 03:04:15PM -0800, Christoph Lameter wrote: >>> >>> >>>>On Thu, 11 Jan 2007, David Chinner wrote: >>>> >>>> >>>> >>>>>The performance and smoothness is fully restored on 2.6.20-rc3 >>>>>by setting dirty_ratio down to 10 (from the default 40), so >>>>>something in the VM is not working as well as it used to.... >>>> >>>>dirty_background_ratio is left as is at 10? >>> >>> >>>Yes. >>> >>> >>> >>>>So you gain performance by switching off background writes via pdflush? >>> >>> >>>Well, pdflush appears to be doing very little on both 2.6.18 and >>>2.6.20-rc3. In both cases kswapd is consuming 10-20% of a CPU and >>>all of the pdflush threads combined (I've seen up to 7 active at >>>once) use maybe 1-2% of cpu time. This occurs regardless of the >>>dirty_ratio setting. >> >>Hi David, >> >>Could you get /proc/vmstat deltas for each kernel, to start with? > > > Sure, but that doesn't really show the how erratic the per-filesystem > throughput is because the test I'm running is PCI-X bus limited in > it's throughput at about 750MB/s. Each dm device is capable of about > 340MB/s write, so when one slows down, the others will typically > speed up.
But you do also get aggregate throughput drops? (ie. 2.6.20-rc3-worse)
> So, what I've attached is three files which have both > 'vmstat 5' output and 'iostat 5 |grep dm-' output in them.
Ahh, sorry to be unclear, I meant:
cat /proc/vmstat > pre run_test cat /proc/vmstat > post
It might just give us a hint what is changing (however vmstat doesn't give much interesting in the way of pdflush stats, so it might not show anything up).
Thanks, Nick
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